• Volume 60,Issue 2,2023 Table of Contents
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    • >Insights and Perspectives
    • Introduction and Analysis of NSFC Projects of Soil Science in 2022

      2023, 60(2):305-316. DOI: 10.11766/trxb202211020599

      Abstract (693) HTML (1038) PDF 3.73 M (1489) Comment (0) Favorites

      Abstract:The NSFC proposals and grants of soil science in 2022 were analyzed which mainly focused on three secondary application codes related to soil science in Environmental Geoscience discipline, including D0701 Environmental Soil Science, D0709 Fundamental Soil Science, and D0710 Soil Erosion and Soil Fertility. Furthermore, the applications, acceptances, peer reviews, project recommendations, grant funding, research teams, project’s scientific attributes and keywords data cloud for various types of NSFC projects were also analyzed comprehensively to provide reference for future applications.

    • >Reviews and Comments
    • Research Progress on the Microstructure and Constituents of Fe-Mn Nodules in Soil

      2023, 60(2):317-331. DOI: 10.11766/trxb202107130360

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      Abstract:Ferromanganese concretions (or Fe-Mn nodules), as a specific type of soil neoformations, are formed during pedogenic processes. The structure of concentric rings in soil Fe-Mn nodules can be used to reconstruct paleoclimatic conditions and soil-forming environments. The constituents of soil Fe-Mn nodules can provide nutrients and energy for microbial metabolism that in turn affect the transformation, fixation and mobilization of soil nutrients and heavy metals. This article reviewed the research progresses on soil Fe-Mn nodules during the past several decades, including(i) the formation mechanism and process of Fe-Mn nodules and the associated influencing factors; (ii)differences in the microstructure and constituents of Fe-Mn nodules in different regions; and(iii) effects of Fe-Mn nodules on nutrient transformation and heavy metal sorption. Future research priorities include(a) study of the formation rates and environmental thresholds of Fe-Mn nodules during different stages of soil development; (b) establishment of the evolution models of Fe-Mn nodules in different soil-forming environments; and(c) elucidation of the mechanisms of stabilization and release of soil nutrients and heavy metals by Fe-Mn nodules. This will facilitate understanding of the pedogenic processes and elemental biogeochemical cycling in the Earth’s Critical Zone and provide a basis for quantitative evaluation of soil quality and function under changing environments.

    • Research Progress on Microbial-Mediated Mitigation of Nitrous Oxide Emissions from Agricultural Soils

      2023, 60(2):332-344. DOI: 10.11766/trxb202106170315

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      Abstract:Nitrous oxide(N2O), an important greenhouse gas, has a global warming potential of 265 times higher than that of an equivalent concentration of carbon dioxide. The N2O has a long atmospheric lifetime and does deplete the ozone layer in the stratosphere. Agricultural soil is an important source of N2O, which has a characteristic of diverse generation paths, multiple impact factors, and complicated regulation processes. Mitigation of N2O emissions from agricultural soils has long been the hotspot of research in this field. N2O-reducing bacteria harboring N2O reductase can reduce N2O to dinitrogen(N2), which is the only known sink of N2O consumption as a primary substrate in the biosphere. The direct use of microorganisms to decrease N2O emissions from agricultural soils is an emerging technology. We elaborated on the biological sources and sinks of N2O emissions from agricultural soils in detail, paying special emphasis on the screening and application strategies of microorganisms that can mitigate N2O emissions. There are three strategies for the direct use of microorganisms to decrease N2O emissions from agricultural soils: (1) application of the prepared microbial inoculum directly to the agricultural soil; (2) combination of the prepared microbial inoculum with fertilizers or other carriers before being applied to the soil; (3) construction of the microbial community with N2O mitigation effect, and then direct application to the soil or in combination with a carrier before being applied to the soil. We summarized two ecological mechanisms of microbial-mediated mitigation of N2O emissions from agricultural soils. One mechanism involved employing N2O-reducing bacteria containing nosZ gene to directly convert N2O to N2 in order to mitigate N2O emissions from agricultural soils. The other mechanism utilizes plant growth-promoting rhizobacteria to alter the community composition, abundance and activity of the N2O-reducing bacteria and indirectly mitigate N2O emissions from agricultural soils. We also discussed the environmental factors that affect the reduction of N2O to N2 by directly using microorganisms and the potential challenges. The biological process of reducing N2O to N2 is affected by many environmental factors, including the availability of NO3- and carbon sources, oxygen concentration, moisture content, temperature, pH and copper concentration. Among them, Cu availability and pH are some of the most important factors that determine the activity of N2OR. Several issues need to be addressed in future studies. For example, there are only a limited number of strains that have been screened with N2O mitigating effects. It remains unknown whether the inoculum colonizes roots or survive in the environment after the inoculation. The microbial ecological mechanisms are poorly understood; such as, how the inoculum achieve their beneficial effects in environments. Moreover, we lack effective technical means to regulate the inoculum to fully exploit their beneficial effects. Further, the methods to evaluate N2O mitigating effects also need to be improved. Finally, prospects on the application of microbial-mediated mitigation of N2O emissions from agricultural soils were suggested. The review provides an important technical reference for achieving the agricultural carbon neutrality strategic goal in China.

    • Promising Applications of Artificial Humification and Its Products in Agricultural Production

      2023, 60(2):345-354. DOI: 10.11766/trxb202110080469

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      Abstract:Humic substance (HS) is the major component of soil organic matter and chemically can be depicted as a collection of diverse, relatively low-molecular-mass components of organic molecules, forming dynamic associations and stabilized by hydrophobic interactions and hydrogen bonds on the nanometer scale. The physiological effects of HS are widely documented and summarized as a result of enhancing nutrient use efficiency, aiding assimilation of both macro and micronutrients, and stimulating plant growth by induction of carbon, nitrogen, and secondary metabolism. Based on the formation mechanism and multifunctional properties of natural HS, scientists have applied HS to extracellular electron transport, environmental repair, cellular stress, and plant growth promotion. However, due to the limited supply of natural extraction sources of HS such as lignite, weathered coal, and peat, the market supply-demand of HS is increasing year by year. Given that the molecular structures of lignin and its derivatives are similar to natural HS, it is important to ask the question of whether artificial humification methods can be used to accurately control the oxidative decomposition of large-molecular lignin and/or the radical coupling of its small-molecular derivatives to synthesize humic-like products. Another important factor to consider is whether the synthesized products can match and even surpass the effects and functions of commercial HS. This creative idea has great commercial value and application prospects for realizing a continuous increase in agricultural production and income in China. In this review, the sources, structure characteristics, and functional attributes of natural HS were briefly generalized. The latest concept, mode, and mechanism of artificial humification by the use of macromolecule lignin and its derived-small phenolics are discussed, and the differences between artificial humification products and natural HS are also compared. It is confirmed that both artificial humification products and natural HS have C, H, O, N, and S elements, but the content of N in natural HS is higher than that of artificial humification products. In addition, they both contain phenolic -OH, -COOH, and aromatic components. Compared with artificial humification products, natural HS has more aromatic structures, but fewer oxygen-containing groups. Moreover, the application values of artificial humification products in agricultural production were explored. The agronomic effects of artificial humification products are extremely significant. On the one hand, artificial humification products can ameliorate soil physicochemical properties, increase soil fertility and water-holding capacity. On the other hand, these humic-like products are also able to serve as plant nutrient pools for promoting crop growth and development, improving nutrient use efficiency and crop yield. Therefore, the practical applications of artificial humification products are of great significance for agricultural production. At present, artificial humification techniques such as chemical oxidation, hydrothermal reaction, bio-composting, Fenton reaction, and fungal laccase-catalyzed oxidation have displayed a great application potential in the precise control of humic-like substance synthesis. In particular, fungal laccase exhibits unique advantages in humic-like substance synthesis due to its dual mechanism of oxidative decomposition and radical coupling. The humification reactions induced by fungal laccase have the characteristics of high catalytic efficiency, simple and controllable operating conditions, low energy requirements, and environmental protection, thus can be used to synthesize a variety of humic-like products. Also, the advantages and disadvantages, process paths, and main control factors of these humic-like substance synthesis technologies are summarized. Hence, this study provides theoretical support and technical guidance for researchers to conquer the artificial humification of lignin and its derivatives and the technology bottleneck of its large-scale application.

    • >Research Articles
    • Spatiotemporal Variation of Fertility Quality of Chinese Paddy Soil Based on Fuzzy Method in Recent 30 Years

      2023, 60(2):355-366. DOI: 10.11766/trxb202107050343

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      Abstract:【Objective】The spatiotemporal variation characteristics and driving factors of fertility quality of paddy soils in China were studied. 【Method】 Based on the long-term monitoring data (1988-2017) of the Ministry of Agriculture and Rural Affairs in the main rice areas in China, the fertility quality of paddy soil in the main rice areas was evaluated by the fuzzy mathematics (Fuzzy) method, and combined geo-statistics with machine learning. 【Result】In the recent 30 years, the soil fertility index (SFI) of the rice area in China increased significantly (P < 0.05). From the prophase (1988-1999) to the interim (2000-2009) and from the interim to the recent (2010-2017), the national paddy field SFI increased by 6.9% and 17.7% on average, respectively. The characteristics of SFI varied in different time stages in the main rice regions. The rice regions in southwest China and the middle reaches of the Yangtze River showed a significant increasing trend of slow in the early period (from prophase to interim) and fast in the later period (from interim to recent) (P < 0.05). Also the rice fields in South China and the Yangtze River Delta showed a rising trend of fast in the early stage (from prophase to interim) and stable in the later stage (from interim to recent). There was a decreasing trend in the earlier period (from interim to recent) and increasing in the later period (from interim to recent) in the Northeast rice region. At the recent, the distribution of paddy SFI in China showed a trend of higher in the north and lower in the south, higher in the east and lower in the west. The area with SFI exceeding the national average (0.61) accounted for 50.2%. Compared with the SFI of the rice region in the interim, the SFI showed an overall upward trend in the recent. The increased area accounted for 69.3%. The relative importance of soil organic matter and total nitrogen to SFI change (19.4%-60.3%) was much higher than that of other indexes (0.1%-13.3%) in all rice areas in the prophase and interim (except the middle stage of the northeast rice region). The relative importance of soil Olsen-P, available potassium and soil pH (except the northeast rice region) increased in all rice regions, and the relative importance of soil Olsen-P content increased to the maximum in the middle reaches of the Yangtze River. However, the relative importance of soil total nitrogen and soil organic matter content was greater in all rice regions.【Conclusion】In conclusion, the spatiotemporal variation characteristics of soil fertility quality and the changes of key driving factors at different stages and in different rice regions should be comprehensively considered. The balanced fertilization measures should be optimized to prevent and control soil acidification and to improve the fertility quality of paddy soil.

    • Acidification Resistance of Tobacco-planting Soils Derived from Different Parent Materials in Wuling Qinba District, China

      2023, 60(2):367-377. DOI: 10.11766/trxb202107010339

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      Abstract:【Objective】 This study aimed to investigate the effects of parent materials and initial soil pH on the resistance of soils to acidification. 【Method】 Twelve soil samples derived from different parent materials under tobacco-cultivation were collected from Wuling Qinba District, China. The pH buffering capacity (pHBC) of the different soil samples was determined by acid-base titration in the pH range 4.0 to 7.0. Additionally, the changes of soil pH, soluble Al and exchangeable Al following acid input were investigated through simulated acidification with HNO3. 【Result】 The results showed that soil pHBC was significantly affected by parent materials and initial soil pH. Due to the depletion of buffering substances in soils by acidification, the pHBC of weakly acidic yellow-brown soils and yellow soils (11.79-45.15 mmol·pH-1·kg-1) was lower than that of neutral yellow-brown soils and yellow soils (23.74-141.53 mmol·pH-1·kg-1). Therefore, the decrease in soil pH and increase in soil active Al in neutral yellow-brown and yellow soils were slow during the simulated acidification, indicating a weak acidification potential. Among the neutral yellow-brown soils and yellow soils, the soil pHBC increased with an increase in carbonate content in soils. The soils derived from limestone and carbonate parent materials reserved more carbonate (21.14 and 1.18 g·kg-1), which led to a higher pHBC than soils derived from quartzite and siliceous parents. However, the opposite tendencies were observed in the weak acid yellow-brown soils and yellow soils. In weak acid soils, the exchangeable base cations played the major pH buffering role due to the exhaustion of carbonate. Compared with the soils derived from limestone and carbonate parents, soils derived from quartzite and siliceous parents contained more clay and organic matter, which provided more H+ exchangeable sites and was thus beneficial to buffer exogenous acid and slow down the activation of soil aluminum during acidification. Among the 12 tested soils, the acidic yellow-brown soil derived from pelite was extremely sensitive to exogenous acids due to the lowest pHBC (11.79 mmol·pH-1·kg-1). When 6 mmol·L-1 HNO3 was added, the pH of the acidic yellow-brown soil derived from pelite was lower than that of the acid yellow-brown soils derived from carbonate parents and siliceous parents (by 0.74 and 1.10 pH units, respectively). Correspondingly, the soluble Al and exchangeable Al in the acid yellow-brown soil derived from pelite were significantly higher than those in the acid yellow-brown soils derived from carbonate parents and siliceous parents. 【Conclusion】 The acid yellow-brown soil derived from pelite presented the highest potential acidification risk. Thus, it is necessary to pay more attention to the acidification trend of soils derived from pelite and improve the resistance of these soils to acidification through the application of organic fertilizer. These findings provide significant guidelines for the management of soil acidification during continuous tobacco cultivation in the Wuling Qinba region, China.

    • Dewaterability of Frozen-melt Brackish Water and Its Soil Salt Leaching Efficiency Based on HYDRUS-1D

      2023, 60(2):378-389. DOI: 10.11766/trxb202105250272

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      Abstract:【Objective】Soil salinization is of growing concern in China since it severely restricts agricultural development and poses ecological and environmental risks in arid and semi-arid regions. High soil salinity causes deterioration of soil physical and chemical properties and inhabits plant growth. Given the wide distribution of brackish groundwater in arid and semi-arid regions, brackish irrigation is an effective approach to mitigate the shortage of freshwater resources. In addition, the naturally low air temperature in winter makes it possible to combine brackish irrigation and freeze-melt processes to enhance salt leaching and thus help reduce soil salinity. 【Method】 To reveal the dewaterability of frozen brackish water, the melting rates and temporal changes in water quality of ice blocks frozen from brackish water were compared under two air temperatures(10 ℃, 15 ℃)and four initial salinity gradients(freshwater, 0.5 g·L-1, 2 g·L-1, and 4 g·L-1). Furthermore, to evaluate the salt leaching efficiency through saline soil by melted brackish water, the HYDRUS-1D model was applied to simulate the one-dimensional soil water and salt transport patterns under two scenarios: direct infiltration using brackish water and infiltration using desalinized water melted from brackish ice frozen at different initial salinity gradients.【Result】 Our results show that: (1)The amount of melting water per unit time of brackish water ice body increased first and then decreased, and the melting rate was faster and peaked at higher value when the initial salinity was greater; (2)Freeze-melt could effectively desalinize brackish water by selectively discharging brine with great salinity at the early stage of melting, and the dewaterability was greater at lower initial salinity with an average dewaterability > 75%; (3)The HYDRUS-1D simulations on saline soil illustrated a better leaching efficiency when infiltrated with water melted from frozen brackish ice than directly applying unfrozen brackish water, and the leaching effect of brackish ice meltwater was better at 10 ℃ than at 15 ℃.【Conclusion】 These findings proved the applicability and effectiveness of combining brackish groundwater with freeze-melt treatments, as a new water-saving and salt-controlling mode, to help mitigate freshwater shortage and reduce soil salinization risk in the arid and semi-arid of China.

    • Study of the Water Infiltration Processes using ERT and TDR in Layered Soils

      2023, 60(2):390-398. DOI: 10.11766/trxb202104280227

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      Abstract:【Objective】Soil water is the link between surface water and groundwater, and the carrier of solute transport in soil, which plays a key role in the Soil-Plant-Atmosphere Continuum (SPAC) system. The movement of soil water is a very complex process, especially in the layered structure. Heterogeneity of texture and pore of layered soil changes hydraulic characteristics at the interface of the soil layer, and thus the soil water movement and solute transport differ significantly from that in homogeneous soil.【Method】To fully understand the process of soil water movement, infiltration was evaluated using electrical resistivity tomography (ERT) in soil with a stratified profile. A field infiltration test was performed on an 8.7-m-long transect and successive measurements using ERT allowed determining resistivity changes as infiltration progressed. In the meantime, the soil water content was measured by time domain reflectometry (TDR) at the point scale, and the quantitative relationship was established between resistivity and water content. In addition, the soil water contents derived by ERT were validated with the soil water content derived by the drying method. 【Result】Results showed that the Multi-electrode resistivity method can fairly monitor the process of soil water movement, identify the depth of water infiltration, 90 cm in our study, and quantitatively retrieve the profile water content. The movement of soil water is mainly vertical downward with a weak horizontal flow. A good correlation between resistivity and soil moisture measurements revealed the capability of resistivity measurements to infer soil moisture spatial and temporal variability with root mean square error (RMSE) equal to 0.042 cm3·cm-3 for loam and 0.041 cm3·cm-3for clay loam. However, when the soil water content was lower than 0.15 cm3 cm-3, the electrical resistivity changed greatly with the increase in water content, while the soil water content was higher, the resistivity did not change significantly with water content. When the soil water reached the interface (~30 cm) between loam and clay loam, the soil water did not immediately move to the lower layer. According to the established relationship between resistivity and water content, it is estimated that at the soil interface, when the mass moisture content reached 0.136 g·g-1 at the upper layer of the soil, the water infiltrated the lower layer. Unexpectedly, abnormally increased resistivity appeared under the area of the soil water infiltration, which was presumably caused by the significant difference in resistivity of soil adjacent layers during the infiltration process. 【Conclusion】 In comparison with TDR, the resistivity method gives information integrated on a greater volume of soil and the measurements are easier and quicker to be carried out without disturbing the soil. Therefore, this method can be considered as an alternative tool to be employed for qualitative and quantitative soil moisture monitoring in the field. Also, this study provides a new method for quantitative analysis of the movement of soil water at a layered interface.

    • Experimental Study on the Improvement of Soil Moisture Characteristics of Coal Mine Dump by Fly Ash and Arsenic Sandstone

      2023, 60(2):399-408. DOI: 10.11766/trxb202104140197

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      Abstract:【Objective】Reclaimed soil in an open-air coal mine is highly infertile due to its poor physical and chemical characteristics. The reclamation methods usually improve soil moisture content which has important theoretical and practical significance for the reclaimed mine soil.【Method】This experiment was conducted in the Heidaigou open-air coal mine in the Inner Mongolia Autonomous Region, in which the following treatments were applied; fly ash, arsenic sandstone and dump soil. A certain mass ratio gradient was set, including 13 blended treatments denoted as L3F1, L4F1, L5F1, L1S1, L2S1, L3S1, L4S1, L5S1, L1F1S1, L2F1S1, L3F1S1, L4F1S1, L5F1S1, in which the L, F, S represented three kinds of controlled treatments. The volumetric water content of each compound soil under different water suctions was measured by the centrifuge method for the drawing of the water characteristic curve using the Gardner fitting model. The specific water capacity, field water capacity, wilting coefficient and effective water content of each treatment were also calculated.【Result】The results showed that the Gardner model was appropriate to fit the water characteristic curves of the 13 compound soils. The addition of fly ash increased the content of fine soil particles (i.e., clay and silt) by 24.11% to 37.19%, respectively, and improved the water holding capacity and water supply of soil. Furthermore, the addition of arsenic sandstone improved the water holding capacity of soil but failed to improve the water supply performance. The water holding capacity of L1F1S1 treatment and water supply performance of L3F1 treatment was the best, which was 47.6% and 40.23% higher than that of the dump soil, respectively. The field water holding capacity and available water content of LF combined treatment and LFS combined treatment were enhanced with the increase of additive proportion. When the mass ratio of soil and fly ash in the dump was 3:1 (L3F1), the maximum field water holding capacity was 18.02%. On the other hand, the maximum available water content was 13.1% with the mass ratio of soil, fly ash and arsenic sandstone of 1:1:1 (L1F1S1).【Conclusion】Overall, the comprehensive utilization of fly ash and arsenic sandstone is beneficial to improve the soil water characteristics of coal mine dump, and the soil water holding capacity and water supply capacity is better when the clay and silt contents of the soil are within the range of 30%-35%. In this experiment, the mass ratio of 1:1:1 is the best.

    • Effects of Paddy Rice Cultivation History on Soil Physical Properties and Water/ Nitrogen Distribution in the Intermediate Zone between Paddy Field and Bund

      2023, 60(2):409-423. DOI: 10.11766/trxb202104150199

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      Abstract:【Objective】Lateral seepage around the paddy bund is an important pathway for water loss in the paddy rice field. Revealing the evolution process of soil physical properties and the characteristics of soil water and nitrogen distribution in the intermediate zone between paddy field and bund is important. This is crucial for understanding the mechanism of water and nitrogen loss in the intermediate zone. 【Method】The intermediate zone between paddy field and bund with a cultivation history of 2 y, 19 y, and >100 y was considered for this study. Soil properties, including soil bulk density, equivalent porosity, soil-water characteristic curve, saturated hydraulic conductivity (Ks), soil water content and soil nitrogen concentration were determined through field sampling and laboratory analysis. The evolution process of soil physical properties and the mechanism of water and nitrogen loss in the intermediate zone were evaluated. 【Result】The paddy rice cultivation history affected soil physical properties. With the cultivation history extending, the difference in soil bulk density between paddy field plow layer (PL) and paddy bund surface layer(SL), and between paddy field plow pan(PP) and paddy bund hard pan (HP) increased. The microporosity(with diameter <0.03 mm) in PL and SL increased with paddy rice cultivation history, while the macroporosity (with diameter >0.3 mm and >0.03 mm) decreased in other soil layers for both in paddy field and bund. This showed a quicker decrease rate in the paddy field than for the bund. Differences in soil water holding capacity (SWHC) between paddy fields and bund were identified in the measured water suction range (0~100 kPa). Similar SWHC was observed between PL and SL in both 2 y and 19 y paddy rice fields, while a much higher SWHC was found in PL than SL in >100 y paddy rice fields. Also, a similar SWHC between PP and HP were identified in both 2 y and 100 y paddy rice fields, while a much higher SWHC was found in PP than HP in the 19 y paddy rice field. A higher SWHC was observed in the paddy bund subsoil layer (BSL) than the paddy field subsoil layer (FSL) for both 19 y and >100 y paddy rice fields. With paddy rice cultivation history extending, the differences in Ks between PL and SL decreased, while it increased between PP and HP. For example, the Ks in PP was 1.10, 6.90, and 6.32 times that in HP for 2 y, 19 y, and >100 y paddy rice fields, respectively. Also, a much higher Ks was observed in bund than the field for >100 y paddy rice field, while no significant differences were observed for the other two fields. The soil water and nitrogen distribution patterns were affected by the evolution of soil physical properties in both paddy fields and bund. Also, a higher soil water content and soil nitrogen concentration were observed in 19 y and >100 y paddy rice fields than in 2 y paddy rice fields, which was mainly accumulated in the PL for both 19 y and >100 y paddy rice fields. 【Conclusion】For the young paddy rice field, soil water and nitrogen rapidly seepage from both paddy fields and bund identically. With the paddy rice cultivation history extending, soil water and nitrogen are expected to be lost more quickly from the paddy bund than the field. For >100 y paddy rice fields, the PP in the newly built bund will degrade with time, then the paddy bund will be the key site for soil water and nitrogen lose again. Therefore, the soil water and nitrogen loss from the paddy bund should be given more attention, and the focus should be more on old paddy than young paddy rice fields.

    • The Surface Properties of Lou Soil with Different Phosphorus Levels and Their Effects on the Loss of Phosphorus

      2023, 60(2):424-434. DOI: 10.11766/trxb202107070349

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      Abstract:【Objective】This study was designed to identify the surface electrochemical characteristics of Lou soil with different phosphorus levels and to study the transportation processes of phosphorus in the soil solid-liquid phase. Also, the relationship with soil surface properties was investigated and the change of surface properties of Lou soil under controlled electrolyte concentration and its internal relevance with phosphorus loss were clarified.【Method】The number of soil particles and phosphorus loss of Lou soil under different electrolyte concentrations was obtained by rainfall simulation test. The surface chemical properties of the soil were determined by combined measurement and analysis of surface properties. Also, the surface electrochemical properties of Lou soil with different phosphorus levels and their effects on soil phosphorus loss under the condition of long-term localized application of phosphate fertilizer were studied. 【Result】The results show that: (1) The total phosphorus content of Lou soil treated with long-term application of phosphate fertilizer is 2.46 times larger than that of Lou soil treated without phosphate fertilizer. However, the former’s surface potential, surface charge density, surface electric field strength, specific surface area, and surface charge number were lower than those of Lou soil without the application of phosphorus; (2) For Lou soils with different phosphorus levels, the cumulative loss of soil particles and phosphorus tended to intensify with an increase in the surface potential (absolute value) of soil particles. There was a linear positive correlation between the cumulative loss of particulate phosphorus (PP) and the cumulative loss of soil particles; (3) The surface potential(absolute value)of Lou soil with lower phosphorus content was larger than that of Lou soil with higher phosphorus content, and the electrostatic repulsion between soil particles was larger, resulting in poor stability of soil aggregates and more cumulative loss of soil particles and their attached phosphorus; (4) Under the experimental conditions, the phosphorus loss in Lou soil was dominated by PP, accounting for 81%-99% of total phosphorus loss, and the proportion of dissolved phosphorus was extremely low.【Conclusion】Long-term application of phosphate fertilizer can change the surface properties of Lou soil, and effectively maintain the stability of soil aggregates and attached phosphorus during rainfall. This study provides a new idea for environmental risk prevention and control of the long-term phosphorus application soil.

    • Screening of Low-Cd-accumulating Rice Varieties and Derivation of Soil Cd Safety Threshold in Southern Jiangsu

      2023, 60(2):435-445. DOI: 10.11766/trxb202108140363

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      Abstract:【Objective】There are urgent requirements to solve the problem of cadmium (Cd) pollution in farmlands and rice. With the rapid economic development in Southern Jiangsu, Cd pollution in soil and rice caused by industrialization has significantly increased. The purpose of this study was to screen low-Cd-accumulating rice varieties from main cultivated varieties in Southern Jiangsu, and to deduce the local soil Cd threshold for the safe production of rice. 【Method】In this study, field plot experiments and field investigation were conducted at different sites and years. Eighteen local main cultivated rice varieties in Southern Jiangsu were selected, including 15 Japonica conventional varieties: Wuyunjing 30(WYJ30), Suxiangjing 100(SXJ100), Changnongjing 10(CNJ10), Nanjing 3908(NJ3908), Nanjing 5055(NJ5055), Changnongjing 12(CNJ12), Nanjing 46(NJ46), Changnongjing 11(CNJ11), Jiahua 1(JH1), Changxiangjing 1813(CXJ1813), Changnongjing 8(CNJ8), Wukejing 7375(WKJ7375), Zaoxiangjing 1(ZXJ1), Yangyujing 3(YYJ3), Zhennuo 19(ZN19), and 3 Japonica three-line hybrid varieties: Changyoujing 6(CYJ6), Changyou 4(CY4) and Changyoujing 11(CYJ11). The differences of Cd bioconcentration factor(BCF) in rice grains among these varieties were comprehensively compared, and the soil Cd safety thresholds at different pH ranges were deduced using the species sensitivity distribution(SSD) method. 【Result】The results showed that the order of grain Cd accumulation among tested varieties was consistent between field investigation and plot experiments. There were significant differences in Cd BCF among 18 main cultivated rice varieties, with a maximum difference of 4.7 times. The Cd accumulation in some varieties like NJ46 and NJ3908 was stably low. Based on the SSD method, the deduced safety thresholds of soil total Cd to protect 95% of rice varieties from exceeding the standard (0.2 mg·kg-1) (GB 2762-2017) under soil pH 5.0~6.5, 6.5~7.5 and 7.5~8.5 in southern Jiangsu were 0.52, 0.80 and 1.78 mg·kg-1, which were higher than the current risk screening value(GB 15618-2018). It was verified that the deduced thresholds remarkably improved the accuracy of Cd risk prediction of rice in Southern Jiangsu. 【Conclusion】Based on field plot experiments and field investigation, numerous local main cultivated rice varieties including NJ46 and NJ3908 were screened with stably low Cd accumulation capacity, stable yield and high quality, and therefore are suggested for cultivation in Cd contaminated paddy fields in Southern Jiangsu. The deduced local safety thresholds of soil total Cd to protect 95% of rice varieties from exceeding the standard were higher than the current risk screening value. The results have important implications for the safe utilization of Cd contaminated paddy fields in Southern Jiangsu.

    • Effects of Drying and Oxidation Stage on the Forms and Availability of Cd and As in the Soil-rice System

      2023, 60(2):446-457. DOI: 10.11766/trxb202105190264

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      Abstract:【Objective】The chemical behavior of cadmium (Cd) and arsenic (As) in paddy soils is opposite to the change in biological efficiency. This presents serious challenges to the simultaneous control of Cd and As pollution in rice. Previous studies have found that continuous flooding of Cd polluted fields can reduce the bioavailability of Cd while reducing soil water content can mitigate the bioavailability of As. However, the forms and availability of Cd and As in the soil-rice system under the process of drying (oxidation) remain unclear. 【Method】In this study, continuous sampling during the process of drying (days 0, 3, 5, and 7 of the drying process) under tillering period of rice in a pot experiment was carried out. At the same time the soil samples were analyzed for pH, Eh, soluble organic carbon, available Cd and As, and the different forms of Cd and As. Also, the plant samples were analyzed for iron plaque in the rice roots and Cd and As concentration in the rice tissues. 【Result】 The results showed that: during the process of drying, the Cd concentration in all parts of rice tissues increased with a reduction in water content, and the Cd concentrations in roots and shoots were increased by 109% and 183%, respectively, under the second drying stage(drying for 5 days) compared with the control treatment (P < 0.05). The concentration of As in rice roots decreased firstly and then increased with the reduction of water content. Compared with the control treatment, the concentration of As in rice roots was decreased by 41.96 mg·kg-1(P < 0.05). Also, the shoot As concentration was decreased by 12% and 18%, respectively, under the second and third stages of drying (drying for 5 and 7 days) (P < 0.05). The Cd, As and Fe concentrations in the Fe plaque increased by 96%, 16% and 16%, respectively(P < 0.05). With the decrease of soil water content, soil Eh, soil soluble organic carbon, and soil available Cd extracted by DTPA were increased, but soil pH, soil available As, and soil available Fe extracted by DTPA were decreased. In addition, reducing soil water content promoted the transformation of residual Cd to acid extractable and reducible Cd, resulting in the increase of oxidizable As concentration. It was also observed that when the soil moisture content was 33.6%, the bioavailable Cd and As concentration remained relatively low. 【Conclusion】Reasonable water management practices can reduce the bioavailability of Cd and As in soil. This is due to the changing soil Eh, pH, soluble organic carbon and available Fe, and the promotion of adsorption and fixation of Cd and As by the iron plaque. However, it is worth noting that while exploring water management methods to reduce the bioavailability of Cd and As, the growth and development of rice cannot be ignored. Our study contributes to the wealth of knowledge aimed at improving the remediation of Cd and As contaminated soils.

    • Human Bioavailability and Health Risk Assessment of Heavy Metals in Soils from a Mining Area of Southwest China

      2023, 60(2):458-468. DOI: 10.11766/trxb202107200287

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      Abstract:【Objective】Ingestion of soil is a pathway of human exposure to several environmental contaminants, including several heavy metals. Risk assessment of soils has typically been performed on total concentrations of target heavy metals. However, it may overestimate the potential adverse effects. To refine exposure risk, bioaccessibility and bioavailability measurements have been employed in many studies. Bioaccessibility tests are used to measure the bioaccessible fractions of contaminants in soils while bioavailability evaluates the fraction of heavy metals that reach the systemic circulation. These tests are both considered accurate approaches to evaluate the potential health risk of contaminants. However, there are few studies on the health risk assessment of heavy metals from mining soils via bioavailability. 【Method】In this study, five soil samples from the mining area of Wenshan, Yunnan Province were collected and the levels of Cd, Pb, Zn and Cu were analyzed using inductively coupled plasma mass spectrometry (ICP-MS). The bioaccessibility of Cd, Pb, Zn and Cu was detected using in vitro digestion method (SBRC), and the relative bioavailability (RBA) of Cd was determined by BALB/c mice models. The health risks of the four heavy metals in mining soils were assessed based on total, bioaccessible, and bioavailable data, respectively.【Result】The results showed that Cd pollution in this study area was serious, with the content being 2.06 mg·kg-1, which was 9.36 folds higher than the limit of soil background value in Yunnan. The bioaccessibility of Cd, Pb, Zn and Cu in the gastric phase was 24.29%-50.55%, 7.68%-17.87%, 24.61%-32.18%, 7.75%-37.87%, respectively, while in intestinal phase, they were 22.78%-44.32%, 1.64%-5.22%, 14.10%-28.11%, 8.51%-31.49%, respectively. As evident, the bioaccessibility of Cd was the highest among the four heavy metals. The RBA of Cd measured in vivo was 1.31%-48.39% in the liver, 2.83%-8.58% in the kidney, and 4.60%-50.95% in the liver and kidney. Compared with a single endpoint, Cd-RBA in the liver and kidney provided better repeatability and were ideal target organs for the determination of Cd-RBA. In vivo-in vitro correlation showed that the bioavailability of Cd determined by SBRC had a poor potential to predict Cd-RBA in contaminated soils from the mining area in China. Health risk assessment of the mining soil based on the target heavy metals, bioaccessibility and bioavailability revealed that the assessment using the total heavy metals had greater human health risk, while the data based on bioaccessibility and bioavailability showed a significantly reduced risk. 【Conclusion】The health risks assessment based on the total heavy metals in soil may be overestimated, and the establishment of a new method based on the bioavailability data will be more accurate. Our results provide a scientific basis for the health risk assessment of contaminated soils in China.

    • Characteristics and Influencing Factors of the Catalytic Transformation of Ferrihydrite by the Structural Fe(II) in Reduced Montmorillonite

      2023, 60(2):469-478. DOI: 10.11766/trxb202111020410

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      Abstract:【Objective】Fe(II)-induced transformations of amorphous ferrihydrite to more crystalline iron oxide phases is a widely occurring geochemical process in soils under reduced conditions, and play an important role in regulating the biogeochemical processes of nutrient elements and pollutants. As one of the main species of Fe(II) in soils, Fe(II)-containing clay minerals are ubiquitous in soils under reduced conditions. However, the catalytic properties of structural Fe(II) in clay minerals for ferrihydrite transformation and its influencing factors are still not fully understood.【Method】In this study, the transformation of ferrihydrite induced by the structural Fe(II) in reduced montmorillonite (rSWy-2), which was produced by a chemical method, were investigated at neutral pH under anoxic conditions. Also, the influencing factors including types of cations and anions, organic matter, and As(III) on the transformation were studied.【Result】The X-ray diffraction (XRD) and chemical extraction analyses results showed that the structural Fe(II) in rSWy-2 with low Fe content can catalyze the transformation from ferrihydrite to more crystalline lepidocrocite, and 83.3% of initial ferrihydrite converted to lepidocrocite after mixing reaction for 96 h. XRD, high-resolution transmission electron microscope (HRTEM), scanning transmission electron microscopy (STEM) and surface adsorbed Fe(II) content analyses showed that the processes of ferrihydrite transformation induced by the structural Fe(II) in rSWy-2 mainly included three stages: Firstly, positively charged ferrihydrite nanoparticles adsorption on negatively charged rSWy-2 surface through electrostatic interaction. Secondly, interfacial electron transfer from the structural Fe(II) in rSWy-2 to the adsorbed ferrihydrite and partly reducing Fe(III) to surface adsorbed Fe(II). Finally, the surface adsorbed Fe(II) catalyzed the transformation of ferrihydrite to more crystalline lepidocrocite phases. HRTEM analyses showed that the formed lepidocrocite phases presented nanoplates with a size range of 100~200 nm. XRD and chemical extraction analyses results showed that Na+ and Cl- ions in the mineral suspension had a weak effect on the catalyzed transformation from ferrihydrite to lepidocrocite phases by the structural Fe(II) in rSWy-2. In contrast, Ca2+, SO42-, organic matter, and As(III) all had obvious inhibition on the transformation due to their strong interactions with minerals.【Conclusion】The structural Fe(II) in rSWy-2 could catalyze the transformation of ferrihydrite to lepidocrocite at neutral pH under anoxic conditions, and coexisting divalent cations and anions, As(III), and organic matter could inhibit the transformation reaction. The results provide a theoretical basis for further understanding the role of iron-bearing clay minerals in regulating abiotic transformations of iron oxides in soils under anaerobic reduction conditions.

    • Effects of Iron Oxide on the Adsorption and Desorption of Se(Ⅳ)in Selenium-Rich Soils of Guangxi

      2023, 60(2):479-490. DOI: 10.11766/trxb202110250434

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      Abstract:【Objective】The influence of free iron oxide on the adsorption and desorption of Se(Ⅳ) in a lateritic red soil and red soil in the selenium-rich area of Guangxi was studied. 【Method】The adsorption and desorption characteristics of Se(Ⅳ) in the soil before and after removing free iron oxide were compared by isothermal adsorption and desorption experiments. Zeta potential, scanning electron microscopy and energy spectrum analysis and Fourier transform infrared spectroscopy was used to explore the influencing mechanism.【Result】The results showed that the adsorption process for Se(Ⅳ) fitted the Langmuir and Freundlich models, with correlation coefficients ranging between 0.920~0.995. After removing free iron oxide of red soil and lateritic red soil, the zeta potential became more negative and changed from -24.42 and -18.06 mV to -33.06 and-26.43 mV. Also, the specific surface area was decreased. This observation correlated with the lower adsorption capacities of the soils after the removal of free iron oxide. Hence, the order of maximum adsorption capacity was: lateritic red soil (1 399 mg·kg-1) > red soil (1 336 mg·kg-1) > DCB-treated lateritic red soil (444 mg·kg-1) > DCB-treated red soil (352 mg·kg-1). The desorption rates of the tested soils were between 2% and 7%, while that of DCB-treated soils were higher than that of the original soils. The FTIR peak fitting analysis showed that the soils reacted with selenium mainly through the oxygen-containing groups such as -OH, Fe-O and C=O. After the removal of iron oxide, the effect of Fe-O in the adsorption was weakened or disappeared.【Conclusion】Free iron oxide can significantly increase the adsorption capacity and strength of soil for Se(Ⅳ) and reduce the release of Se(Ⅳ) by its physical and chemical properties and surface groups.

    • Effects of Key-stone Microbe Based on Co-occurrence Networks on Wheat Yield in the Soils with Straw Returning

      2023, 60(2):491-502. DOI: 10.11766/trxb202107200372

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      Abstract:【Objective】The mechanism of the inconsistent effects of straw application on crop yield is still unclear, and the main reason may be related to the insufficient understanding of the effects of straw application on soil microbial community composition and its abundance changes on crop yield.【Method】A pot experiment to study the growth of wheat was carried out using red and yellow-cinnamon soils. Three levels of straw application were set for each soil, namely 0, 10 and 30 g·kg-1 soil(S0, S10, S30). Based on bacterial-fungal co-occurrence networks, the Path analysis model was used to evaluate the contribution of microbial ecological clusters, enzyme activities and chemical properties to wheat yield. 【Result】The results showed that although the content of available nutrients, soluble organic carbon, microbial biomass carbon and enzyme activities(amylase, invertase, polyphenol oxidase, urease, acid phosphatase, dehydrogenase) increased significantly in both soils, the wheat yield increased with the increase in straw dosage in the red soil but decreased in yellow-cinnamon soil. Compared with S0, S10 and S30treatments increased wheat grain yield and above-ground biomass by 33%-44% and by 73%-85% in the red soil; and decreased wheat grain yield and above-ground biomass by 22%-25% and by 55% in the yellow-cinnamon soil, respectively. The abundance of two key ecological clusters within the bacterial-fungal co-occurrence network, enzyme activities and soil chemical properties had positive effects on wheat yield in red soil while the abundance of two key ecological clusters had a larger positive effect on wheat yield in yellow-cinnamon soil. The straw application significantly increased the abundance of Aspergillus, a key microorganism positively correlated with wheat yield in red soil, while significantly decreased the abundance of Bacillus, Burkholderia, and Basidiobolus, which were positively correlated with wheat yield in yellow-cinnamon soil.【Conclusion】The combined effects of straw application, an increase in key microbial abundance, enzyme activities, and improvement of soil chemical properties was responsible for improving wheat yield in the red soil. In the yellow-cinnamon soil, the decrease in wheat yield was mainly related to lower key microbial abundance, whose effect was superior to the potential positive effects of improved soil chemical properties and enzyme activities. These results suggest that the change in the abundance of key microorganisms has an important influence on the variation of crop yield after straw returning.

    • Seasonal Variations in Methane Production Potential and Methanogenic Pathway in a Permanently Flooded Rice Field

      2023, 60(2):503-511. DOI: 10.11766/trxb202107050347

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      Abstract:【Objective】The mitigation of methane (CH4) emission has long been the focus of governments and researchers worldwide. Rice fields are an important source of CH4 emission, and CH4 production is the precondition of CH4 emission. It is mainly produced by acetate fermentation and CO2/H2 reduction. Substantial CH4 emissions are observed from the permanently flooded rice fields, thus leaving a high mitigation potential of emission. However, reports on the seasonal variation of the methanogenic pathway in these rice fields are limited.【Method】Fresh soil samples from four rice growth stages (tillering stage, booting stage, heading stage, and maturity stage) were collected from a permanently flooded rice field in Ziyang City, Sichuan Province, China. The CH4 production potential (MPP) was observed through anaerobic incubation experiments. Both stable carbon isotope technique and methyl fluoride (CH3F, 2%) inhibition method were used to quantify the carbon isotopic fractionation factor for the conversion of CO2to CH4(α(CO2/CH4)), as well as to quantitatively estimate the relative contribution of acetate-dependent methanogenesis (ƒacetate).【Result】The addition of CH3F significantly decreased the CH4 production, and the MPP reached the highest value at the maturity stage, ranging from 3.22 to 12.71 µg·g-1·d-1. The δ13C-value of produced CH413CH4) varied from -66.83‰ to -59.62‰, which was much more positive than that of produced CH4 with CH3Faddition (from -90.83‰ to -82.26‰). The α(CO2/CH4)reached its maximum and minimum values at tillering and booting stages, respectively, which was between 1.064 and 1.076. The ƒacetate(30%-61%) decreased sharply from 54%-61% at the tillering stage to 30%-35% at the booting stage and then increased to 54%-61% at the maturity stage. Further analysis showed that the seasonal variation of MPP was positively correlated with the variation of soil dissolved organic carbon (DOC) content, and the seasonal variation of ƒacetate was significantly related to variations of acetate content.【Conclusion】Distinct seasonal variations were observed in both MPP and methanogenic pathways of the permanently flooded rice fields, which was mainly affected by soil DOC content and acetate content, respectively.

    • Effects of Long-term Organic Fertilizer Instead of Chemical Fertilizer on Organic Carbon Stability of Paddy Soil

      2023, 60(2):512-522. DOI: 10.11766/trxb202105260276

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      Abstract:【Objective】The effects of long-term and different proportions of organic fertilizer on the content of labile/non-labile carbon components and the stability of the chemical structure of organic carbon in paddy soil were determined.【Method】This study selected a long-term positioning test of paddy soil developed by Quaternary red clay as the research object. Five treatments were selected: no fertilizer treatment (CK), chemical fertilizer treatment (100F0M), 30%, 50% and 70% of the nitrogen in chemical fertilizer treatment was replaced by organic fertilizer (70F30M、50F50M、30F70M). During the study, we measured the content of total organic carbon, labile/non-labile organic carbon pool, the chemical structure of organic carbon, and soil properties.【Result】 The contents of organic carbon, labile carbon and non-labile carbon in paddy soil were increased by different proportions of organic fertilizer instead of chemical fertilizer. The contents of labile carbon were 70F30M, 30F70M > CK, 50F50M, and the contents of non-labile carbon were 30F70M > 70F30M, 50F50M > 100F0M > CK. With an increase in the replacement proportion of organic fertilizer, the proportion of non-labile carbon gradually increased from 69.67% to 77.26%. Under different organic fertilizer substitution ratios, the chemical structure of soil organic carbon remained the same, and the relative content percentage shows the same trend: aromatic carbon (33.28% ~ 37.79%), alkyl carbon (27.81% ~ 31.19%), alkoxy carbon (16.19% ~ 20.10%), carbonyl carbon (10.35% ~ 12.07%), and carboxyl carbon (2.52%~ 5.75%).Alky, carbonyl, and carboxyl carbons made up the largest, second largest and lowest proportions, respectively.With the increase in the proportion of organic fertilizer instead of chemical fertilizer, the aromaticity increased from 0.40 to 0.44 while the ratios of aliphatic carbon/aromatic carbon and alkyl carbon/alkoxy carbon decreased, suggesting an increase in the stability of organic carbon. PH (P< 0.05) and Cation exchange capacity (CEC, P< 0.01) were the main influencing factors of activated carbon content. pH, CEC, Alkali hydrolyzed nitrogen (AN) and carbon input(organic fertilizer, rice root and rice stubble) (P< 0.01) were the main influencing factors of non-labile carbon content.【Conclusion】30F70M not only improved the content of labile and non-labile carbon but also increased the aromaticity of organic carbon, which is more conducive to improve the stability of soil organic carbon.

    • Responses of Soil Carbon Pool of Abandoned Grassland on the Loess Plateau to Two-years Warming and Increased Precipitation

      2023, 60(2):523-534. DOI: 10.11766/trxb202106120307

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      Abstract:【Objective】Global climate change has had a huge impact on the carbon pool of terrestrial ecosystems, and the changes in average temperature and precipitation patterns pose severe challenges to the management of terrestrial carbon pools. This is especially serious in the Loess Plateau where the ecological environment is fragile. Therefore, understanding the changes of different fractions of soil organic carbon pool and the stability of this carbon pool under the background of the warming and humid climate of the Loess Plateau is of great significance. This will be of importance in the sustainable development of the carbon pool in fragile ecological areas and the evaluation of regional ecological benefits.【Method】In this study, natural abandoned grassland in the loess hilly region was taken as the research object. Soil warming and increased precipitation were artificially simulated and the changes in vegetation community, soil carbon fractions content and distribution, carbon pool stability index under warming (W), precipitation (P50%) and their interactions (WP50%) were analyzed. 【Result】The results showed that: (1) P50% significantly increased the vegetation Gleason richness index, W and WP50% significantly reduced the Gleason richness index (P < 0.05) and the leaf organic carbon content under the WP50% was significantly higher than the control treatment. Both P50% and WP50% treatments reduced the Shannon-wiener diversity index and pielou evenness index. (2) Soil organic carbon (SOC), acid hydrolyzable organic carbon (AHC), easily oxidizable carbon (EOC), dissolved organic carbon (DOC) content and their distribution ratio of the two sampling years under P50% were significantly higher than the control treatment. Also, warming based on P50% can further increase soil EOC, microbial biomass carbon (MBC) content and distribution ratio. However, only the soil MBC showed a significant difference compared with the control under W treatment. Recalcitrant organic carbon (ROC) presents an opposite trend to AHC. The correlation analysis showed that there was a significant correlation between soil total organic carbon and each active carbon fractions (P < 0.05). (3) The soil carbon pool activity (CA), carbon pool activity index (CAI), and carbon pool management index (CPMI) under WP50% treatment were higher than other treatments. The sensitivity index of AHC and MBC was relatively high under the treatment of climate change.【Conclusion】In summary, the total organic carbon, the content and distribution ratio of active fractions of organic carbon, and the stability of the carbon pool in abandoned grassland of the loess-hilly area will be significantly improved under background warming and humid climate. This will contribute to the healthy development of the carbon pool. From a sensitivity point of view, AHC and MBC can be used as early important indicators of changes in soil organic carbon under warm and humid climates in the future. This study provides a theoretical and scientific basis for research on climate change and carbon pool management in fragile ecological areas.

    • Optimization of Denitrifier Method for Determination of Soil NO2--15N Abundance

      2023, 60(2):535-545. DOI: 10.11766/trxb202106070299

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      Abstract:【Objective】NO2- is a key intermediate product of several nitrogen transformation processes in soil, with low concentration and fast transformation. The combination of denitrifier method and mass spectrometry technology has been widely used in the 15N isotope analysis of NO3- or NO2- in water. 【Method】This paper aims to optimize the culture and reaction conditions of Stenotrophomonas nitritireducens denitrifier method, and to realize the specific, rapid and accurate determination of NO2--15N abundance in soil extract. Thus, we will optimize the culture method and reaction conditions, and bacterial concentration of denitrifier method, then applied this method to the soil extract. 【Result】There was no significant difference in the 15N isotopic determination of NO2- samples between aerobic shaking with seed solution and microaerobic incubation of single colonies. The seed solution ensures the stability of different batches of bacteria and aerobic incubation reduces the incubation time from 7~8 days to 12~15 h. High purity N2 or He purging for 0.5 h can effectively remove O2 and blank nitrogen, but the cost of N2 purging is lower. The transfer of N2O gas to the dry gas cylinder did not affect the accuracy and precision of the determination results. However, it can prolong the sample preservation time and reduce alkali steam corrosion of the instrument pipeline. Also, using bacteria in logarithmic growth phase and adjusting the reaction system to an OD600 value of 0.3~0.9 will ensure their denitrification efficiency and reduce variation between batches. Using no more than 1 mol·L-1 KCl to extract the soil will ensure the extraction efficiency and reduce the impact on the viability of the bacteria.【Conclusion】The optimized method can accurately determine the 15N abundance of NO2- in different types of soil with a measurement accuracy of δ15N of 5~20 nmol and natural abundance value of NO2- between 0.1‰~0.9‰, with most values being less than 0.5. Also, the experimental period was greatly shortened, simplified steps and save cost.

    • Community Assembly and Functional Potential of Habitat Generalists and Specialists in Typical Paddy Soils

      2023, 60(2):546-557. DOI: 10.11766/trxb202105310284

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      Abstract:【Objective】Great differences exist in the utilization of carbon and nutrients between habitat generalists and specialists, which play unique roles in the cycle of soil energy and nutrients. At present, the research on farmland microorganisms mainly focuses on the whole bacteria, fungi, archaea or other functional communities, and the understanding of habitat generalists and specialists in farmland ecosystems is still lacking. Therefore, this study was designed to explore the community structure, assembly mechanism and possible functions of habitat generalists and specialists in typical paddy soils in eastern and southwestern China. 【Method】Sixteen surface soil samples (0~20 cm) were collected from the eastern area (Jiangsu, Anhui, Shanghai) and southwestern area (Guizhou and Yunnan) of China according to the data of the second soil survey, and their physico-chemical properties and next generation high-throughput sequencing were analyzed. 【Result】The results showed that 3.28% of all OTUs were classified as habitat generalists while 9.07% as habitat specialists. There were significant differences in species composition between habitat generalists and specialists. At the level of phylum, the proportions of habitat specialists in Chloroflexi, Actinobacteria, Nitrospirae, Firmicutes and Planctomycetes were higher than those of habitat generalists. The analysis of the community assembly process based on the β diversity null model showed that habitat generalists and specialists were dominated by deterministic process. Compared with habitat specialists, habitat generalists were more affected by the stochastic process. The environmental factors driving the community structure variation of habitat generalists and specialists were different. pH, mean annual precipitation, clay content and total nitrogen were the main factors driving the community structure variation of habitat generalists, while the community structure variation of habitat specialists was dominated by pH and clay content. By analyzing the co-occurrence network and robustness of habitat generalists and specialists, it was found that the habitat specialists’ network had more connections, more complexed structure and stronger robustness. The functional prediction by FAPROTAX showed that biological nitrogen fixation mainly existed in habitat generalists. 【Conclusion】Information derived from the community structure, environmental driving factors, assembly process, co-occurrence network characteristics and related functions of nitrogen metabolism of habitat generalists and specialists, provides a theoretical basis for the evolution and regulation of bacterial communities in paddy fields.

    • Characteristics of Protist Community in Banana Rhizosphere Soil and Interactions between Protists and Pathogens of Fusarium wilt Disease

      2023, 60(2):558-567. DOI: 10.11766/trxb202107140361

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      Abstract:【Objective】As a serious banana disease and a significant limiting factor in banana production worldwide, banana Fusarium wilt disease is caused by the fungal pathogen Fusarium oxysporum f. sp. cubense race 4(FOC). The rhizosphere microbiome of plants is a key barrier that defends plant roots from an invasion of soil-borne pathogens. Most studies of the rhizosphere microbiome have focused on bacterial and fungal communities. However, as an important component of the rhizosphere microbiome, the rhizosphere protist community has been neglected in the regulation of microbiome and plant health. This study was conducted to explore the characteristics of the soil protist community in healthy and diseased plants and the interactions between protists and pathogens through field experiments of continuous cropping of bananas.【Method】In this paper, high-throughput Illumina MiSeq sequencing was applied to analyze the differences of soil protist community structure and composition among different treatments in field experiments.【Result】Results show that the relative importance of protists in predicting pathogenic Fusarium number was 47.19%, suggesting that protists might be the best predictor for pathogen number than culturable bacteria and fungi. The diversity and richness of the rhizosphere protist community decreased during plant growth and was lower in diseased plants. The composition and community structure of protists differed between healthy and diseased plants in rhizosphere soil. Also, the relative abundance of phagotrophic protists was highest compared to other functional groups in all soil samples, showing an increasing trend throughout plant development and enriched in diseased plants. Before the heading stage, the relative abundance of Bacillariophyta_X_unclassified, a phototrophic protist, was highest in healthy plants but decreased at a later stage. In healthy plants, the relative abundances of Group-Te and Cercomonas, phagotrophic protists, were higher at the heading stage. It was also observed that diseased plants showed a higher relative abundance of Pythium while phagotrophic protists, particularly Cercozoa protists, had significant correlations with Fusarium than other protistan communities. Group-Te and Cercomonas, two Cercozoa taxa, were negatively linked with the pathogen. In contrast, pathogens in diseased plants were positively linked with Pythium, which was a plant pathogenic protist. 【Conclusion】Protists in the rhizosphere soil demonstrated a greater impact on pathogens. The community characteristics of protists in rhizosphere soil changed in the process of plant growth and differed between healthy and diseased plants. Particularly Group-Te and Cercomonas were negatively linked with the pathogen, which might have potential in the prevention and control of banana wilt disease. Future research should focus on(i)the isolation and purification of phagotrophic protists negatively related to pathogens, (ii)exploring the mechanism of phagotrophic protists and pathogens, and(iii)investigating the internal connection with other beneficial microorganisms in rhizosphere soil to improve the efficiency of controlling banana wilt.

    • Contributions and Influencing Factors of Soil Fauna to Litter Decomposition under Different Land-use Patterns

      2023, 60(2):568-576. DOI: 10.11766/trxb202110090441

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      Abstract:【Objective】Soil fauna play an important role in litter decomposition processes through activities such as burrowing and feeding. Besides the quality of litter, climate parameters have been identified as major factors affecting the role of soil fauna on litter decomposition. However, the contribution of soil fauna in litter decomposition and the role of climatic parameters under different land-use patterns are still not clear.【Method】In this paper, meta-analysis was used to quantify the effect sizes of soil fauna on litter decomposition rates. We established a meta-analysis database by collecting the results of 56 publications both in Chinese and English in China from papers published until May 31, 2021. The effect size of soil fauna on litter decomposition rates among three different land-use patterns (i.e., forest, grassland, and farmland), was further tested by a random-effects model. Correlations were tested between environmental factors -temperature, precipitation, litterbag size, latitude, and experimental duration -and the effect size of soil fauna on litter decomposition rates.【Result】Soil fauna increased litter decomposition rate by an average of 8.10%, reaching a significant level among different land-use patterns. Among the three different land-use patterns, the effect of soil fauna on litter decomposition rates was strongest in the farmland (12.36%). Environmental factors were closely correlated with the effect size of soil fauna on litter decomposition rates.【Conclusion】In the forest, effect size significantly increased with temperature, including the average temperature in January and July, and the mean annual temperature (P < 0.01). In grassland, however, the effect size of soil fauna decreased with temperature (P > 0.05). The effect size significantly increased with mean annual precipitation but significantly decreased with altitude (P < 0.01). Besides, with the increase of litterbag size and experimental duration, the effect size values of soil fauna on litter decomposition significantly increased and decreased, respectively.

    • Regulation of Phosphorus Levels on the Interaction Balance between Endophytic Fungus Phomopsis liquidambaris and Rice

      2023, 60(2):577-586. DOI: 10.11766/trxb202108030397

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      Abstract:【Objective】The interaction between endophytic fungi and plants is affected by many factors, among which the content of phosphorus in soil plays a key role in regulating the interaction between them. However, it is not clear whether and how phosphorus fertilizer in the environment affects the interaction between endophytic fungi and plants.【Method】In this study, the symbiont of Phomopsis liquidambaris(B3)and rice(Oryza sativa L.)was used as the experimental model. Three different phosphorus levels, i.e., low phosphorus(LP), medium phosphorus(MP), high phosphorus(HP), uninoculated endophytes treatment(E-), and inoculation endophytes treatment(E+)were designed in this experiment. Physiological indexes of rice and B3 colonization were detected under different phosphorus levels to explore how phosphorus regulates the interaction between B3 and rice in the outdoor experiment and greenhouse experiment.【Result】In the outdoor experiment, B3 significantly(i)increased the phosphorus uptake ability of rice, (ii)promoted rice growth in the whole growth period and the formation of rice grain in the mature stage, and(iii)increased the rice yield by 7.05% under LP treatment. In MP and HP treatments, the effect of B3 was weakened. In the greenhouse experiment, under LP treatment, compared with the uninoculated treatment, the root/shoot ratio of rice inoculated with B3 increased by 22.48%, and resulted in an enhanced phosphorus uptake ability of rice by 39.98%. There was a significant enhancement of photosynthesis in rice and the accumulation of carbon sources such as sucrose, glucose and fructose in rice roots. However, the colonization of B3 was significantly limited under this condition. Also, consistent with this result, the biomass of B3 cultured in the LP medium was significantly lower than that in MP and HP mediums, which meant that B3 would be significantly affected by the phosphorus concentration. In the MP and HP treatments, B3 had high colonization in rice roots, but had little effect on rice growth.【Conclusion】Phosphorus concentration affects the symbiotic relationship between B3 and rice. In LP treatment, B3 colonization was low in rice roots, but it significantly promoted plant growth. In MP and HP treatments, B3 had high colonization rates in rice roots but with no significant effect on rice growth. Understanding the influence of phosphorus fertilizer on the interaction between endophytic fungi and plants will be helpful to improve the utilization efficiency of phosphorus fertilizer and microbial resources, which will provide a new path for the sustainable development of agriculture.

    • Effects of Seawater Rice Rhizosphere Effect on Soil Ammonia-oxidizing Microorganisms in Coastal Saline-alkali Soil

      2023, 60(2):587-598. DOI: 10.11766/trxb202109170340

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      Abstract:【Objective】The special environment of coastal saline-alkali land restricts the transformation and utilization of soil nitrogen. Microorganisms in saline-alkali paddy environment mediate ammonia oxidation in rice rhizosphere in a key process of soil nitrogen cycling. However, due to research blindness and outdated technology, the effect of seawater rice rhizosphere effect on the microbial community structure of ammonia oxidation in coastal saline-alkali soil is rarely reported. 【Method】In this study, the saline tolerant rice species ‘Haidao 86’ was used as the experimental material for the pot experiment. The pot experiment was conducted with low (2 g·kg-1) and high (6 g·kg-1) salt concentrations. Soil physicochemical properties and microbial biomass were measured and analyzed, and high-throughput sequencing of ammonia-oxidizing microorganisms was conducted to analyze the effects of different treatments of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) community structure in the rice rhizosphere.【Result】Results showed that after 70 days of rice growth, pH of rhizosphere soil decreased by 0.82 and 0.70, soil organic matter (SOM) content decreased by 6.41 g·kg-1and 4.46 g·kg-1, humus (HU) content increased by 5.76 g·kg-1 and 4.45 g·kg-1, total nitrogen (TN) content decreased by 0.46 g·kg-1 and 0.37 g·kg-1 for low and high salt concentrations, respectively. Rice rhizosphere effect significantly increased soil microbial biomass carbon, microbial biomass nitrogen and microbial respiration intensity, reaching peak values on the 55th day of planting with 850.0 mg·kg-1, 72.2 mg·kg-1 and 231.9 mg·kg-1·d-1 for high salinity treatment and 546.1 mg·kg-1, 53.7 mg·kg-1 and 171.2 mg·kg-1·d-1 for low salinity treatment, respectively. The rhizosphere effect had no noticeable influence on the Chao1 index, Shannon index and Simpson index of AOA. At the genus level, the dominant bacteria of AOA were norank_c_environmental_samples_p_Crenarchaeota, unclassified_k_norank_d_Archaea, and Nitrososphaera. The rhizosphere effect of seawater rice significantly affected the richness, diversity and abundance of AOB in coastal saline-alkali soil. It can significantly increase the abundance of environmental_samples_f_Nitrosomonadaceae and Nitrosospira. Also, correlation analysis between the AOB community and soil environment showed that environmental_samples_f_Nitrosomonadaceae and Nitrosospira had a significant positive correlation with HU and a significant negative correlation with pH. 【Conclusion】The results of this study indicate that planting tolerant rice species can improve nutrient cycling in coastal saline-alkali land, and the rhizosphere effect of saline-alkali tolerant rice mainly affects the community structure of AOB in acidic soil.

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