• Volume 58,Issue 5,2021 Table of Contents
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    • >Insights and Perspectives
    • Soil fauna and soil health

      2021, 58(5):1073-1083. DOI: 10.11766/trxb202012290717

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      Abstract:Soil animals play important roles in conservation of soil health. Their diversity can sensitively reflect soil disturbances caused by human activities and climate change and they also influence the above-ground ecosystem through interactions with other below-ground components. Inadequate knowledge on taxonomy of soil animals as well as the complexity of soil food web greatly limit the research on the links between soil animals and soil health. We here review recent advances in soil animals as indicators of soil health, point out the significance of soil animals and soil food webs in maintenance of healthy soil, and propose to improve the establishment of genome and functional traits database. Soil health assessment that integrates the diversity of soil animals, functional traits and soil food web structure is thought to be most promising.

    • >Reviews and Comments
    • Microbiome-mediated Transformation Mechanism and Regulation Principle of Mixed Organic Pollutants in Soils: Progress and Perspective

      2021, 58(5):1084-1093. DOI: 10.11766/trxb202009300458

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      Abstract:Polluted soils contain a mixture of contaminants. The microorganisms-mediated degradation processes and mechanisms appear to be more complicated than those in a controlled lab environment given that the soil microbiota is diverse and versatile in ecological functioning. In recent years, many researchers have highlighted the adaptation mechanism of the microbiome, biotransformation processes, design of synthetic microbial systems, in situ remediation by microbiomes in mixed organic pollutants contaminated soils, which are important for an understanding of the processes and mechanisms underlying soil remediation. The degradation of mixed organic pollutants is believed to be mainly controlled by the soil microbial community and influenced by their complex ecological networks. Due to the advantages of microbiomes(metabolic synergy and functional complementarity), this paper systematically analyzes the research progress and development trend of microbiome-mediated transformation and remediation mechanisms in organic pollutant co-contaminated soils and presents an outlook on the development of microbiomes in green and sustainable in situ bioremediation of mixed contaminated soils.

    • Status and Prospect of Farmland Soil Environmental Quality Management in China

      2021, 58(5):1094-1109. DOI: 10.11766/trxb202009220533

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      Abstract:The protection and management of environmental quality in farmland soils is an important prerequisite for food security and sustainable agricultural development. In recent years, with the rapid development of China's economy and society, the problems of degradation of farmland soil quality and soil pollution have become increasingly prominent. After decades of efforts, China has established a soil environmental quality management system with the "Law on Prevention and Control of Soil Pollution" and the "Action Plan for the Prevention and Control of Soil Pollution". These are the core policies and relevant supporting management methods, standards and technical specifications. This paper briefly(i)states the status and causes of degradation of environmental quality in farmland soils, (ii)reviews the development process of farmland soil environmental protection and environmental management, (iii)summarizes the latest progress of farmland soil environmental quality management and standard system construction in China, (iv)compares and analyzes the international experience of farmland soil environmental management, and(v)proposes strategies and prospects of environmental management of farmland soils in China. This study can provide references for China's farmland soil environmental protection, pollution control and environmental management.

    • Research Progresses on Remediation of Organic Contaminated Soils with Electrochemical Technologies

      2021, 58(5):1110-1122. DOI: 10.11766/trxb202004280200

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      Abstract:The serious situation of soil pollution is now threatening the safety of agricultural products, human health and ecological environment, even though, increasing attention has been paid to remediation of soils polluted with organic chemicals in recent decades. The organic chemicals commonly found in the soil include mainly pesticides, petroleum hydrocarbons, antibiotics and so on. As an effective in-situ remediation technology, the technology of electrochemical remediation can be used to remediate organic chemical polluted soils without disturbing their ecological environments. In this paper a review is presented on recent advances in the research on use of electrochemical technologies to remediate polluted soils, including electrokinetic and microbial electrochemical system. Effects of electrode materials, operation conditions and electrode arrangement on pollutant removal efficiency in the process of electrokinetic remediation are analyzed. A conclusion has been reached that both the technologies of electrokinetic and microbial electrochemical remediations can be used to remediate organic chemical-polluted soils, when used in combination with certain facilitating agents, such as surfactants, co-solvents, nanoparticles, and oxidizing chemicals, or with in-situchemical oxidation, bioremediation and phytoremediation, all of which show a synergistic effect on the removal or elimination of organic chemical contaminants in soil. Especially, relationships between variation of the soil microbial communities in the process of soil remediation and removal of pollutants in the process of microbial electrochemical remediation are discussed. Generally speaking, both electrokinetic remediation and microbial electrochemical remediation show good effects of removing organic pollutants from soils, and the former is better than the latter. All the experiments so far carried out show that the adoption of binary metallic oxidation electrodes, nanoparticles modified carbon felt electrode and that optimized electrode arrangement and reactor configuration can improve the pollutant removal efficiency, and the optimization of applied voltage, electrolyte type, operation time, external and internal resistance, electrode area and interval between electrodes can also significantly improve the electrochemical remediation efficacy. Compared with deionized water, Na2SO4, citric acid and NaOH, the NaCl and KH2PO4 are preferable electrolyte during the electrokinetic remediation. Organic pollutants removal efficiency increases with decreasing internal resistance and external resistance. Addition of biochar, carbon fiber, graphene oxide and sand can increase the soil conductivity and mass transport capacity. And microbial electro-remediating cells without external resistance is one of the hot spots in future researches on electrochemical technologies. With the decreased electrode interval, both electricity generation and removal of organic pollutants increase. Microbial community analysis shows that the microorganisms at the anode decrease in diversity and increase in homogeneity in the process of the microbial electrochemical system removing organic pollutants from soil. The exoelectrogens play an important role in the anode cell, while degrading bacteria are effective in the soil near the cathode and anode. In addition, the coupling of electrochemical remediation with chemical oxidation, phytoremediation and bioremediation may make it possible to extrapolate the technology of electrochemistry on a large scale. In order to realize the practical application of the technology to remediation of organic-chemical polluted soils, it is necessary to improve the electrochemical remediation technology by optimizing configuration of the reactor, soil conductivity and electrode materials in the future. Moreover, it is anticipated that mechanisms of the electrochemical remediation technology and characteristics of the functional microbial community relative to type of organic pollutants will be one of the hot spots in the research on microbial electrochemical remediation.

    • Progress and Prospect of Biological Soil Crusts in Loess Plateau

      2021, 58(5):1123-1131. DOI: 10.11766/trxb202007140391

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      Abstract:Loess Plateau is one of the most serious areas of soil erosion in the world and a typically sensitive and ecologically fragile area. It is also key areas of ecological restoration and ecological civilization in China. Biological soil crusts(BSCs)are complex organic integrities of cyanobacteria, bacteria, eukaryotic algae, fungi, lichen and moss, gluing loose soil particles together, and are critical component of the surface system in arid and semi-arid areas, which plays an important role in soil and water conservation, nutrients accumulation and ecological restoration of the Loess Plateau. In this paper, BSCs types and their successional process were reviewed. We summarize microbial and moss diversity in BSCs, and their environmental influencing factors, ecological function, artificial BSCs and ecological restoration in Loess Plateau. The existing problems in previous research were also discussed. In further research, the authors suggest microbial diversity can change from descriptive work to mechanical exploration. More attention should be paid to microbial functional groups in BSCs and their driving effects on the biogeochemical cycle of key elements of C, N and P. Inter-specific relationship among microorganism, spore plant and seed plant should be included in BSCs in Loess Plateau. Future research should focus on the mechanism how cyanobacteria, eukaryotic algae and mosses in BSCs response to key environmental factors such as culture substrate, water, temperature, nutrients and pH. The technical system of artificial BSCs cultivation should be established in laboratory and field, which will lay a theoretical foundation for ecological restoration. This review will be benefited to BSC research in Loess Plateau in China.

    • Release of Nitrous Oxide from Soil Aggregates and Its Microbial Mechanism

      2021, 58(5):1132-1144. DOI: 10.11766/trxb202008240311

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      Abstract:Nitrous oxide (N2O), a potent greenhouse gas, is produced and reduced mainly under the mediation of functional microorganisms in soil. In terrestrial ecosystems, soil is an important source of N2O emission. Soil aggregates, a key structural component of the soil, consist of sand, silt, clay (primary particles), organic matter (binding agents) and pore spaces. According to the hierarchy theory, soil aggregates can be divided into four fractions by size, that is, large macroaggregates (>2 mm), small macroaggregates (2-0.25 mm), microaggregates (0.25-0.053 mm) and silt plus clay-sized particles (<0.053 mm). Large macroaggregates are high in pore connectivity and oxygen diffusion rate, fast in turnover, and rich in organic matter, and microaggregates high in water retention capacity and stable carbon content, and capable of protecting microorganisms from being predated. Hence, soil aggregates different in size may offer heterogeneous microhabitats for fungi and bacteria. And each independent microhabitat could be regarded as a biogeochemical reactor producing greenhouse gas. Nitrifiers and denitrifiers, which carry functional genes amoA, narG/napA, nirK/nirS, are identified as the major contributors to N2O production. However, N2O reduction is primarily a single process catalyzed by N2O reductase, encoded by nosZI and nosZII genes, which are present in bacteria and archaea capable of complete denitrification and acting as non-denitrifiers in N2O reduction to N2. These microorganisms are distributed separately in polymerized reactors different in size, driving N2O production and transportation as affected by soil moisture status, substrate availability, and porous connectivity. However, so far little is known about community structure of the nitrifiers and denitrifiers in aggregates relative to particle size and its influences on N2O emission. Nowadays, a numerous of studies have been reportedly devoted to soil N2O emission characteristics in different ecosystems, but limited knowledge was achieved on N2O emission and relative contribution of soil aggregates relative to size fraction. Therefore, with the clarification of functional microbial distribution at the aggregate scale, hot-spots of N2O production and reduction in soil microhabitats could be specified. In this review, advances in the recent research are summarized on divergence of N2O emission from soil aggregates. Large macroaggregates and small macroaggregates were found emitting more N2O than microaggregates did. However, studies were also found reporting conversely that microaggregates emitted N2O more vigerously. Papers in the literature also reported relationships between aggregate turnover(the formation, stabilization and disintegration of soil aggregates)and microbial structure dynamics. Bacteria contribute strongly to the formation of both macro- and microaggregates, while fungi play an important role in the formation of large macroaggregates. Hence, the mechanisms of soil microbes producing and reducing N2O in soil microhabitats could be summed up. A large number of studies have shown that ammonium oxiders are abundant in macroaggregates (>0.25 mm) and a dominant denitrifier community in microaggregates (<0.25 mm), and environmental factors affect N2O emission via redistributing these functional microorganisms. Based on the current results, discussions are done of some perspectives for future investigations: potential hot-spots for soil N2O production at the aggregate scale as heterogenetic living niches existing in soil aggregates different in size, critical values of key environmental parameters impacting soil N2O production and reduction, and holistic research on functional gene groups and enzymes instead of some individual gene due to the complex participation of soil microbes in N2O production and reduction. It is expected that this study will provide a reference for modeling and parameter optimization and a solid theoretical basis for mitigation of N2O emissions.

    • >Research Articles
    • Study on Effect of Surface Wind Erosion on Hillslope Water Erosion in Regions of Typical Thin Layered Mollisol at Early Stages

      2021, 58(5):1145-1156. DOI: 10.11766/trxb202005040112

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      Abstract:[Objective] Soil erosion in the Mollisol region of Northeast China is a compound process occurring under the joint effect of water, wind, freeze-thaw and other external forces. However, so far researches on compound soil erosions in this region are still far from adequate, thus affecting precise implementation of soil erosion control measures. [Method] This paper applied a research strategy of combining an indoor wind erosion simulation experiment with a simulated rainfall experiment to analyze effects of early-stage surface wind erosion on hillslope water erosion in the Mollisol region of China.[Result] Results showed:(1) the foregoing wind erosion significantly delayed the generation of hillslope runoff, but it significantly increased hillslope runoff and soil loss (P<0.05), with both rates rising with wind velocity;(2) The foregoing wind erosion did have a positive effect on the following hillslope water erosion, and contribution of the former to the latter was increased significantly with rising wind velocity, but declined with rainfall intensity from 24.2%, 45.4%, 80.3%, down to 17.5%, 26.3% and 46.3%, respectively, when the wind was at 9, 12 and 15 m·s-1, respectively; (3) The contribution of wind erosion was attributed on the one side to the effect of wind erosion reducing soil erosion resistance by lowering shear strength and impactness of the surface soil, and on the other side, to the effect of wind erosion causing generation of aeolian sand particles that impacted and scrubbed the land surface in their wind-driving movement, thus forming micromorphology of wind erosion dents, and in turn altering runoff paths on the slope, speeding up pooling of runoff on the slope, increasing flow velocity and reducing flow resistance, and eventually intensifying surface runoff erosion and sediment transport. In addition, the foregoing wind erosion also provided erosion material for the later hillslope water erosion. [Conclusion] All the findings of this study not only enrich the theory of compound soil erosion, but also provide a scientific basis for soil erosion control in the Mollisol region of China.

    • Effects of Content of Soil Rock Fragments on Calculating of Soil Erodibility

      2021, 58(5):1157-1167. DOI: 10.11766/trxb202004200185

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      Abstract:[Objective] Rock fragments (>2mm diameter)are an important component of soil, and its presence has a significant impact on soil erosion and sediment yield. So it is essential to take into full account content of the rock fragments for accurate calculation of soil erodibility factor (K). [Method] In this paper, based on the data available of the content of rock fragments and classes of soil texture with a resolution of 30 arc-second, influence of the content of rock fragments, including rock fragments in the soil profile (RFP) and gravels on the surface of the soil (SC), on K was assessed at a global scale, using the equation (Brakensiek, 1986) of the relationship between saturated hydraulic conductivity and grade of soil permeability, and the equation (Poesen) of soil erodibility attenuation under a rock fragment cover. [Result] Results show: (1) The existence of rock fragments in the soil increased K by 4.43% and soil permeability by 5.68% on average in grade and lowering soil saturated hydraulic conductivity by 11.57% by reducing water infiltration rate of the soil and increasing surface runoff. The gravels on the surface of the mountain land and desert/gobi reduced K by 18.7% by protecting the soil from splashing of rain drops and scrubbing of runoff; so once the content of rock fragments in the soil profile and gravels on the surface of the land are taken into account in calculation, soil K may be 5.52% lower; (2) In the areas dominated with the effect of rock fragments, about 62.7% of the global land area, soil K decreased by 0.0091(t·hm2·h)·(hm-2·MJ-1·mm-1)on average, while in the area affected mainly by rock fragments in profile, about 31.1% of the global land area, soil K increased by 0.0019(t·hm2·h) (hm-2·MJ-1·mm-1); and (3) The joint effect of rock fragments in profile and gravels on the surface reduced the soil erosion rate by 11.8% in the 6 sample areas. [Conclusion] The presence of RFP increases soil K while the presence of SC does reversely. The joint effect of the two leads to decrease in soil erosion. In plotting regional soil erosion maps, it is essential to take both of the two into account so as to improve accuracy of the mapping.

    • Methods and Measures for Soil and Water Conservation in Hilly Red Soil Regions in South China: A Case Study of Xiaoyang Watershed in Ningdu

      2021, 58(5):1168-1178. DOI: 10.11766/trxb202003290139

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      Abstract:[Objective] After nearly 70 years’ development, the strategy of zoning for soil and water conservation has gradually got matured with natural environment, socio-economic situation, type of soil erosion, and other factors in the region taken into account, thus becoming an effective tool for strategic planning. However, local management agencies now require more locale-specific schemes to guide soil management practices, and zoning of small watersheds for soil and water conservation is an effective method, very helpful for comprehensive control of soil erosion. Moreover, zoning uses an evaluation- and cluster-based approach to sort and organize spatial units according to their similarity from a technical perspective. There are many kinds of clustering algorithms applied to zoning, and each has its own characteristics that may be desirable in some applications, but undesirable in others. Therefore, it is significant to evaluate the schemes of zoning for soil and water conservation designed with different clustering algorithms. [Method] In this study, the Pressure-State-Response(PSR)model was used as a framework for establishing a tri-mensional indicator system that included soil erosion sensitivity, the soil erosion status, and ecosystem services. The Xiaoyang watershed is a small watershed typical of the hill red soil regions in South China, suffering severe soil erosion. Taking the Xiaoyang watershed as an example, evaluation and comparison was made of the schemes of zoning for soil and water conservation designed with the 3 commonly-used clustering algorithms, i.e. k-means clustering algorithm(k-means), Self-organizing Maps(SOM)and Iterative Self-organizing Data Analysis(ISODATA), in dispersion, diversity and practicability, in light with the 3 evaluation indices, i.e. Silhouette Coefficient(SC), Shannon's Diversity Index(SHDI)and Perimeter-Area Fractal Dimension Index(PAFRAC). [Result] Results show that the k-means algorithm was the most reasonable for the zoning. The final schemes were adjusted through filtering, trend-surface analyses and comparison with other schemes to ensure continuity and integrity of the zoning and in the end 5 kinds of zoning were screened out. [Conclusion] Zoning for soil and water conservation should take into account characteristics of a region so as to make a suitable arrangement for vegetation rehabilitation in terms of the surface landscape. This study provides a framework within which region-specific soil erosion measures can be planned, and it has also laid down a scientific basis for local management agencies to implement region-specific erosion control measures and improve their integrative management benefits.

    • Hydraulic Characteristics of Concentrated Flow Relative to Angle between Rock and Slope in the Karst Trough Valley Area

      2021, 58(5):1179-1189. DOI: 10.11766/trxb202002140046

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      Abstract:[Objective] Outcrops of rock in the Karst Trough Valley Area often form angles with slopes, thus making micro landforms complicated and altering significantly paths of concentrated flows, and consequently bringing about great changes in characteristics of the flows. So, it’s important to study hydraulic characteristics of the concentrated flows relative to angle between rock strata and slope. [Method] To that end, a flume experiment, designed to have combinations of three slope gradients (10°, 15° and 20°), three flow rates (5, 7.5 and 10 L·min-1) and six angles between the rock and slope (30°, 60°, 90°, 120°, 150° and 180°(0°)), was carried out. [Result] Results show that hydraulic characteristics of a concentrated flow varied significantly with the angle between the rock and slope. Reynolds number (Re) of the concentrated flow changed with duration of the scouring in a complicated trend, but exhibited a significant trend of increasing with rising flow rate, and an insignificant one of changing with slope gradient. And it varied in the range of 517~3 343 in the experiment. Darcy-Weisbach friction coefficient (f) of the concentrated flow increased with rising slope gradient and with scouring going on, but decreased with rising flow rate. And it varied in the range of 0.62~5.70 in the experiment. The relationship of Re with angles between rock and the slope was not significant, but that of f varied with combinations of slope gradient and flow rate. The coupling relationship between f and Re was influenced significantly by the angle between the rock and the slope. It could be better described with a logarithmic equation when the angle between the rock and the slope was narrower than 90°, and with a power function equation when the angle between the rock and the slope was wider than or equal to 90°. In the experiment, when Re<1 791, the relationship between f and Re was not apparent, but when Re>1 791, it was significantly positive.[Conclusion] To sum up, f and Re and their relationship vary with the angle between the rock and the slope. The findings in this experiment may provide strong data support for establishing soil erosion prediction models in the Karst Trough Valley Area.

    • Cl- Transport and Its Influencing Factors in Soil as Affected by Soil Texture and Vegetation Cover in Loess Plateau

      2021, 58(5):1190-1201. DOI: 10.11766/trxb202002200059

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      Abstract:[Objective] To evaluate nutrient cycling and pollutant migration in the Earth’s Critical Zone, it is essential to understand solute transport in soils. Despite extensive studies on processes and mechanisms of solute transport, little is known about how to characterize and regulate solute transport in soils different in soil texture and vegetation cover. This study is oriented to investigate Cl- transport parameters and their influencing factors in typical sandy and loamy soils different in vegetation cover in the Liudaogou and Gechougou watersheds in the northern part of the Loess Plateau. [Method] Six undisturbed soil columns(7 cm in diameter and 25 cm in height)different in texture(sandy and loamy, abbreviated as S and L, respectively)and in vegetation cover(arbor, shrub and grass, abbreviated as AR, SH and GR, respectively)were collected by hand to keep it as intact as possible in the Liudaogou and Gechougou watersheds located in Shenmu City, Shaanxi Province. With these columns, an indoor experiment was carried out to determine Cl- penetration curve in each soil column and experiment data were simulated with the convection-dispersion equation. Cl- transport parameters, including time the penetration begins(TS), time the penetration ends(TE), average pore-water velocity(V)and hydrodynamic dispersion coefficient(D), were obtained with the aid of the CXFIT software. Relationships between soil properties and Cl- transport parameters were analyzed with Pearson’s test. [Result] Results show that TS, TE, V and D of Cl- all varied with soil texture, vegetation type and soil depth in the range of 12~80 min, 75~480 min, 0.52~1.98 cm·h-1 and 0.75~2.55 cm2·h-1, respectively. TS and TE was the lowest in the 0~20 cm soil layer and increased with increasing soil depth, while V and D both exhibited a reverse pattern. Mean V and D in 0~1 m soil profiles the same in texture varied with vegetation cover, showing a decreasing order of S-AR > S-GR > S-SH and L-AR > L-SH > L-GR, while mean TS and TE did too, but showing opposite trends, which could be attributed to the differences caused by different vegetation covers in root biomass distribution in 0~1 m soil profiles either sandy or loamy in texture, which in turn affected the number of macro-pores, connectivity density and preferential flow pathway. On the other hand, mean V and D in 0~1 m soil profiles the same in vegetation cover varied with soil texture, showing orders of S-AR > L-AR; S-SH > L-SH and S-GR > L-GR, while mean TS and TE did too, but showing reverse patterns, which could be ascribed to the difference in soil mechanical composition that significantly affected the size and distribution of soil pores. Sandy soil was higher in number of macro-pores and more homogeneous in pore distribution, which was conducive to formation of preferential flow in soil profiles. In contrast, loamy soil was more complex in porosity formed of micro-pores and higher in adsorption force and capacity that inhibited Cl- transport. Soil bulk density, number of macro-pores, connectivity density, and contents of organic carbon, sand, silt and clay were all significantly related to V, TS and TE, indicating that soil properties were major factors affecting Cl- transport in soils, even though different in soil texture and in vegetation cover. [Conclusion] All the findings in this study may serve as references for rational vegetation distribution in soils different in texture in the Loess Plateau region to reduce soil nutrient loss and improve service functions of the ecosystem.

    • Multi-scale Spatial Distribution Characteristics of Soil Salinity in Root-zone in Film-mulched Drip-irrigated Cotton Fields in Anjihai Irrigation Zone, Xinjiang

      2021, 58(5):1202-1213. DOI: 10.11766/trxb201912060666

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      Abstract:[Objective] The knowledge about the spatial distribution characteristics of salinization is the basis for amelioration of saline-alkali soils. Compared with research based on a single scale, nested multi-scale research can be more effectively applied to analyzing structural characteristics of the spatial variation of soil salinity with higher accuracy in describing scale-dependent self-correlated variation of salinization. [Method] In the Anjihai Irrigation Zone of Xinjiang, soil samples were collected by soil layer in the film-mulched cotton fields for analysis of rules and variabilities of the spatial distribution of salt content in the root zone of the drip-irrigated cotton field under film mulch with the method combining conventional statistics and geo-statistics, as affected by nested scales of 4 km, 500 m and 100 m. [Result] Results show that, for any scale, soil salinity in the root-zone of the cotton fields in the zone is generally low, with the mean being in the range of 1.52-1.87 g·kg-1, moderate in variability, and characterized by continuous variation and accumulation at the bottom of the root zone. Non- and mildly-salinized soils dominate the irrigation zone and are mainly located in the south-east part of the zone, where the surface water is low in salinity and the ground is quite high with unobstructed drainage, while relatively seriously salinized soils are mainly distributed in areas where the ground water is shallow and overflows, and in areas that are affected by the leakage from reservoirs in the plains and trunk canals. As influenced by some structural factors (e.g. terrain landform) and stochastic factors (e.g. artificial activities), soil salt content in this area shows a significant spatial self-correlation in distribution. With increasing scale, nugget-to-sill ratio of the geo-statistical analysis model decreases, and related distance, self-correlation and impact of structural factors increase, but the influence of random factors weakens. On the contrary with declining research scale, spatial distribution becomes more explicit, and the impact of random factors intensifies. [Conclusion] For those relatively seriously salinized areas, it is advisable to employ the nested multi-scale root-zone sampling strategy so as to get a full knowledge about characteristics of the spatial distribution and variability of soil salinity in the drip-irrigated cotton fields under film mulch.

    • Numerical Inversion-based Simulation of Hydraulic Properties of Stratic Soi

      2021, 58(5):1214-1223. DOI: 10.11766/trxb202005030212

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      Abstract:[Objective] Soil hydraulic properties refer mainly to soil hydraulic conductivity and water characteristic curve, which are essential basic data for numerical simulation of soil water movement. Most soils in nature are present in layers, but in the simulation experiments so far done, the soils were assumed to be homogeneous in texture. [Method] Through an indoor soil column infiltration test, vertical one-dimensional water movement was numerically simulated with the aid of Hydrous-1D software, and numerical inversions were carried out with the variables (cumulative infiltration volume, pressure head and combination of both) measured in the infiltration process in the indoor soil column infiltration test as objective functions to optimize the parameters obtained of the hydraulic properties of each layers of the static soil. In the light of the evaluation indices, such as determination coefficient (R2), root mean square error (RMSE), coefficient of variation (CVS), correlation matrix of the optimized parameters and standard error of the optimized parameters, effects of different measured variables on non-uniqueness and accuracy of the solution were discussed. [Result] Results show that when the objective functions contain only the cumulative infiltration, although the value simulated through inversion is in good agreement with the measured value, R2 reached up to 0.999 and RMSE was less than 0.005 62 cm, but the parameters verified with the measured value of pressure head was not so good in accuracy, with R2 being only 0.338, and RMSE less than 0.096 5 cm. When the pressure head was used as the objective function for parameter optimization and solution, the fitting degree R2 reached up to 0.855 and RMSE was less than 0.038 4 cm; the use of the measured value of cumulative infiltration in verification was quite good in effect, with R2 reaching up to 0.99 and RMSE being less than 0.038 56 cm. The simulation of hydraulic properties of the soil through inversion using cumulative infiltration and pressure head in combination was quite high in goodness of fit, with R2 reaching more than 0.905 and RMSE less than 0.035 4 cm. Although in the inversion, the indices, R2 and RMSE, were inferior to that using cumulative infiltration, the inversion reduced correlation and standard error of the optimized parameters to a certain extent, thereby reducing the solution Non-uniqueness, and improving accuracy of the solution. [Conclusion] The above conclusions may serve as reference for selection of measured variables for use in the inversion for parameters of transient flow in the static soil.

    • Estimation of Soil Organic Carbon Based on Spectral Similarity Matching

      2021, 58(5):1224-1233. DOI: 10.11766/trxb202003010082

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      Abstract:[Objective] The objective of this study is to explore effective strategies for predicting soil organic carbon (SOC) in local areas with high precision based on the spectral similarity indexes in the global spectral library. It goes specifically as follows: (1) to compare different spectral similarity indexes obtained separately with three different similarity matching algorithms (Euclidean Distance, Mahalanobis Distance and Spectral Angle Mapper) in effect on prediction accuracy; (2) to compare calibration sets different in size in effect on prediction accuracy; and (3) to compare different modeling strategies (PLSR modeling and the assignment strategy) in effect on prediction accuracy. [Method] From the global spectra library a total of 245 China samples were selected to form a prediction set and the remaining 3 537 samples a reference set. From each spectrum in the prediction set, different numbers of similar spectra (5, 10, 20, 30, 40, 50, 100, 150, 200, 250, 300, 400 and 500) were picked out separately with the three different similarity matching algorithms for comparison between the spectra selected by the different similarity algorithms. Based on the reference sets of different sizes selected by the different algorithms, PLSR models were built to predict SOC contents, and effects of the similarity matching algorithms and size of the modeling set on prediction accuracy were evaluated using R2, RMSE and RPD. Then a similarity matching algorithm with the highest prediction accuracy was selected and on such a basis, comparison was performed between the different modeling strategies in effect on prediction accuracy: Ⅰ, Prediction with PLSR modeling; Ⅱ, Prediction with direct assignment. [Result] Compared with the overall model, the three similarity algorithms greatly improved prediction accuracy. Among the three, the SAM model was a bit higher than the other two in prediction accuracy (R2 =0.75, RPD=1.73). The low prediction accuracy might be attributed to the wide distribution of the soil samples in the global soil spectral library that caused marked variation. Size of the modeling sets did have a great impact on modeling accuracy, and the optimal size for the three similarity algorithms varied in the range of 400~500 (0.71<R2 <0.75, 1.56R2 >0.6, RPD>1.4). However, when the modeling set was big in size (>50), the PLSR modeling strategy was higher in prediction accuracy (R2 >0.6, RPD>1.4). [Conclusion] Compared with the global model, the models based on the three spectral similarity indices all significantly improve SOC prediction accuracy. In general, the spectral angle algorithm is slightly better than Euclidean distance and Mahalanobis distance; type of the similarity algorithm, size of the modeling set and method of the modeling all have a great impact on precision of the SOC prediction.

    • Research of Contaminated Sites Based on Knowledge Graph Analysis and Its Development Trend

      2021, 58(5):1234-1245. DOI: 10.11766/trxb202002280030

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      Abstract:[Objective] Contaminated sites have become a serious environmental problem in China. It is particularly important to safely reclaim and utilize the contaminated sites. In order to make clear status of the current researches on contaminated sites, bibliometric analysis was performed. [Method] Knowledge graph analysis tools, like VOSviewer, CiteSpace and HistCite, were used in the bibliometric analysis of countries, institutions, journals, first authors and hot spots of the publications and important papers contained in the core collection database of the Web of Science.[Result] Results show: (1) The research on contaminated sites involves cooperations between multiple countries. In the past five years, China has come out to be the country with the largest number of papers in this field, and in close cooperation with many countries. Paper contributing institutions in this field, like the Chinese Academy of Sciences, the Oak Ridge National Laboratory, the University of Waterloo, the University of Chinese Academy of Sciences and the Helmholtz Environmental Research Center, have been cooperating extensively; (2) The Science of the Total Environment, Chemosphere, Environmental Science & Technology, Environment Science and Pollution Research, etc. are the main journals publishing papers in this field, and Naidu Ravi, Huang Guohe, and Megharaj Mallavarapu are on top of the list of highly productive scholars. Among the top 10 important papers on contaminated sites, bioremediation-related content occupies a quite high proportion; (3) In the keyword clustering network, key words of papers were sorted into four clusters: biological toxicology research on the contaminated sites, soil heavy-metals contamination and remediation technologies, pollutants environmental behavior and water remediation, and bioremediation of organic pollutants contaminated sites; (4) The bibliometric analysis shows that soil and groundwater are important pollutants carriers in the contaminated site. Remediation of the soil and groundwater in the contaminated site should deem the two as one. The neglect of either one in remediation may cause secondary pollution in the contaminated site. Heavy metals and polycyclic aromatic hydrocarbons are the pollutants of great concerns. Searching of efficient and green remediation technologies are still the focus of future work in the contaminated site research field. [Conclusion] It is still not optimistic for China to address the issue of remedying heavy metals contaminated sites, which calls for further efforts to intensify and implement corresponding works. Therefor it remains to be a focus in solving the problem of contaminated sites to explore high-efficiency green remediation technologies and it will be of great application value to improve the application of bioremediation technology to remedying contaminated sites.

    • Distribution of Heavy Metals and Microbial Community Structure in Soils High in Geological Background Value

      2021, 58(5):1246-1255. DOI: 10.11766/trxb202001030003

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      Abstract:[Objective] Farmland soils high in geological background value are widely distributed in China, complex in pedogenesis, and diverse in geological age and parent material. Consequently, by far little is known about mechanisms of their pollution of heavy metals and ecological risks of the pollutants. The purpose of this study was to explore distribution of the heavy metals in paddy soils typical of the karst area of Guangxi, and key factors that affect structures of the microbial communities in the soil, in an attempt to provide certain data support for establishment of a heavy metals risk assessment system for high-geological background-valued farmland soils of the secondary enrichment type. [Method] Samples of four types of paddy soils different in geological age and type of parent material were collected separately in Maling, Luoxu, Suxu, and Yunbiao of Guangxi for analysis of contents and distribution of heavy metals, physical and chemical properties, and the functional diversity of the microbial communities therein. And redundant analysis(RDA)was performed to obtain key environmental factors that impact structures of the soil microbial communities in those soils. [Result] Results show: (1) Development stage and type of parent material significantly affected basic physiochemical properties of the soils, and also concentrations of heavy metals in the soils. The soil in Maling started its development earlier and so was higher in organic matter content and heavy metals enrichment. Derived from carbonate limestone, the soil was also higher in soil Cd than all the other three types of soils derived from Quaternary fluvial sediments; (2) The four types of paddy soils were sharply different in soil microbial activity and microbial community structure. The soil in Suxu was significantly lower in soil microbial carbon source utilization capacity and microbial diversity index than the other three types of soils, where the amine and amino acid utilizing microbes were higher in metabolic activity; However, the soil in Suxu was higher in metabolic activity of the saccharides and polymeric carbon utilizing microbes; (3)RDA shows that soil organic matter, pH, total As, Cd, Pb, and available As were the main environmental factors that caused differentiation of the soil microbes in utilizing carbon sources; and(4)Correlation analysis shows that intensity of the soil microbes utilizing amines, amino acids, and phenolic acids was positively related to soil organic matter and pH, and negatively to As, available Cu, and available As. [Conclusion] For the four types of soils high in geological background value, parent material and geological age are the two factors that determine physical and chemical properties of and distribution of heavy metals in the soil, which then in turn influence community structure and activity of the microorganisms in the soil.

    • Three-source Partitioning of CO2 Emissions from Maize-planted Soil Using 13C Labeling and Natural Abundance

      2021, 58(5):1256-1266. DOI: 10.11766/trxb202005070111

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      Abstract:[Objective] In crop fields of calcareous soil, it is found that CO2 released from rhizosphere may come from as many as three sources, i.e., root respiration, decomposition of soil organic carbon (SOC) and dissolution of soil inorganic C (SIC), so partitioning of the released CO2 by source is the premise of quantifying soil C balance.[Method] A pot experiment was designed and carried out to have 18 pots of summer maize (Zea mays L.), of which three had maize plants treated with 13CO2 pulse-labeling for 7 hours, separately, at the elongation, heading and grain-filling stages, while in the other, the crop grew in nature as CK. On the 27thday (d) after each labeling, samples of the soils and plants were collected from the pots for analysis of total C and 13C in shoot, root, soil and emitted soil CO2. Comparisons were made between the treated pots and CK, trying to partition the emitted CO2 by source.[Result] Results show that the contribution of root respiration to soil CO2 emission decreased from 66.7% at the elongation stage to 25.8% at the grain-filling stage. Over the whole vigorous vegetative growth period, root- and soil-derived CO2 did not differ much in contribution to soil CO2 emission, and the contribution of SOC decomposition and SIC dissolution to soil CO2 emission reached about 30% and 20%, respectively. The contribution of maize growth to input of soil C in the form of roots and rhizodeposits was high enough compensate for the release of soil total C (SIC + SOC), hence the soil functioned as a C sink.[Conclusion] All the findings in this experiment demonstrate that SIC dissolution is more important than previously presumed in stabilizing the global C pool and regulating CO2 concentration. If the contribution of SIC dissolution to soil CO2 emission in calcareous soil were neglected, overestimation of the contribution of SOC decomposition is likely to occur, which will inevitably affect quantification of the priming effects of SOC and assessment of the soil C balance.

    • Effect and Mechanism of Periphyton Affecting Ammonia Volatilization in Paddy Field

      2021, 58(5):1267-1277. DOI: 10.11766/trxb202005060218

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      Abstract:[Objective] Periphyton, a kind of microbial aggregates, grows generally at the water-soil interface in paddy fields, and plays an important role in regulating nitrogen cycling in the paddy fields. This study was conducted to explore effects of perphyton on ammonia volatilization, and relationships of ammonia volatilization with microbial community structure of the periphyton and physico-chemical properties of the surface floodwater in the paddy field, so as to provide certain theoretical guidance and technical support for controlling ammonia emission in paddy fields. [Method] In this study, a field experiment was carried out with two treatments designed to be: 1) with normally growing periphyton, and 2) with terbutryn (C10H19N5S) added to regulate growth of periphyton. Each treatment had three replicates. Ammonia volatilization was monitored after the application of base fertilizer (BF), tillering fertilizer (TF) and heading fertilizer (PF), separately, for analysis of changes in ammonia volatilization rate as affected by periphyton, calculation of ammonia flux volatilized throughout the entire rice growing season, and determination of relationships of ammonia volatilization with microbial community structure of the periphyton and physico-chemical properties of the surface floodwater, in an attempt to establish a model to predict ammonia volatilization from paddy fields based on periphyton growing in the field.[Result] Results show: 1) the cumulative loss of ammonia via volatilization reached 24.75±1.05 kg·hm-2 and 10.92±0.07 kg·hm-2 in the treatment with periphyton growing normally, after the application of base fertilizer (BF) and tillering fertilizer (TF), respectively, and was 71.3% and 39.3%, respectively, lower in the treatment with terbutryn added, however, it did not differ much between the two treatments at the panicle stage (); 2) Ochrophyta, Cryptomycota and Nematoda were the dominant eukaryotic microbes in the study area (accounting for 42.6%-67.6%) while Proteobacteria, Bacteroidetes, Chloroflexi (14.6%-0.8%), Acidobacteria were the dominant prokaryotic microbes (accounting for 52.1%-58.1%); and 3) Correlation analysis shows that biomass of the periphyton, nitrogen content in the floodwater and wind speed are the key factors affecting daily ammonia volatilization flux. [Conclusion] To sum up, the effect of addition of C10H19N5S changing community structure of the periphyton can effectively reduce ammonia volatilization from paddy soil by modulating growth of the periphyton during the periods after the application of base and tillering fertilizers, and hence alleviate N loss via ammonia volatilization.

    • Effects of Coal Mining Subsidence on Soil Microbial Community in the Loess Plateau

      2021, 58(5):1278-1288. DOI: 10.11766/trxb202003160122

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      Abstract:[Objective] Mining activity has posed a great impact on the environmental background of the areas. To meet the challenge, the near-natural recovery method might be the only feasible way. Soil microorganisms not only affect the material cycling and transformation of soil substances, but also play an important role in the restoring ecological systems. So to improve resilience of the mining ecological system, it is essential to clarify how soil microbial communities adapt to the environment and evolve therein; how do different microbial groups interacted with each other; and which of their functional groups are more suitable for the arid and barren environments? Moreover, knowledge about interactions between soil microbial communities and their responses to coal mining subsidence are crucial to ecological restoration and resilience of the ecosystems in the semiarid and/or damaged mining areas. However, so far little has been done in this aspect. [Method] To explore the structures and compositions of soil microbial communities, and mechanisms of reciprocal feedback between the dominant microbial groups and the environment in the subsidence areas of the Dongping (DP) and Dalita (DLT) Coal Mines, the methods of high-throughput sequencing and molecular ecological network were adopted. [Result] Results showed that the subsidence environment posed a significant impact on the diversity, structure and distribution of the soil microbial communities. Actinobacteria remained to be the dominant phylum in the DP and DLT Coal Mines, whereas DP had two more dominant phyla than DLT did, namely Nitrospira and Cyanobacteria. In the soil of DP, RB41, Solirubrobacter, Roseiflexus, Gaiella and Lysobacter were the genera > 1% in relative abundance, while in the soil of DLT, Nocardioides, RB41, Solirubrobacte and Roseiflexus were. The molecular ecological networks in the soil of the DP Coal Mine interacted more complicatly than those in the soil of the DLT Coal Mine, with more network nodes and connections. The soil microbial groups in the soil of the DP Coal Mine belonged to the phyla of Actinobacteria, Chloroflexi, and Thaumarchaeota, whereas the key species in the soil of the DLT Coal Mine were of the phyla of Proteobacteria and Actinobacteria. Moreover, significant relationships were observed between network structure and soil properties in both mining areas. For instance, among the DP networks, Modules 1, 2 and 3 were significantly and positively related to the soil EC, while module 2 was to the soil water content; Module 6 was significantly and positively related to soil available phosphorus, and Module 1 was to nitrate-nitrogen content; Modules 6 and 7 were significantly and positively related to soil dehydrogenase activity, while Modules 2 and 5 were to activity of soil urease. And among the DLT networks, Module 1 was significantly and positively related to pH and soil temperature, while Module 2 was to soil temperature only; Modules 1 and 2 were significantly and negatively related to soil water content and clay percent, whereas Module 5 was to nitrate-nitrogen content, but reversely.[Conclusion] In order to adapt to the nutritional infertility of the poor soil in the subsided mining areas, soil microbial communities tended to interact with each other. Holophagae and Aquabacterium were found to be the dominant species, and/or applicable to restoration of the ecological systems in the mining areas in the future.

    • Dual Functions of Bacteria Colonized on AM Fungal Hyphae-Fixing N2 and Solubilizing Phosphate

      2021, 58(5):1289-1298. DOI: 10.11766/trxb20200316004

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      Abstract:[Objective] The objectives of this study were to investigate whether the bacteria colonized on the surface of extraradical hyphae of arbuscular mycorrhizal (AM) fungi do have the dual functions of N2 fixation and phosphates solubilization. AM fungi release through extraradical hyphae exudates that are rich in carbohydrates and hence provide a habitat for bacteria to colonize in. The interaction between AM fungi and bacteria in the hyphosphere has been found to promote mineralization of organic phosphorus and organic nitrogen, enhance nutrient acquisition of AM fungi, and further improve nutritional status of the host plant. However, it remains unclear whether the bacteria colonized on the surface of extraradical hyphae of AM fungi do have the dual functions of N2 fixation and phosphate solubilization. [Method] AM fungal hyphae were collected with PVC tubes and nylon mesh in a maize field, and then N2-fixing bacteria colonized on the surface of extraradical hyphae of AM fungi were isolated with a nitrogen-free selective medium, purified for DNA extraction and identified through 16S rRNA gene sequencing. NifH gene of the bacterial DNA was amplified with specific primers. Nitrogenase activity of the N2-fixing bacteria was determined via acetylene reduction assay, phosphorus solubilizing ability was by measuring the diameter of the bacteria colony and phosphorus solubilizing halo in NBRIP solid medium, and IAA (Indoleacetic acid) secretion ability was with the Salkowski colorimetric method. [Result] Twenty-three strains of N2-fixing bacteria, capable of surviving in N-free media, were isolated from the surface of AM fungal hyphae, and sorted into nine different genera (Paenibacillus and Bacillus in Firmicutes; Microbacterium and Arthrobacter in Actinobacteria; Sphingomonas, Brevendimonas, Variovorax, Xenophilus and Hydrocarboniphaga in Proteobacteria), among which, bacteria in Brevendimonas (six strains, accounting for 26% of the total) and Microbacterium (six strains, for 26%)were the highest in relative abundance; eleven strains showed nitrogenase activity, peaking up to (2.97 ±1.32) nmol·mg-1·h-1; twelve strains did the ability to solubilize organic phosphorus (phytin); eleven strains did the ability to solubilize sparingly soluble inorganic phosphates (tricalcium phosphate); eight strains did the ability to solubilize both phytin and tricalcium phosphate, with the former being higher than the latter; sixteen strains did the ability to secrete IAA, peaking up to (688.00 ±19.17) μg·mL-1; and six strains did all the functions, nitrogenase activity, phytin solubilization, sparingly soluble inorganic phosphates solubilization, and IAA secretion, and they belong to five different genera (Brevundimonas, Microbacterium, Sphingomonas, Paenibacillus, and Arthrobacter) and among them, Paenibacillus was the highest in ability of N2 fixation, sparingly soluble phosphates solubilization, and IAA secretion. [Conclusion] A variety of strains of bacteria with nitrogen-fixing, sparingly soluble phosphates-solubilizing and growth-promoting ability are found colonized on the surface of extraradical hyphae of AM fungi in the maize field. These bacteria with multiple functions may have the ability to greatly expand the pathways for mycorrhizae to absorb nutrients in the soil.

    • Effects of Raising Duck in Paddy Field on Soil Nutrients and Rice Pests and Diseases Control

      2021, 58(5):1299-1310. DOI: 10.11766/trxb202003050694

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      Abstract:[Objective] A field experiment was conducted in paddy fields in the Dianshan Lake region of Shanghai to explore effects of raising ducks in paddy fields on soil nutrients, field surface water, rice growth, and control of weeds, pests and diseases.[Method] The experiment was designed to have three treatments and one CK, i.e. duck-raising in paddy field (DR); No duck-raising in the fields irrigated with drainage from Treatment DR(ND); No duck-raising in the field irrigated with drainage from Treatment DR, plus manual weeding (NDW); and rice cultivated as usual with irrigation of normal water (CK). No pesticides and herbicides were applied during the whole experiment to prevent their disturbances to rice and duck growth, soil nutrients and enzyme activities.[Result] Results show that all of the treatments, no matter whether it was directly affected by duck raising (Treatment DR), or indirectly affected by irrigation (Treatments NDW and ND), increased the content of soil nutrients. Treatment DR was the highest in the effect, being 11.0%, 17.6%, 11.5%, 13.3% and 13.6% higher in content of total nitrogen, alkali-hydrolyzable nitrogen, available phosphorus, readily available potassium and organic matter in the soil, respectively, than CK during the rice growth season, and Treatment NDW and ND was 9.8%, 8.4% and 10.9% higher, and 3.4%, 2.1%, and 7.1% higher than CK, respectively, in content of alkali-hydrolyzable nitrogen, available phosphorus, and readily available potassium. Meanwhile, stirring of the flooding water by ducks in Treatment DR increased the content of dissolved oxygen and hence microbial biomass and activities of soil enzymes, especially the activity of urease, catalase and sucrase, by 21.0%, 20.6% and 13.9%, respectively. Thanks to the activities and fecal excretion of ducks in Treatment DR, the flooding water in Treatments DR, ND and NDW was significantly higher than that in CK (P<0.05) in nutrient content. Especially on September 26, the flooding water in Treatment DR peaked up to 1.18 mg·L-1, 0.40 mg·L-1 and 0.93 mg·L-1, respectively in contents of ammonium nitrogen and available and total phosphorus, or was 137%, 177% and 316% higher than in CK. Pecking, trampling and stirring of the ducks in the fields effectively inhibited weed growth, reducing the density of weeds by 91.6% on average in Treatment DR. In contrast, the density of weeds was the highest in Treatment ND and then in Treatment NDW and in Treatment DR, suggesting that the effects of regular manual weeding in the paddy fields on weed control were not so good as that of raising ducks in paddy fields. All the treatments, DR, NDW and ND, stimulated tillering of the rice significantly or by 34.2%, 32.1% and 33.6%, respectively. The incidence of rice leaf rollers, sheath blight or rice planthoppers was significantly lower in Treatment DR than in CK and the other treatments. Moreover, Treatment DR was also significant in yield increasing effect, with grain yield reaching up to 8 507 kg·hm-2, 1.42 times that of CK. [Conclusions] In general, raising ducks in paddy fields can improve soil fertility, inhibit weeds and reduce incidence of rice pests and diseases effectively, which is beneficial to minimizing the utilization of pesticides, herbicides and chemical fertilizers in paddy fields and controlling non-point source agricultural pollution. Moreover, it helps produce high quality healthy food-rice and ducks.

    • Metagenomics Based Study on Community Characteristics of Ammonia-Oxidizing Microorganisms in Acid Forest Soil

      2021, 58(5):1311-1321. DOI: 10.11766/trxb202002020040

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      Abstract:[Objective] The acidic forest soils are characterized by low pH and depletion of NH3 substrate, which are considered to be the major factors shaping ammonia-oxidizing communities (AOA, AOB and comammox) in soils. However, so far little is known about the relationship between absolute and relative numbers of the three kinds of ammonia oxidizers in acidic forest soils, especially due to the absence of reliable real-time quantitative PCR (qPCR) primers for the newly identified comammox Nitrospira. Here this study aims to develop a strategy to determine abundance of comammox Nitrospira, and to investigate community structures of AOA, AOB and comammox in the acidic forest soils. [Method] Absolute abundances of amoA genes of AOA, AOB and comammox in soils were measured with the aid of qPCR. However, Ntsp-amoA-162F/359R and comaB-244F/659R, the two primers for comammox amoA genes would trigger significant non-specific amplification, and hence overestimation of the abundance of comammox. Therefore, qPCR data of comammox amoA genes needs to be calibrated with the semi-quantitative method based on agarose gel electrophoresis of the qPCR products, and by the metagenomic sequencing of the total soil DNA. Furthermore, community compositions of AOA, AOB and comammox in the acidic forest soils were also characterized with the aid of metagenomic sequencing. [Result] The qPCR demonstrate that the amoA genes of AOA and AOB were 2.61×106 and 1.45×106 copies·g-1, respectively in abundance, while the qPCR of comammox amoA gene exhibits significant non-specific amplification, and the calibrated data show that comammox amoA gene, was about 1.38×106 ~1.47×106 copies·g-1 in abundance. Furthermore, Group 1.1b was the predominant one, accounting for ~88.07% of AOA, while the classical 1.1a-associated acidophilic group accounted only for ~11.93%. Of AOB, Nitrosospira was the major group, accounting for ~63.96%, while Nitrosomonas made up ~36.04% of the total AOB gene sequence. Clade B was the major group, accounting for ~63.39% of the comammox, while Clade A accounted only for ~36.61% of total comammox amoA gene sequence and exclusively fell into Sub-clade A.1. [Conclusion] In this study, a strategy for determining the absolute abundance of comammox amoA genes in soil was developed based on comprehensive utilization of the techniques of qPCR, semi-quantitation and metagenomic analysis. In the acidic forest soils studied the ratio of comammox to AOA and AOB in amoA gene abundance is found to be ~0.55 and ~0.98, respectively. Group 1.1b, Nitrosospira and Clade B is the major group of AOA, AOB and comammox, respectively, in the soils, and the potential molecular mechanisms underpinning the adaption of these ammonia oxidizers to the environments low in pH and short of NH3 substrates warrant further physio-biochemical studies of pure cultures.

    • >Research Notes
    • Prediction of Soil Free Iron Oxide Content Based on Soils Munsell Color

      2021, 58(5):1322-1329. DOI: 10.11766/trxb202004130048

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      Abstract:[Objective] Soil iron oxides are found in almost all the types of soils and are good indicators reflecting variation of the environment thanks to their high variability in concentration. Soil iron oxides, mostly in the form of free iron, function as important mineral binders in the soil and have a significant effect on color of the soil. Soil color is an important soil property, described by Munsell color space, in the soil taxonomy. Both soil color and free iron are indicators in the soil taxonomy, and fairly related to each other, but so far few papers have been reported on quantitative relationships between them. [Method] Since soil color is an indicator reflecting genesis and evolution of a soil, it is often used to invert and predict soil properties via modeling. Therefore, in this paper, the typical soil series in the hilly region of Central Sichuan were taken as the research object for analysis of relationships between soil Munsell color and free iron content in the soils. On this basis, a BP neural network model is established to explore differences between the Munsell color based model and the traditional spectral model in predicting soil free iron content. [Result] Results show significantly positive relationships of free iron content with hue, value and chroma of soil Munsell color. When the Munsell color prediction model had 4 neurons in the single hidden layer, the determination coefficient R2 of its test set was 0.94, its standard deviation RMSE 4.20, and its relative analysis error RPD 4.37; When the spectral prediction model had 6 neurons in the single hidden layer, its R2 was 0.98, RMSE 3.35, and RPD 5.99. Both models have demonstrated a high level of goodness of fit and accuracy, though the spectral model is slightly higher than the color model. [Conclusion] Munsell color can be used to predict soil free iron content effectively, but by comparison, the spectral model is a bit higher in goodness of fit and prediction accuracy, which may be attributed to the fewer neurons in the input layer of the color model and the dispersion degree of free iron oxides dispersion degree. Color information is easy to obtain, for some historical soil literature, which do have color data, but lack the data of free iron contents, the Munsell color-based prediction model can be used to figure out an approximate content of free iron in the soil.

    • Soil Fauna Community Dynamics of Phyllostachys violascens Litter in Different Decomposition Environments

      2021, 58(5):1330-1340. DOI: 10.11766/trxb202008210182

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      Abstract:Soil fauna is an important component of soil ecosystems, which plays a significant role in the decomposition of biological remains, modification of soil properties, enhancement of material recycling, and energy conversion in soils. Forest fire is a common phenomenon disturbance on the terrestrial ecosystem which destroys the biology diversity above and under the forest floor. Soil fauna is also affected intensively by forest fire although it is often ignored. To understand the effects of Phyllostachys violascens after fire disturbance on soil fauna community characteristics, Phyllostachys violascens with fire disturbance were selected as the research object and Phyllostachys violascens without fire disturbance were used as a reference. Litter bags were used to study the characteristics of the soil fauna from May to November 2016. The litter bags were collected each month and stored in soil fauna sealing black bags, then, soil fauna was separated and collected from the litter bags by the Tullgren method. The results showed that the decomposition process occurred in three stages. The first stage represented the primary decomposition stage, in which the litter mass loss rate was higher. In the second or middle decomposition stage, the litter mass loss rate was slower compared to the first stage. Lastly, in the third stage or final decomposition stage, the litter mass loss rate increased again. Importantly, a total of 824 individuals of soil fauna, belonging to 34 orders were collected. Phytoseiidae, Formicidae, Nematodes, and Isotomidae were the dominant species and accounted for 49.27% of the collected total animals. The rangeability of soil fauna species was lower than the individual number. The results of statistical analysis showed that there was no significant difference between the burned plot and reference plot on the soil fauna species during litter decomposition. Also, the Pielou index, Shannon-Wiener index, and Margalef index increased as the decomposition continued while the Simpson index declined. There was a significant relationship between soil individual number, species number, and precipitation respectively. Furthermore, PCoA analysis results showed that fire disturbance was the main factor that affected litter decomposition during the primary stage while cumulative time controlled the final stage. The results of these experiments indicated that litter decomposition was delayed by the forest fire, but the decomposition rate and soil fauna community recovered with time.

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