Abstract:The NSFC proposals and grants of soil science in 2021 were analyzed by focusing on the three secondary application codes related with soil science, including D0701 Environmental Soil Science, D0709 Fundamental Soil Science, and D0710 Soil Erosion and Soil Fertility in Environmental Geoscience Discipline. Furthermore, the applications, acceptances, peer reviews, project recommendations, grant funding, research teams and project's scientific attributes for various types of NSFC projects were analyzed comprehensively in order to provide scientific guidance for future applications in the field of soil science.

Abstract:Extracellular polymeric substances (EPS) are high-molecular-weight natural polymers secreted by microorganisms into their surrounding environment. EPS establish the structural and functional integrity of biofilms, and are the key component that determines the physicochemical properties of biofilms. As the basis of bacterial life activities, EPS can provide an ideal environment for chemical reactions, nutrient capture, and resistance to environmental stress. EPS secreted by bacteria can benefit soil functions through its properties (such as adhesiveness, hydroscopicity, and complexation), e.g. increasing aggregate stability, enhancing water retention, and fixing heavy metals. Therefore, in-depth research on soil EPS is key to better management of biologically mediated nutrient turnover and soil health. To this end, this review first introduces the concept, components, and controlled factors of EPS, and especially emphasizes the interdisciplinary consistency of the term EPS. The paper then summarizes the soil ecological functions of bacterial EPS:1) Act as cells' protection layer, such as preventing cell desiccation, maintaining the stabilization of extracellular metabolism, and affecting the adhesion process of cells on the solid surface; 2) Regulate biological response, such as assisting the symbiosis between bacteria and plants, resisting the sterilization of antimicrobial compounds, and influencing genetic material transfer; 3) Alleviate abiotic stress, such as enhancing the water-holding capacity of soils, improving salt tolerance of organisms, and adapting microbes to extreme environments; and 4) Improve the overall functions of soil, such as driving soil particle aggregation, promoting soil aggregates stabilization, achieving the capture of nutrients and extracellular storage, holding heavy metals and reducing them by acting as electron transfer media and electron donors, and adsorbing organic pollutants and accelerating their biodegradation. Finally, we suggest future research opportunities of bacterial EPS in soil:a) Promote the specific extraction and accurate analysis of soil EPS; b) Clarify the relative contribution of EPS-protein and Glomalin-related soil proteins to soil aggregates stability; c) Evaluate the relative importance and correlation between soil EPS and other soil health indicators; d) Consider the benefits of bacterial EPS for the research and development of novel biofertilizers. This review calls for more scholars to pay attention to and study bacterial EPS and its functions in soil ecosystems, and to explore its potential application in the development of environmental-friendly agriculture.

Abstract:Soil-borne pathogenic bacteria threaten health of the soil-plant system and sustainable development of the agriculture. The soil ecosystem is complicated with tangled interactions of abiotic and biotic factors. Before contacting and invading the host plant root system, pathogenic bacteria often encounter sudden changes in abiotic factors, such as pH, oxygen content and types and contents of nutrients or biotic stresses, like competition, parasitism and predation of other soil microorganisms. Pathogenic bacteria have numerous capabilities, like biofilm formation, metabolism, movement, virulence, DNA repair, and resistance to bacteriophages, antibiotics, and other environmental stresses, which are essential for them to survive in the soil and infect host plants. To adapt to the complicated and variable soil biotic and abiotic environment, pathogenic bacteria have to trade off dynamically between survival and virulence so as to maintain the balance between survival, spread, proliferation and infection in soil and to maximize their adaptability in the soil environment. A systematic understanding of the processes and mechanisms of the survival-virulence trade-off of soil-borne pathogenic bacteria is the key to establishing efficient and precise ecological prevention and control strategies. To this end, this review first introduces the trade-off theory and survival-virulence trade-off of soil-borne pathogenic bacteria. Soil-borne pathogenic bacteria allocate more resources either on survival or virulence for adaption to for adaption to the environment. It is costly for pathogenic bacteria to resist stresses while producing virulent secretions. They have to reduce the production of virulent secretions to achieve the ability to resist stresses. Viable but non-cultivable status and phenotypic conversion are two typical phenomena of the survival-virulence trade-off in soil borne pathogenic bacteria to optimize the balance of viability and pathogenicity. In the second section, this paper elaborates on how abiotic and biotic factors influencing the survival-virulence trade-off of the pathogenic bacteria in the soil environment. Soil is a complex and constantly changing living habitat for bacteria, containing various stressful factors to threaten the existence of pathogenic bacteria. Harsh environmental factors can trigger resistant systems in the bacteria, which is rather costly, thus leading to a cut-down investment in the expression of virulent features. So this paper analyzed and summarized those phenomena from the perspective of mechanism. In the third section, this paper focused on this trade-off pattern in the process of pathogenic bacteria moving from bulk soil to root surface:migration from soil to rhizosphere, colonization and proliferation in rhizosphere and invasion to root surface, since the infection of host plants is a continuous and dynamic process varying temporally and spatially. The last section of this paper proposed several research topics relative to the survival-virulence trade-off theory:1) explore impacts of biotic and abiotic factors on survival and virulence characteristics of pathogenic bacteria; 2) enhance the understanding of the multi-level interactions in the rhizosphere micro-food web and the superimposition of biotic and abiotic factors on the survival-virulence trade-off in soil-borne pathogenic bacteria; 3) investigate the molecular mechanism of adaptive evolution of pathogenic bacteria. This paper calls for establishment of an ecological prevention and control strategy for soil-borne diseases based on the survival-virulence trade-off theory. Moreover, it is expected that this paper may provide certain theoretical reference for sustainable development of the green agriculture.

Abstract:Potassium-solubilizing bacteria (KSB) often attach onto the surface of minerals, and secrete acid substances capable of dissolving insoluble nutrients, such as potassium, phosphorus, and silicon from the minerals, facilitating plant nutrients uptake. The functioning potassium solubilizing mechanisms of KSB are key for soil quality improvement and sufficient nutrient utilization that are essential for sustainable agriculture. Recently, mechanistic understanding of the KSB functions and their potentials on sustainable agriculture has become a hot spot of focus of the study on soil microbiology both in domestic and abroad, and this trend is likely to continue in the near future. Numerous studies have attempted to address the biological and/or physiological mechanisms of potassium-solubilizing processes (e.g., acidolysis, enzymolysis and polysaccharides complex dissolution, etc.), and to identify key factors regulating their performance in both lab-scale and field-scale experimental systems. Nevertheless, microscopic mechanistic understanding of the functioning biophysical interactions between KSB and soil minerals remains largely unknown, and remarks a key for untangling the black-box potassium-solubilizing processes. This paper reviews recent advances on KSB diversity and specification, and their implication performance, focusing on biofilm formation processes onto potassium-bearing minerals at single-cell level and the driving forces, and their consequent functions on soil potassium utilization. Numerous advanced analysis technologies and cutting-edge methodologies devoting on KSB investigation are summarized.

Abstract:Ecological effects mainly refer to the damage to the environment caused by various anthropogenic activities. These usually cause structural and functional change in the ecosystem. With the increased use of plastics, plastic pollution has become a very important part of environmental pollution management. Their ecological effects and prevention have been the focus of pollution prevention in recent years. Plastics are difficult to degrade in the environment, and after the physical effects of weathering, they eventually form plastic particles less than 5 mm in diameter called microplastics. This has led to the ecological effects and degradation methods of microplastics receiving much attention in recent years. In addition to plastic waste pollution, the use of disposable plastic products, plastic film, and other plastic agricultural materials can also cause microplastic pollution in the soil. Microplastics in the soil will expand the pollution scope by lateral and vertical migration. Lateral migration mainly refers to the diffusion of microplastics in the surface layer of soil through wind and surface water, while vertical migration refers to the diffusion of microplastics to deeper layers of soil through soil organisms, water or various enrichment methods. The migration of microplastics increases the degree of microplastic pollution in soil and creates a great challenge for soil microplastic management. In this review, the ecological effects of microplastics are reviewed in terms of soil environment, soil microorganisms, plant, and food chain. The deposition of microplastics in plants and food chain is discussed, and the risk of microplastic enrichment along the food chain is analyzed. The ecological effects of soil microplastics come from three main sources:the main components of the plastic, additives in the plastic synthesis process, and the pollutants that the microplastic absorbs from the environment. Microplastics can directly change the physicochemical properties of soil, affect the function and structure of soil microbes, and accumulate in plants, thereby affecting their health. Also, microplastics can enter the animal and human bodies through diet, drinking water, and respiration. Additionally, soil microplastics can be dispersed into the air by way of dust lifting and thus be inhaled by animals at different levels of the food chain through respiration. Once inside the organism, microplastics may enter the circulatory system of the organism and thus accumulate in various parts of the organism. Microplastics have been found in many livestock products, but their entry channels into animals still need to be studied. Even though the content is very low, these microplastics may accumulate in the human body in large quantities through the food chain. The biodegradation methods of microplastics, especially the degradation mechanisms of fungi and bacteria are discussed in detail. Currently, studies have found that insects, bacteria and fungi in soil can degrade microplastics, and all of these organisms could also be good countermeasures to deal with soil microplastic pollution. Finally, based on the summary of the existing research on ecological effects and biodegradation of microplastics, the review analyzes and outlooks the future research directions and priorities of soil microplastics.

Abstract:Microplastics (MPs) refer to plastic debris with a dimension <5 mm and possess high chemical stability, small particle size, and strong mobility. Once they enter the soil environment, MPs can exhibit long-term retention, act as a vector for soil contaminants, and even pass through the food chain by plant enrichment, causing serious damage to the environment and human health. Due to the complexity of soil substrates and the limitations of analytical techniques, there is still a big gap in the study of soil MPs. Research on soil MPs analysis technology is the basis for uncovering the migration and transformation mechanism in soils and to evaluate the ecological risks of MPs. In this study, worldwide research progress on the separation, extraction, and identification of MPs in environmental samples is reviewed. Moreover, the advantages and disadvantages of these methods and their applicability to soil samples are discussed. Finally, the development direction of future research on the analysis technology is suggested. The most common separation method, density separation, is simple and effective for extracting MPs, but it cannot remove organic matter or separate plastic debris <50 μm. Though it may damage MPs structures, the newly developed pressure fluid extraction (PFE) still has good application prospects because of its low cost, high automation and efficiency. Other alternative methods (e.g. oil extraction and magnetic separation) are rarely used, and their applicability to soil samples remains unclear. Also, digestion methods with different intensity are reported to cause different degrees of damage to the MPs structures, and the enhancement of organic matter digestion efficiency is usually at the cost of MPs recovery. The existing identification and quantification methods include (i) visual identification methods with the aid of a microscope, (ii) FTIR and Raman based spectral analysis methods, and (iii) the MS or chromatography coupled thermal analysis methods. When applied to soil MPs identification, these methods exhibited shortcomings such as time consumption, size and amount limitations of MPs sample, and damage to MPs structures. The combination of different technologies is expected to address these shortcomings. Importantly, due to the selection biases of researchers on available separation and analysis methods, the results of different studies were difficult to be compared horizontally. Considering the existing deficiencies in current analytical methods, the future research focus should be (1) to establish standard methods for soil MPs extraction, identification, and quantification; (2) to explore suitable analytical methods for small MPs (<50 μm); (3) to develop separation/identification/quantification methods that do not damage MPs structures and are less time-consuming.

Abstract:[Objective] Soil organic carbon mineralization is an important process of terrestrial carbon pool turnover, and its subtle changes will cause drastic fluctuations in the global carbon pool. This study aims to highlight the current status and development trend of international soil organic carbon mineralization research and to provide theoretical groundwork for the dynamic changes of global carbon pools.[Method] A total of 1 045 publications related to soil organic carbon mineralization; published from 1982 to 2020, were obtained from the Science Citation Index-Expanded database (SCIE). Then the Cite Space, VOS viewer, and GIS were used to sort out and visualize high-productive countries, research institutions, and researchers based on indicators such as the number of papers, total citation, average citation of articles, and total link strength.[Result] The number of papers related to soil organic carbon mineralization have been increasing since 1982; especially between 1993 to 2019, and there is potential for continuous growth. As evident, research on soil organic carbon mineralization was very popular in the US with the largest number of publications and very high Betweenness Centrality. The annual average publications on soil organic carbon mineralization in China were higher than those of the US after 2016. However, the average cited times were very low due to the lack of innovation. Therefore, there is a call to strengthen the innovation and international influence of scientific research in this field. The Chinese Academy of Sciences had the largest number of literatures for all research institutions. However, both France and Germany demonstrated the highest Betweenness Centrality, which means that they commanded the greatest influence in the world. The research on soil organic carbon mineralization was carried out based on the integration of knowledge between different disciplines and fields. It was observed that most of the existing studies focused on (i) the effect of soil types, environmental factors, and land use types on the amount and temporal and spatial distribution characteristics of soil organic carbon mineralization, and (ii) the global carbon mineralization and turnover.[Conclusion] Despite the growing trend in the literature, more studies that explore the internal mechanism and model of soil organic carbon mineralization under different environmental factors and land use conditions are needed. These studies should provide accurate parameters and ways for global carbon storage, turnover, and cycle models. Meanwhile, it is also necessary to initiate more studies related to priming effects, soil erosion, and exogenous carbon/nitrogen additions.

Abstract:Lakes in arid areas play an important role in keeping the balance between ecology and environment of the fragile ecosystems therein. Unfortunately, the lakes in the arid regions of China are shrinking, drying up or even disappearing, due to changing environment and human's abusive use of water and soil resources.[Objective] With the lake drying up, ground water table keeps on going down in depth and aeolian sediments build up, and consequently soil forming process begins to change. However, so far little has been reported in the literature about formation and development of sediment-derived soils in such an environment.[Method] In this study, both pedological and geochemical methods were applied to investigation of characteristics and mechanisms of salt and calcium carbonate accumulating in sediment-derived soils in the Gurinai Playa.[Result] Results show that the accumulation of various kinds of salts was the primary pedogenic process. Soil salt content, Na⁺ concentration and sodium absorption ratio in the surface soil layer of the sediment-derived soils in the Gurinai Playa reached 13.15~650.50 g·kg^-1, 186.9~12 114.7 mmol·L^-1 and 22.3~890.5(mmol·L^-1)^1/2, respectively, which suggest that the studied soils experienced strong salinization. In addition, with the Gurinai Playa drying up the accumulation of salts in the soils evolved from modern salinization to residual salinization, and soil salt composition gradually changed from Cl^-, and Cl^--SO₄^2- types in the initial stage of the soil evolution to SO₄^2--Cl^-, and SO₄^2- types in the more advanced stages. Given that Sr and Ca are quite similar in geochemical behavior, the analysis of composition of Sr isotopes demonstrates that secondary CaCO₃ predominated in soil total CaCO₃ accounting for over 80%. Moreover, soil CaCO₃ in this area was primarily derived from calcareous sediments and groundwater. The continuous provision of Ca²⁺ by groundwater led to strong accumulation of CaCO₃and formation of Calcicpan.[Conclusion] Interpretation of the accumulation of salts and CaCO₃ in soils during the drying progress of the Gurinai Playa provides a scientific basis for evaluating soil formation and evolution in relation to environmental changes, and for predicting changes in soils and ecological environment of the terminal lake areas of inland rivers.

Abstract:[Objective] The upper section of the Hanjiang River flows along the edge of the northern subtropical monsoon climate zone and the catchment is extremely sensitive to climate change. A special landform of asymmetric four-level terraces has been developed in the catchment, and the terraces are covered with loess deposits varying in thickness. In this study, two profiles, located in HPC (Huangpingcun) and GKZ (Gaokezhan) on the third-order terrace in the upper reaches of the Hanjiang River were selected as the research objects.[Method] Soil samples were collected from the two profiles for determination of magnetic susceptibility and frequency susceptibility, and furthermore, some representative samples were tested for measurement of thermomagnetic curves, IRM acquisition curves, and hysteresis loops.[Result] Results suggest that the main magnetic minerals in the paleosol and loess of the terrace are magnetite and maghemite, and a certain amount of hematite and goethite. Superparamagnetic and pseudo-single domain ferrimagnetic minerals are the major contributors of enhanced susceptibility. Profile GKZ is lower than Profile HPC in content of low-coercivity ferromagnetic minerals, and contains more ferromagnetic minerals in loess. But in Profile HPC section, little difference in content of magnetic minerals was found between loess and paleosol.[Conclusion] The lower magnetic susceptibility in the upper reaches of the Hanjiang River is related to the high effective precipitation in this area. Seasonal abundant precipitation makes soil moisture supersaturated in the area. In the locally humid reducing environment, a large volume of strongly ferromagnetic iron oxides in the soil are dissolved and transformed into weakly magnetic minerals that persist stably in humid environments. The transformation of magnetic minerals leads to lower magnetic susceptibility in the upper reaches of Hanjiang River strong in soil formation than in the Loess Plateau weak in soil formation. When the soil forming condition remains to be a reducing environment, the higher the effective precipitation and the reduction degree, the more the strongly magnetic iron oxide dissolved and the lower the magnetic susceptibility, which is the reason why GKZ is higher in precipitation larger than HPC section, but lower in magnetic susceptibility.

Abstract:[Objective] Interrill erosion is a complex process triggered jointly by a number of influencing factors, and the effects of each factor vary with changes in other factors.[Method] Based on the data published and available in the literature and multiple linear regression analysis, the effects of soil type, rainfall intensity, slope gradient and slope length on interrill erosion process and the interactions were quantitatively analyzed.[Result] Results show that rain intensity, slope gradient, interrill erodibility and slope length were the main factors contributing to interrill erosion rate positively, and among the factors, rain intensity was the major one, contributing (62.93%). Analysis of the interactions between these factors shows that the increase in slope length promoted the contribution of the factor of slope gradient, but inhibited the contribution of the factor of rain intensity. The increase in slope gradient promoted first and then inhibited the contribution of the factor of slope length, but did reversely the contribution of the factor of rain intensity. The slope gradient of 20% was the inflection point of the interaction between slope length and rain intensity, changing from positive to negative. The increase in rainfall intensity interacted positively with slope length and slope gradient. However, slope gradient was more dependent on rainfall intensity than slope length.[Conclusion] During the process of interrill erosion, the interactions observed between the factors of soil type, rainfall intensity, slope gradient and slope length were either positive or negative. All the findings in this study are of great significance for revealing mechanism of rill erosion and predicting soil erosion on slopes.

Abstract:[Objective] The black soil region of Northeast China is an important commodity grain production base in China. The long-term high-intensity utilization and unplanned agricultural cultivation measures led to serious soil fertility decline, grain yield reduction, and soil erosion in some areas. Soil erosion and land degradation in the black soil region seriously restricted the development of the agricultural economy in Northeast China. This threatened national food production and security. Ridge tillage is the most common cultivation method in the black soil region of Northeast China. However, the long-term effects of different ridge direction on slope erosion and deposition has never been evaluated. Therefore, understanding soil erosion and deposition on slopes with different ridge directions is of great significance for the development and deployment of soil and water conservation measures and the control of soil loss in the black soil area of Northeast China.[Method] ¹³⁷Cs is an artificial radionuclide produced by atmospheric nuclear test and nuclear leakage. Its half-life is 30.2 years. The nuclide is strongly adsorbed by clay particles in soils after settling to the surface, and it is difficult to be leached by water. This implies that it only moves mechanically with soil particles. Therefore, it is regarded as a good tracer and widely used in the study of soil erosion rate. In this study, ¹³⁷Cs tracer technology combined with wavelet analysis method was used to study the spatial variation of soil erosion and deposition rate on slopes with different ridge planting directions.[Result] (1) The average concentration of ¹³⁷Cs on slopes with horizontal and longitudinal ridges was 1 802 Bq·m^-2 and 1 770 Bq·m^-2, respectively, which were lower than the local background value of 2 417 Bq·m^-2. (2) The slope erosion and deposition rate were between -700 to 2 662 t·km^-2·a^-1 with an average of 1 341 t·km^-2·a^-1 on slopes with horizontal ridges, and that on slopes with longitudinal ridges was between -726 to 3 327 t·km^-2·a^-1 with an average of 1 466 t·km^-2·a^-1. These results showed that the average erosion rate of slopes with horizontal ridges was lower than that with longitudinal ridges. (3) Different periodic change of erosion and deposition on 140 m long slopes were observed. There was a 45 m main period and a 75 m secondary period of erosion and deposition on slopes with longitudinal ridges, while only a 45 m main period was observed on slopes with horizontal ridges.[Conclusion] The periodic change of erosion and deposition on slope surface is mainly caused by the change of transportation capacity of rainfall-runoff and snowmelt runoff along the slope. Different degrees of soil erosion occurred on slopes with different ridge directions, and the slopes with horizontal ridges showed less soil loss than those with longitudinal ridges. Although soil erosion in the study area was mild, its average soil erosion rate is far greater than the allowable soil loss (200 t·km^-2·a^-1)in the black soil region of China. Thus, there is an urgent need for comprehensive control of soil erosion in this area. This study could provide a scientific and theoretical basis for the precise layout of soil and water conservation measures such as; (i) reinforcement of ridge platform and (ii) arrangement of contour plant belt on strong erosion positions of slopes. This could prevent the destruction of ridges by runoff and effectively reduce slope soil erosion.

Abstract:[Objective] The Huang-Huai-Hai plain is one of the most important agricultural production bases in China. Nevertheless, there exists very little information on the current status of soil organic matter content in cultivated land across this area or on the in-depth analysis of spatial distribution characteristics for regional soil organic matter management.[Method] Based on the spatial autocorrelation theory and soil property data from 267 sample sites, this study investigated the spatial distribution structures of soil organic matter content in dry farmlands in the Huang-Huai-Hai plain and analyzed the spatial autocorrelations between organic matter and soil clay particle content/soil bulk density.[Result] The results showed that the regional average values of soil organic matter contents of dry farmlands in the Huang-Huai-Hai plain were 20.11±6.46(0~10 cm), 14.76±5.11(10~20 cm), 9.96±4.14(20~30 cm), and 8.03±3.45(30~40 cm)g·kg^-1, which corresponded to levels 3 to 5. In the soil layers between 0~10 cm and 10~20 cm, the high-value areas were mainly distributed in the traditional agricultural production areas such as Piedmont plain of Taihang Mountain, and Yellow River Irrigation Area in Shandong, and lime concretion black soil areas in southern Henan and northern Anhui also included. As the depth of the soil layer increased, the spatial distribution structure of soil organic matter content became obvious. In the soil layers between 10~20 cm, 20~30 cm, and 30~40 cm, the Hebei plain and Northwestern Shandong showed LL(Low-Low)and HL(High-Low)distribution characteristics while Henan and Anhui regions mainly demonstrated HH(High-High)and LH(Low-High)type distributions. The specific distribution of spatial autocorrelation types in the different soil layers was different. Also, the mathematical relationship between the contents of soil organic matter and clay particles was significant(P<0.05), but there was no significant difference in the relationship between organic matter content and soil bulk density. Among the two-variable local spatial autocorrelation types; organic matter/clay particle content and organic matter/bulk density, (i)the LL/HL types were mainly distributed in Hebei plain and Northwestern Shandong region; (ii)the HL type was concentrated in the Piedmont plain of Taihang Mountain and Yellow River Irrigation Area in Shandong for 0~10 cm and 10~20 cm soil layers; and(iii)the HH/LH types were mainly distributed in Henan and Anhui, with a distinct distribution in the different soil layers.[Conclusion] Soil organic matter content in dry farmlands in the Huang-Huai-Hai plain showed apparent spatial autocorrelation characteristics. The spatial autocorrelation structure between soil organic matter content and clay content was more distinct. Importantly, the spatial autocorrelation structure in 0~10 cm and 10~20 cm soil layers was more obvious than that in 20~30 cm and 30~40 cm soil layers. The observed dissimilarities in distribution structure in the spatial horizontal and vertical soil layering directions were attributed to regional differences in agricultural management.

Abstract:[Objective] Soil total nitrogen is closely related to soil quality and fertility. It is of great significance to know the spatial distribution characteristics of soil total nitrogen for the implementation of precision agriculture management.[Method] The spatial distribution of total nitrogen in the surface soil of Xunwu County was predicted and mapped by using two methods:random forest and random forest plus residuals kriging. These methods were combined with multi-source auxiliary variables such as (i) terrain factors, (ii) geographical coordinate, (iii) remote sensing factors, (iv) climate factors, (v) distance factors, and (vi) soil physical or chemical factors. Also, the prediction accuracy of the two models was compared after 100 times of repeated operation.[Result] Our results show that the mean values of the decision coefficient (R²=0.6291) and concordance correlation coefficient (CCC=0.7613) of the random forest model were higher than those of the random forest plus residual kriging method (R²=0.5719, CCC=0.6881). Also, the mean values of the mean absolute error (MAE=0.1570 g·kg^-1) and root mean squared error (RMSE=0.2108 g·kg^-1) were lower than those of the random forest plus residual kriging method (MAE=0.1682 g·kg^-1, RMSE=0.2267 g·kg^-1).[Conclusion] Importantly, adding residual to the random forest model did not improve its accuracy. These results suggest that the random forest model can be used as a new method for predicting soil properties, and it provides technical support for the implementation of agricultural management.

Abstract:[Objective] Soil is an important carrier of cultivated land and the basis of agro-ecosystems. Farmland soil health not only affects crop production and food quality, but also regulates local climate and water environment quality, and sustains biological safety. Therefore, the assessment of farmland soil health should take into account both soil productivity and eco-environment effects. As a living ecosystem, soil varies in health status region, time and management practice. A reasonable assessment index system and a set of evaluation methods are the premise and foundation of soil health assessment. The aim of this study was to gain a deeper understanding of the connotations of soil function and soil health, and to explore the actual state of farmland soil health in China.[Method] In this paper, the Fengqiu, Luancheng and Yucheng ecological experiment stations in the Huang-Huai-Hai Plain, the most important grain producing area in China, were selected as objects for the study. Soil functions were generalized into five groups:crop production, water holding and purification, nutrient transport and buffering, carbon sequestration, and habitat and diversity. By referring to the German Müncheberg index scheme, evaluation indices were divided into basic ones and hazard ones. Out of each function, basic indices were selected in the light of their attributes, inherent or dynamic, separately, and got combined with region-specific restricting indices that might threaten soil functions to form a soil health assessment index system for agro-ecosystems. Among basic indices, the inherent attributes were classified or selected from climate, topography, hydrology and soil conditions, and the dynamic attributes were divided into three aspects including physical, chemical and biological indices. On considering that the restricting indices impact their inherent and dynamic properties to varying extent, two multipliers were defined for the two separately. In the end, based on the established soil health assessment index system, soil health of the three typical agro-ecosystems were assessed using the gray correlation analysis method.[Result] Results show:(1) The three agricultural ecological experiment stations all gained quite high soil health scores in the assessment, but still differed slightly, showing an order of Yucheng > Luangcheng > Fengqiu; (2) Farmland management methods also affect soil health, so the farmlands with no fertilizer applied were certainly lower than fertilized ones in productivity; in the Yucheng station, No.2 auxiliary plot applied with fertilizer and straw was the highest in soil health score among all the comprehensive and auxiliary observation fields; and (3) R² between the crop production function and the maize yield reached 0.60.[Conclusion] (1) On the whole, the typical farmlands of the Huang-Huai-Hai Plain are fairly good in soil health, displaying an order of Yucheng > Luancheng > Fengqiu. (2) By referring to the Müncheberg index scheme, the revised soil health assessment framework has assigned different multipliers to the inherent and dynamic properties, which proves to be quite reasonable. So the multi-functionality-based soil health assessment system may serve as reference for further exploring trends of long-term evolution of soil health, and implementing effective management of soil resources.

Abstract:[Objective] Soil aggregate size distribution evolves due to soil aggregation and breakdown processes that occur under natural and anthropogenic impact. A change in aggregate size distribution can alter soil pore structure, which might also influence soil water transportation, gas exchange and microbial activities. Thus, this may potentially affect soil organic carbon (SOC) mineralization. Therefore, this study aimed to investigate the effect of aggregate size distribution on soil pore structure and the SOC mineralization.[Method] Soil samples were collected from a long-term field experimental site with treatments receiving different amount of pig manure (No manure, CK; Low manure, LM; High manure, HM). The soil samples were passed through sieves with 5.0(S₅), 2.0(S₂), or 0.5 mm(S_0.5) openings to create different aggregate size distributions, with the maximum aggregate sizes corresponding to 5.0, 2.0 and 0.5 mm, respectively. The sieved aggregates were repacked into soil columns (diameter 2.9 cm, height 4 cm) with a bulk density of 1.3 g·cm^-3. Soil columns were incubated for 57 days and SOC mineralization was measured during this period. Also, the soil pore structure was quantified using X-ray micro-computed tomography (μ-CT) imaging.[Result] Sieving altered soil aggregate size distribution, which resulted in significantly different soil pore structure in the repacked columns. Compared to S₅ and S₂ treatments, the S_0.5 treatment significantly decreased the image-based porosity (>16 μm) by 83.0%~93.9% and pore connectivity by more than 95%. The differences in macroporosity and pore connectivity between S₅ and S₂ treatmentwere significant for the HM soil, but not for the CK and LM soil. Also, aggregate size distribution had a significant effect on SOC mineralization. The breakdown of aggregates increased SOC mineralization. The cumulative SOC mineralization amount of S_0.5 treatment was 64.2%~79.1% and 14.1%~19.3% higher than that of S₅ and S₂ treatments for the CK and HM soils, respectively. However, there was no significant difference in the cumulative SOC mineralization between the S_0.5 and S₅ treatment in the LM soil. The correlation analysis indicated that the cumulative SOC mineralization amount was negatively correlated with the porosity of 16~30 μm pores.[Conclusion] Sieving broke down large aggregates and decreased soil macroporosity in the repacked soil columns. The breakdown of soil aggregates promoted SOC mineralization partially due to the release of the protected SOC. The change of aggregate size distribution and the resulting alteration of pore structure also correlated with SOC mineralization. This study can serve as a reference for future research related to SOC mineralization and the effect of soil aggregation on this process.

Abstract:[Objective] Soil macropores in oasis croplands account for a low ratio of the total soil porosity, but significantly influence the path and process of soil water infiltration. Therefore, it is necessary to explore or characterize soil macropores and their effects on the water infiltration process in arid regions.[Method] In this study, we selected three types of oasis croplands(piedmont oasis cropland, old oasis cropland, and marginal oasis cropland), using in-situ dye tracer experiment and a helical medical X-ray computed tomography (CT), the characteristics of soil macropores and water infiltration in the three oasis croplands were quantitatively investigated.[Result] The results showed that soil macropores in the oasis croplands were mainly concentrated in the 0-20 cm soil layer. The three-dimensional macropore structure in the old oasis cropland was more complex than those of piedmont oasis and marginal oasis croplands. Also, the soil macroporosity, fractal dimension, branch density, junction density, and connectivity density in the old oasis cropland were significantly (P<0.05) higher than those of piedmont oasis and marginal oasis. Besides, there was a significant (P<0.05) difference in the steady infiltration rates between the three types of oasis croplands. The steady infiltration rates were of the order marginal oasis cropland (0.48 mm ·min^-1) > old oasis cropland (0.28 mm·min^-1) > piedmont oasis cropland (0.16 mm·min^-1). The dyed area of water infiltration in croplands decreased with the depth of the soil layer. Also, the maximum infiltration depth and preferential flow length index in old oasis cropland were significantly (P<0.05) higher than that in piedmont oasis and marginal oasis croplands, and the degree of preferential flow was also the highest.[Conclusion] In oasis croplands of arid regions, the characteristics of soil macropores are the key factors affecting preferential flow. The oasis soils with high macroporosity and connectivity are more likely to produce preferential flow, resulting in soil water moving to the deep soil layer. It was observed that old oasis cropland had higher macroporosity and pore connectivity than the other oasis croplands, and demonstrated the most obvious degree of preferential flow. This study provides an in-situ measurement technique and database for the study of preferential flow in arid regions.

Abstract:[Objective] Hofmeister effects can be seen everywhere, and usually manifested as difference in behavior of covalent ions in solution. Hofmeister effects exist universally in physical, chemical and biological processes, playing an important role in interface interaction. Also, Hofmeister effects are of great scientific significance to ion exchange/adsorption process. The purpose of this paper is to compare Hofmeister effects on the adsorption processes of three kinds of alkali metal cations (Na⁺, K⁺, Cs⁺) on saturated surface of Cu²⁺montmorillonite and to analyze sources of the Hofmeister effects.[Method] An ion adsorption kinetics experiment was carried out using the constant current method to compare the three alkali metal cations (Na⁺, K⁺, Cs⁺) in adsorption on Cu²⁺-montmorillonite surface. The experiment was designed to have the temperature fixed at 298 K, and the binary mixed electrolyte solution composed of XNO₃ + LiNO₃, where X stands for Cs, Na or K. The two electrolytes in the mixed solution was equal in concentration, 0.0001, 0.001 or 0.01 mol·L^-1. In the end, ion-surface Hofmeister energies and apparent charge coefficients were worked out in line with the Gouy-Chapman theory.[Result] (1) The adsorption of Na⁺, K⁺ and Cs⁺ on the surface of Cu²⁺-montmontmoillonite showed a first-order kinetic process under a weak electrostatic force, with average adsorption rate and equilibrium adsorption demonstrating obvious Hofmeister effects in an order of Na⁺ < K⁺ < Cs⁺. For example, when the electrolyte was 0.01 mol·L^-1 in concentration, the average adsorption rate of Na⁺, K⁺ and Cs⁺ on the surface of Cu²⁺-montmorillonite reached 4.48, 8.68 and 11.74 mmol·kg^-1·min^-1 respectively, and the equilibrium adsorption did 129.17, 341.11 and 911.47 mmol·kg^-1respectively in amount within 27 minutes. (2) Hofmeister energy increased with decreasing electrolyte concentration. On the surface of Cs⁺-clay mineral, Hofmeister energy was the main contributor of the interaction, while on the surface of K⁺/Na⁺-clay mineral, classical Coulomb energy was. When the electrolyte concentration was 0.0001, 0.001 and 0.01 mol·L^-1, the ratio of w_Hi (0)/w_Ti (0) of Cs⁺ was 67.00, 65.71 and 58.00%; that of K⁺ 44.24, 43.36 and 42.19%; and that of Na⁺17.88, 9.29 and 8.82%, respectively. (3) Effective charge coefficient decreased with increasing electrolyte concentration. This is because increased electrolyte concentration compresses the electric double layer, thus enhancing its capability of shielding the electric field, and leading to decrease of the electric field in the diffusion layer in intensity, which eventually weakens polarization of the adsorbed ions and reduces effective charge coefficients. The non-classical polarization significantly amplified apparent charge coefficient of the ions. The apparent charge coefficients of Cs⁺, K⁺, Na⁺ worked out in this experiment under the condition of the electrolyte concentration being 0.0001, 0.001, 0.01 mol·L^-1 followed the sequence of Cs⁺ > K⁺ > Na⁺ in Hofmeister effect. Apparent charge coefficient of Na⁺, K⁺ and Cs⁺ increased from the standard +1 value to +1.14, +1.76 and +2.78, respectively.[Conclusion] This study clearly demonstrates that the Hofmeister effects originate mainly from the difference in ion-surface interaction energy between the ions caused by their non-classical polarization. And all the findings in this study provides a new idea for elucidating the mechanisms of interface reactions in such a unique system as soil.

Abstract:[Objective] pH buffering capacity (pHBC) highlights the capacity of a soil to resist changes in soil solution pH as the concentration of protons (H⁺) changes. The most widely used method for determining soil pHBC is acid-base titration. The soil pHBC can be calculated from a buffering curve showing the relationship between H⁺ concentration and soil solution pH. Nevertheless, the fitting method of titration data has a penetrating influence on the result of soil pHBC and needs to be investigated.[Method] In this study, acid-base titration curve was adopted to measure the pHBC of 38 acid purple soils. Four fitting methods (polynomial curve fitting of all data (PA), linear fitting of data between two inflection points (LT), polynomial curve fitting of acid titration data (PAT), and linear fitting of acid titration data (LAT)) were used to fit the soil acid-base titration data and the pHBC estimated from the different fitting methods.[Result] The pHBC obtained by the different fitting methods was different. Also, the different fitting methods produced different R² for the fitting equation. The R² obtained by LAT was lower than that of the other three fitting methods, which result in lower accuracy of the soil pHBC calculated by linear fitting of acid-base titration data. More so, PA produced the best fit for the acid-base titration data. The correlation between soil pHBC calculated by the different fitting methods and soil exchangeable cations(r₁) or effective cation exchange capacity (ECEC, r₂) was as follows:PA(r₁=0.486^**, r₂=0.525^**) > LT(r₁=0.223, r₂=0.245) > PAT(r₁=0.183, r₂=0.220)> LAT(r₁=-0.219, r₂=0.002). There was a significant positive relationship between soil pHBC calculated by the PA method and soil exchangeable cations or ECEC. This revealed that the pH buffering ability of the tested soils at current pH value was mainly controlled by exchangeable cations and weathering of silicate.[Conclusion] Given the importance of soil pHBC, an accurate method for its measurement is essential. Thus, estimating pHBC of soils by fitting the acid-base titration data using PA is a reliable method.

Abstract:[Objective] Rare earths are resources of great strategic significance for national economy. With rapid development of the science and technology, the demand for rare earths is increasing dramatically. However, the environmental pollution caused by mining of rare earth ores has aroused concerns of related countries. In South China, in mining ionic rare earth ores, the technology of in-situ leaching with a large volume of (NH₄)₂SO₄ is commonly used, causing severe chemical pollution of the soil and water environments in the mining areas, especially the pollution of ammonium nitrogen residue in ore tailings. However, the mechanism of ammonium nitrogen changing along with the depth in soil profiles (below 200 cm) of the soil in the rare earth mining areas is still unknown.[Method] Here in this study, a total of 157 soil samples were collected systematically from the upper and down layers of the soil in a hill of ore tailings typical of the Zudong mining area, in South Jiangxi. The mining operation started in 2003 and ended in 2007. The soil samples were analyzed for soil mass moisture content, pH, and ammonium nitrogen content.[Result] Results show:1) serious soil acidification and high concentration of ammonium nitrogen residue were the main environmental pollution problems stemming from the mining activities; 2) after the mining had been closed for 12 years, the mean soil pH followed a decreasing order of raw ore (5.73±0.17) > lower reaches (4.87±0.26) > washing tailing (4.63±0.16) > tailing (3.87±0.32); and the content of soil ammonium nitrogen was 60-204.3 mg·kg^-1, about 12-40 times the background value of the virgin soil; 3)The high concentration of soil ammonium nitrogen has posed great environmental risk; and 4)as soil pH and soil water content are two main factors affecting adsorption and desorption of ammonium nitrogen, in-situ washing is an effective method to remove residual ammonium nitrogen. The higher the soil pH and the more the water flowing through the tailings, the more ammonium nitrogen desorbed from the soil.[Conclusion] All the findings in this study are expected to be able to serve as technical support for exploring mechanism of ammonium nitrogen migration in tailings and management of the pollution. However, up to now, little has been reported about soil conditions affecting ammonium nitrogen migration and transformation in the literature. Further study should be done on mechanisms of interactions between soil texture, particle size gradation, porosity, and permeability in the tailing soil.

Abstract:[Objective] Wuyi rock tea is a kind of traditional brand high-quality oolong tea. However, its quality varies from region to region significantly. This regional difference is mainly attributed to two major factors. One is soil parent material that affects soil nutrient supply, and the other is climate that determines temperature, light intensity, humidity, etc. To date, the focus of this study is to find out the determinants of quality formation of Wuyi rock tea. The purpose of this study was to explore difference in mineral nutrition of Wuyi rock teas from different planting areas, so as to better understand contributions of the nutrients to the quality of Wuyi rock tea.[Method] In this study, tea leaf samples and soil samples were collected from 112 tea gardens in the Wuyi Mountain. The tea gardens were distributed in three planting areas, i.e. Zhengyan, Banyan and Zhou, producing teas, high, medium and low in quality, respectively. The tea leaf samples were analyzed for contents of 12 mineral elements(N, P, K, Ca, Mg, Fe, Mn, Cu, Zn, B, Mo, Ni)and the soil samples were for 5 soil fertility parameters(pH, Alkaline nitrogen, Available phosphorus, Available potassium and Organic matter).[Result] Principal component analysis shows that the tea leaf samples varied evidently in content of mineral elements with the planting area. Random forest analysis shows that among all the mineral nutrients in the tea leaf samples tested, phosphorus, copper and nitrogen differed the most significantly from region to region. Furthermore, in terms of concentrations of the three elements the teas followed a decreasing order of Zhou tea > Banyan tea > Zhengyan tea, while in terms of tea quality, they followed an increasing order of Zhou tea < Banyan tea < Zhengyan tea, which indicates the lower the concentrations of phosphorus, copper and nitrogen, the higher the tea quality. Pearson correlation analysis shows that the 5 soil fertility parameters did not have much correlations with the three nutrient elements in the tea leaves, suggesting that contents of the nutrients in Wuyi rock tea are affected by comprehensive environmental factors, rather than solely determined by soil nutrient status.[Conclusion] To sum up, this study demonstrates that content of phosphorus, copper and nitrogen in tea leaves is somewhat negatively related to quality of Wuyi rock tea, which serve as reference for soil nutrient management of tea gardens.

Abstract:[Objective] Tomato wilt is a kind of destructive soil-borne disease caused by Ralstonia solanacearum. In controlling the disease, rhizosphere probiotic play an important role. They reduce pathogen density and dull disease dynamics via resource competition and competitive exclusion. Their effective colonization in the rhizosphere is the precondition of their wilt controlling effect. As a porous material with adsorptive properties, biochar is used as a promising strategy to improve soil structure and fertility. Moreover, biochar has a great potential to inhibit soil-borne diseases, by transferring root exudates off rhizosphere and providing shelter for probiotics. To improve biocontrol efficiency of the probiotics, it is proposed that biochar is used as a carrier to improve their colonization in rhizosphere.[Method] In this study, wilt controlling effect of the application of biochar and probiotics in combination was evaluated and mechanisms underlying the potential were explored. To the end, a pot experiment was conducted to test three different kinds of biochar (derived from maize straw, pine wood and rice husk, separately) applied together with probiotic Bacillus amyloliquefaciens T-5 in controlling tomato bacterial wilt. And then an in-lab experiment was carried out too to measure efficiencies of the biochars adsorbing of pathogens and tomato root exudates in vitro. In the end, resource competition and direct toxin production of Bacillus amyloliquefaciens T-5 and their effects on pathogens were determined with simulated root exudates.[Result] Among the three kinds of biochar, the one derived from wood biochar is the highest in specific surface area (SSA) and absorption capacity, reaching up to 395.88 mm² and 116.4 mg·g^-1, respectively. The pot experiment showed that the application of biochar, regardless of kind, significantly reduced the incidence of bacterial wilt and pathogen density in rhizosphere. Wood biochar performed the best. When applied alone it decreased the incidence of bacterial wilt by 60.56%, and when applied in combination with probiotic Bacillus amyloliquefaciens T-5, it reduced pathogen density by about 97.42%, while increasing probiotic colonization by about 5.71 times. In exploring mechanisms of such potentials, it was found that 1) Biochar effectively adsorbed pathogen R. solanacearum, and wood biochar was the highest in adsorption capacity, reaching up to 90.00%, and fixing the adsorbed R. solanacearum by 94.66%; 2) Biochars absorbed root exudates, which were used by probiotic T-5 as carbon source for growth, thus inhibiting the growth of R. solanacearum.[Conclusion] As a carrier of probiotics, biochar, once applied together with probiotics can significantly decrease incidence of bacterial wilt by adsorbing root exudates probiotics relay as nutrient resources for growth and restricting the mobile ability of pathogen. The findings in this paper explain the mechanism of combined application of biochar and biocontrol bacteria controlling soil-borne tomato bacterial wilt and may serve as a theoretical basis for developing an environment-friendly, high efficiency and stable biocontrol strategy.

Abstract:[Objective] The use of organic manure as alternative for chemical fertilizer has been recommended as an effective method to mitigate impacts of chemical fertilizer on the agricultural system and improve soil fertility. However, how soil microbial communities respond to the organic input following fertilizer reduction remains poorly understood.[Method] In the current study, the technique of Illumina MiSeq sequencing combined with the construction of molecular ecological networks was used to investigate community composition and structure of the molecular ecological network of the soil bacteria and fungi in the field of fluvo-aquic soil under a wheat-maize rotation system. Furthermore, correlation networks between soil organic manure (SOM) content and bacterial and fungal communities were constructed. The fertilization regimes were designed as follows:conventional chemical fertilizer (NPK), reduced chemical fertilizer (NPKR), chemical fertilizer partly substituted with straw (NPKRS), chemical fertilizer partly substituted with organic manure (NPKRO), chemical fertilizer partly substituted with organic manure and straw (NPKROS).[Result] Results show that chemical fertilizer reduction and organic substitution significantly increased the content of soil SOM. In the case of bacterial community ecological network, compared with Treatment NPK, the treatments of chemical fertilizer reduction and organic substitution increased the numbers of network nodes and edges, but reduced the average path length. Treatments NPKRS and NPKROS were higher than the other three treatments in average clustering coefficient and the network density. With respective to fungal ecological network, Treatment NPKR increased the numbers of network nodes and edges, while the organic substitution treatments increased the average path length and reduced the average clustering coefficient, as compared to Treatment NPK. Treatment NPKR was the highest in network density among all treatments, but the lowest in modularity. Compared with Treatment NPK, all the other treatments increased the proportion of bacterial flora that were significantly and positively related to soil SOM content, but decreased the proportion of fungal flora that were significantly and negatively related to soil SOM content. Some flora, like Proteobacteria, Chloroflexi, Actinobacteria, Gemmatimonadetes, Bacteroidetes, Firmicutes and Acidobacteria, play an important role in the soil bacterial molecular ecological network, while some, like Ascomycota, Basidiomycota and Glomeromycota, do a significant one in the soil fungal molecular ecological network.[Conclusion] In conclusion, partial substitution of chemical fertilizer with organic manure or material increases the bacterial molecular ecological network in size, while simultaneously improving the transferring efficiency of substances, energy and information among species. The substitution with straw enhances interaction between bacterial communities and makes the bacterial community more sensitive to disturbance of the external environmental factors. The reduction of chemical fertilizer increases the fungal molecular ecological network in size and community interaction, and the substitution with organic manure or material improves stability of the fungal community structure.

Abstract:[Objective] Ammonia oxidation is the limited step of nitrification and it is the main microbial pathway of N₂O emissions in aerobic soils. Grassland soils are sinks for CH₄and sources of N₂O. However, the mechanisms of the interactions between methane and ammonia oxidation under various N levels in grassland soils remain elusive.[Method] A microcosm experiment was implemented using Inner Mongolia grassland soils based on DNA-Stable-Isotope-Probing (DNA-SIP), and the methane and ammonia oxidation under various N levels was studied.[Result] The results showed that methane-oxidizing activity was stimulated by low N while it was suppressed by high N application. Contrarily, the nitrification activity was stimulated by both low and high N addition. The Methylobacter lineage of methane-oxidizing bacteria(MOB) were the active players in methane oxidation, while nitrifying communities dominated by Nitrosospira lineage of AOB and Nitrospira-like nitrite-oxidizing bacteria (NOB) were the active players in nitrification. Also, the network analysis unraveled significant negative relationships between active Methylobacter MOB and Nitrosospira AOB/Nitrospira NOB, which suggested competitive interactions between active nitrifiers and methanotrophs in grassland soils.[Conclusion] The interactions between active methanotrophs and nitrifiers can be affected by N levels. Our results provide theoretical support for regulating CH₄ sinks and N₂O sources of grassland soils and for greenhouse gas mitigation.

Abstract:[Objective] Microbial-mediated methane oxidation plays an important role in controlling methane emissions from paddy fields. Atmospheric CO₂ enrichment could change the potential activity, abundance, and community composition of methanotrophs in paddy rhizosphere soil, and consequently, affect their role in controlling greenhouse gas emission. Currently, there are still controversies about the effect of elevated atmospheric CO₂ concentration on methane oxidation potential and methanotrophic communities in paddy fields. Moreover, the atmospheric CO₂ concentration is a slow increase process, rather than a sharp increase to certain concentrations. However, there is still no relevant research on the effect of the slow increase of CO₂ concentration on methanotrophs. Therefore, it is necessary to study the effects and mechanism of slow increase of CO₂ concentration on methane oxidation in paddy soils.[Method] In this study, a slow increase of atmospheric CO₂ concentration (an increase of 40 μL·L^-1 per year with 4 years) (EC) was set up based on the atmospheric CO₂ concentration (AC) automatic control platform. Slurry incubation, high-throughput sequencing, and quantitative PCR on pmoA genes were used to systematically investigate the methane oxidation potential, and the abundance and community structure of methanotrophs in paddy soils under different CO₂ concentrations. This study was carried out during key growth stages (e.g. tillering, jointing, flowering and milky) of rice.[Result] Results show that the variation trend of methane oxidation potential and methanotrophic abundance was consistent, and both were increased with the elevated atmospheric CO₂ during flowering and milky stages but decreased during tillering and jointing stages. Based on the data obtained from all four growth stages, the atmospheric CO₂ enrichment enhanced the methane oxidation potential by 11.7% and increased the abundance of methanotrophs by 53%. Further, the community structure of methanotrophs in soil was changed significantly, with the dominant methanotrophs shifting from type II under AC to type I under EC. There was no single environmental factor that was found to have a significant impact on methane oxidation potential, pmoA gene abundance or diversity. Also, the content of soil inorganic nitrogen (NH₄⁺-N, NO₂^--N and NO₃^--N) was decreased under EC compared with the control. The relatively lower inorganic nitrogen content in paddy soils under EC could alleviate the inhibition of nitrogen on methane oxidation in the paddy field. By stimulating the growth of rice roots and increasing soil carbon input, elevated atmospheric CO₂ can indirectly enhance the production of methane in soils, which in turn increased the methane oxidation potential and abundance of pmoA genes. The increase of rice root exudates and aerenchyma volume under EC might provide a more suitable living environment for type I methanotrophs.[Conclusion] Elevated atmospheric CO₂ promotes the growth of crops, which can subsequently increase both CH₄ and O₂ concentrations in paddy rhizosphere soil, and decrease soil nitrogen level. The combined effects of the above environmental factors could affect the methane oxidation potential, methanotrophic abundance and community structure in our paddy soils. Taken together, our results indicate a positive response of methane oxidation to the slow increase of atmospheric CO₂ concentration in paddy ecosystems, which could help alleviate global warming.

Abstract:[Objective] Soil microorganisms are important regulator of nutrient cycles during plant growth. The species, quantity, and spatial distribution reflect the characteristics and transformation law of habitat soil. There constitute an important index to evaluate soil development status and vegetation succession. In the revegetation areas of arid deserts, most existing studies described the overall restoration, structure and function of soil microorganisms after sand-fixing revegetation construction. However, there are few studies on soil microbial functional groups that indicate the carbon, nitrogen and phosphorus cycles. Thus, this study, looks at soil microorganisms in the revegetation areas replanted in different years in the southeast edge of Tengger Desert. We explored the spatial distribution, seasonal changes, and the effects of different restoration years, plant species, microhabitat and physiochemical properties on the number of soil microorganisms.[Method] Soil composites 0-5, 5-10, and 10-20 cm layers were collectd in January, April, July and Octerber in 2017 under shrubs and between shrubs at Artemisia ordosica and Caragana korshinskii plots established in 1990 and replanted in 2010. The numbers of cellulolytic bacteria, ammonifying bacteria, nitrifying bacteria and phosphorus solubilizing bacteria were determined by dilution culture counting method. Three-factor analysis of variance was used to compare the differences of soil microbial functional groups, microhabitat and soil layer (fixed factors) and sampling time (repeated factor). The redundancy analysis and best fitting were used to explore the relationships between soil physicochemical properties and microorganisms.[Result] The results showed that:(1) with an increase in sand fixation age, the number of microbial functional groups increased significantly, and the number in soil surface layer (0-5 cm) was higher than those of deep layers (5-10 and 10-20 cm); (2) the number of microbial functional groups in soil carbon, nitrogen and phosphorus cycles showed obvious seasonal variations. For example, the number of cellulolytic bacteria distributed in "V" shape was high in summer but low in springwhile ammonifying and nitrifying bacteria showed a trend of slow increase from winter, spring, summer to autumn. Additionally, the number of phosphorus solubilizing bacteria decreased from winter to autumn to spring and summer; (3) the numbers of cellulolytic bacteria and phosphorus solubilizing bacteria in C. korshinskii and A. ordosica plots were higher in 1990 than in 2010 while the numbers of ammonifying and nitrifying bacteria in A. ordosica plot were higher in 2010 than in 1990; (4) total nitrogen and available phosphorus had significant effects on the number of microbial functional groups.[Conclusion] This study showed that important microbial functional groups involved soil carbon, nitrogen, and phosphorus cycles are mainly affected by planting time, microhabitat, and soil layer of sand-fixing revegetation, while short-term fluctuation is controlled by seasons. In the early stages of revegetation, A. ordosica promoted recovery of nitrogen-circulating microorganisms, while, C. korshinskii was more favorable to the recovery of carbon-circulating microorganisms in the late stage. Besides, total nitrogen and available phosphorus contents were the key factors determining the numbers of soil microbial functional groups. Presumably, plant growth and soil microbial reproduction in the late stage of sand-fixing revegetation succession may be mainly restricted by nitrogen. The results of this study provied a foundation for a better understanding of the interaction between microorganisms and soil characteristics in revegetation restoration, and strengthen our understanding of soil carbon, nitrogen and phosphorus cycles in arid deserts.