Abstract:Due to its high efficiency, safety and cost-effectiveness, biofilm remediation technology has been widely used in the removal of refractory pollutants in the environment. Biofilm technology refers to the adhesion, enrichment and reproduction of plankton microorganisms, eventually forming biofilm structure on the surface of the abiotic carrier, which leads to an increase in total biomass density and highly efficient metabolism of hydrophobic and toxic compounds. Compared with planktonic cells, a biofilm matrix can provide microorganisms with stronger resistance to high pressure of survival competition, harsh environmental conditions or harmful toxins. Changes in the local concentration of nutrients in the biofilm matrix and differences in the division of labor among microorganisms can induce differential gene expressions, leading to biofilm cells differing (phenotypically and metabolically) from the planktonic cells. It is beneficial for microorganisms to degrade pollutants through multiple metabolic pathways. Meanwhile, bacteria chemotaxis and flagellar movement can help the microorganism get access to pollutants, thereby improving their biodegradation efficiency. The formation and dispersal of biofilm are regulated by quorum sensing. The generation of extracellular polymeric substances is regulated by signal molecules by quorum sensing (QS), thereby changing the biofilm characteristics and enhancing the bioremediation of pollutants. Quorum sensing is a form of cell-cell communication among microorganisms. Through the diffusion of autoinducers among cellular matrix, bacteria can perceive cell density and species complexity and regulate their gene expressions when the concentration of signal molecules reaches a threshold level. So far, many different structural QS signals have been identified. Although many of them are specific among species, some QS signals can be produced and recognized by many species, thereby allowing inter-species communication. N-acyl-homoserine lactones (AHLs) are often employed as QS signal molecules for many Gram-negative bacteria regulated by members of Luxl/R family genes, while Gram-positive bacteria use processed oligo-peptides to communicate. Biofilm formation and dispersal are genetic processes, therefore, they can be manipulated with synthetic biology tools like other genetic systems. Thus, biofilms and the biodegradation of pollutants may be controlled by manipulating signals. Successful application of a bioremediation process relies upon an understanding of interactions among microorganisms, contaminants and carrier materials. At present, more and more researches focus on pollution remediation using engineering biofilm technology, including in-situ and ex-situ bioremediation technology. During this process, quorum sensing or quorum quenching acts a crucial role. Quorum sensing plays a major role in various microbial physiological functions, such as biofilm formation and biofilm repair in polluted environments. Biofilms provide an optimal environment for cell-to-cell interactions, cell-to-cell exchange of genetic material and signals, and dispersal of metabolites. Biofilm quorum sensing technology exhibits an ideal application prospect in the remediation of contaminated soils. For the biofilm QS system, it is very important to clarify the generation rules of signal molecules among microorganisms, as well as the signal conduction path and its mechanism, which is conducive to the engineering design and application of functional bacteria. However, most of the well-studied QS systems are from Gram-negative bacteria. More research is needed to uncover and study the details of QS in a variety of microbial species, including Gram-positive bacteria and fungi. The role of QS in microbial populations, including QS crosstalk and signal specificity, is another important area of research that will impact strategies to regulate biofilm formation and pollutant elimination. Furthermore, QS signals regulation under defined conditions could contribute to the stability of the microbial community and the repair ability of functional microorganisms. Therefore, synthetic biologists should also focus on engineering mixed flora based on biofilm QS systems.

Abstract:Minerals are critical active components of soils, which can drive the chemical transformation of toxic organic pollutants in the soil environment and reduce the risk of environmental contamination. Previous studies of the interface behavior of toxic organic pollutants on soil mineral surfaces mainly focused on the water environment or mineral suspension systems. Natural soils, especially surface soils, are usually under water-unsaturated conditions. In recent years, the transformation mechanism of toxic organic pollutants on water-unsaturated soil mineral surfaces has attracted considerable attention, and a series of fascinating discoveries have been made. The water-unsaturated iron and manganese minerals, clay minerals and metal ion-saturated clay minerals drive the transformation of the hydrophobic toxic organic pollutants such as polycyclic aromatic hydrocarbons and antibiotics. Water molecules at the interface of water-unsaturated minerals have weak competition with toxic organic pollutants for active sites, and the dehydration process of minerals also makes them change to highly active structures. However, an appropriate increase in the moisture of dry soil minerals can enhance the transformation of toxic organic pollutants. A small amount of water molecules can improve mass transfer and promote the transfer of toxic organic pollutants to the active sites of soil minerals. The different moisture state of soil minerals leads to the formation of various products. The water-unsaturated condition is more conducive to the formation and stability of persistent free radicals and halogenated dioxins. Also, the transformation of toxic organic pollutants is influenced by the catalytic activity of soil minerals and the physical and chemical properties of toxic organic pollutants. In previous studies, electron transfer reaction was considered as the transformation mechanism of toxic organic pollutants on soil minerals. However, free radical catalysis and hydrolysis mechanisms have been gradually discovered. Transition metal elements in iron manganese minerals and metal ion-saturated clay minerals can receive or give electrons during the transformation of toxic organic pollutants. The defects of soil minerals tend to induce the generation of active free radicals. While, the active hydroxyl, Brønsted acid sites, and Lewis acid sites in soil minerals can accelerate the hydrolysis of toxic organic pollutants. The development of detection technology and theoretical calculation has made the relevant mechanisms precise to the mineral crystal type and plane structure. Additionally, Soil, a complex system, is mainly composed of minerals, organic matter, and microorganisms. And organic matter and microorganisms have an important effect on the transformation of toxic organic pollutants at the mineral interface. Most notably, they can promote the reduction of mineral dissolution, and improve the reduction of toxic organic pollutants. Although the transformation and mechanism of toxic organic pollutants at the interface of water-unsaturated minerals have been gradually evident, the breadth and depth of the research need to be further expanded and deepened. It is suggested that future research focus on the following aspects: reveal the transformation of toxic organic pollutants in the natural soil systems, strengthen the research on the reduction and transformation processes, develop the in situ reaction devices and detection methods, and analyze the transformation mechanism of toxic organics at the mineral interface from micro and nanoscale and molecular level.

Abstract:ObjectiveThe increasing emergence and spread of antibiotic resistance genes (ARGs) poses a great threat to global public health. Soil as an important repository and medium for ARGs has attracted extensive attention from many scholars.MethodIn order to have a comprehensive understanding of the research progress and hotspots in the field of soil ARGs, the VOSviewer and CiteSpace software were used to perform a bibliometric analysis of the publications related to soil ARGs published from 2013 to 2022 in "Web of Science" database. More specifically, density visualization analysis, keyword emergence analysis, and contribution network analysis were performed to deeply discuss the types of publications, cited times, country/region and institution of publications, keywords, research directions, and the hotspots and trends of research.ResultOur results demonstrated that the number and cited times of the publications related to soil ARGs increased year by year. The number of publications increased slowly in the first five years (2013—2017), while it rapidly increased in the last five years (2018—2022). Besides, the maximum cited times was 22123 times. China published the most publications in the field of soil ARGs, accounting for 61.40% of the total number of publications, and China cooperated closely with 27 countries, including Australia and the United States. Tetracycline and sulfadiazine were the two main types of antibiotics in the soil ARGs field, and Escherichia coli as model bacterium was a microbial type of great interest in this field. The results of citation burst and co-occurrence analysis indicated that research hotspots of soil ARGs differed significantly in different periods. At first, scholars focused on the understanding and quantification of ARGs. Subsequently, more attention was paid to the source analysis of ARGs and their intrinsic connection with microorganisms. The study of the spread and fate of ARGs has become a hot topic of interest and research for scientists today.ConclusionThis study conducted a comprehensive bibliometric analysis of the relevant publications in the field of soil ARGs. On the whole, the research on the propagation deterrence and reduction techniques of soil ARGs is still insufficient. Future research may focus on the development and improvement of monitoring system as well as mathematical model establishment to assess the environmental risks of soil ARGs and their effects on human health. Besides, prevention-oriented source control of soil ARGs should be expanded. In addition, the diffusion mechanisms and influencing factors should be explored further to lay a solid foundation for ARGs reduction technology.

Abstract:Diversified cropping system is an important strategy of ecological intensive agriculture. It makes significant improvements to soil quality, ecosystem stability and land productivity. Under the background of a global shortage of food supply and cultivated land, diversified cropping system play an increasingly important role in ensuring food security and improving the quality of cultivated land in China. Currently, diversified cropping system has become a research hotspot in soil, ecology, crop, and tillage sciences. Nevertheless, the research about diversified cropping system in the recent period is more focused on improving its biodiversity and ecological functions but with limited understanding of how these diversified systems impact soil quality and productivity due to the changes of soil physical, chemical, and biological properties. In this paper, we analyzed the connotation of diversified cropping system and its significance on promoting soil health and improving ecological service function, and systematically summarized the advances in the effect of diversified cropping system on some soil properties including: (1) Physical properties: We focused on the improvements of soil porosity, aggregates, water binding capacity, etc., by different crop root morphology (biological tillage) and straw inputs under diversified cropping system. (2) Chemical properties: We analyzed diversified cropping system optimized light, water, and heat resources to increase the utilization efficiencies of nitrogen and phosphorus, promote soil organic carbon sequestration, balance soil nutrients and regulate root exudates, mitigate using differences in spatial and temporal niche and nutrient niche between aboveground and underground parts. (3) Biological properties: We reviewed the biological effect and advantageous changes of soil biodiversity under diversified cropping system including soil microbial diversity, arbuscular mycorrhizal fungi, soil enzyme activity, and soil animals. However, there are still some challenges limiting the development of diversified cropping systems including the lack of theoretical systems, production technology and machinery equipment, technological polarization, and policy support. Furthermore, we proposed four areas where future research should focus on improving cultivated land quality by diversified cropping system: (a) Develop multi-functional diversified cropping system including soil amendment, bioremediation, and low-carbon cropping system; (b) Supplement the theoretical system of cultivated land quality improvement under diversified cropping system including crop-soil interaction mechanism and nutrient regulation, ecological versatility and environmental response mechanisms at different scales; (c) Establish appropriate technology system for diversified cropping system including suitable special varieties or their combinations, optimization production technology and machinery equipment; and (d) Applicability and popularization strategies of diversified cropping system including planning and design, policy guidance and financial support to establish typical diversified cropping system in different regions. On the whole, this review provides a reference for the diversified cropping system to play a greater role in the strategy of improving cultivated land quality.

Abstract:ObjectiveTraditional digital soil mapping methods are unable to produce detailed soil maps within a reasonable cost and time. Digital soil mapping is a powerful technique, which is popular and widely used by scholars coupled with environmental covariates to map soil types or properties. The selection of environmental covariates is the key to ensuring the accuracy of mapping. Previous studies have proven that remote-sensing images can be used as auxiliary factors for reasoning mapping. Remote sensing data can provide rich soil landscape information, which is consistent with the core idea of using grids to express spatial changes of soil features in digital soil mapping. Moreover, remote sensing technology can obtain real-time information quickly. However, there are few relevant studies on how principal components and texture information of remote sensing factors contribute to the reasoning process. Thus, determining the weight of remote sensing factors in the reasoning process is the key content of this study, which is tested by the reliability of testing mapping results.MethodChengmagang Town, Macheng City, Hubei Province was selected as the study area. Using Chinese soil classification and soil type map with a spacing of 10 meters, which were extracted from the contour data and remote sensing image using a variety of feature selection algorithms to effective screening of variables, this study conducted the soil digital mapping by reasoning machine learning algorithms. Specifically, the recursive feature elimination screening algorithm, ReliefF algorithm and tree-based feature screening algorithm were used to rank all environmental factors in the whole area, plain and hilly areas of the study area, respectively. Then, it screened the effective environmental variables of environmental factors and analyzed the weight of remote sensing factors in the reasoning process. The factors involved in plant-hill region mapping were explored and their importance was determined. According to the selected relatively stable indicators, the gradient boosting decision tree model after parameter tuning of the Bayesian optimization algorithm based on TPE was used for modeling. Also, the mapping accuracy results after different feature screening algorithms were compared between the whole region and the terrain region to further explore ways to improve the accuracy of soil type mapping.ResultThe soil type inference map was verified by 141 independent field sampling sites. The results showed that the importance of remote sensing factors in the plain area was higher than that in the hilly area and the NDVI and Mean values of the remote sensing factors were relatively stable. The highest accuracy of topographical inference mapping based on the recursive feature algorithm was 75.89%, which was higher than the 13.48% and 4.97% of the ReliefF algorithm and tree-based feature screening algorithm, respectively. In addition, among the mapping results of the three feature screening algorithms, the accuracy of the mapping based on terrain factors was higher than that of the overall region mapping. It suggests that remote sensing factors as an auxiliary means to participate in the reasoning process can effectively improve mapping accuracy.ConclusionThis study uses a feature selection algorithm to select features with a strong correlation with soil types as auxiliary variables in the machine learning model. The method is efficient and cost-effective for soil type prediction. Compared to other methods, the soil type mapping method based on machine learning is advantageous and the feature mining and machine learning algorithms have theoretical significance and practical value.

Abstract:ObjectiveAs the largest Carbon pool in the terrestrial ecosystem, Soil Organic Carbon(SOC) plays an important role in Soil quality and crop yield. Accurate prediction of the spatial distribution of SOC on cropland is essential for the development of agricultural management measures. In the framework of Digital Soil Mapping(DSM), an important method to improve the precision of SOC spatial prediction is to select an effective environmental covariate. In previous studies, the mean values of remote sensing indices and climate variables for a certain period or point in time were usually used as input variables, while temporal characteristics and events were rarely used for SOC prediction. Therefore, in order to reduce the impact of the lack of part of physical information and climate characteristics, phenological variables and extreme climate variables were added in this study. The response characteristics to the spatial variability of SOC of cultivated land and the feasibility of predicting the spatial distribution of SOC were discussed.MethodThe research area of this paper is Shanggao County, Jiangxi province. A random forest model was used, in which remote sensing data, DEM-derived variables, phenological parameters and climatic characteristics were selected as environmental covariates, and the model results were corrected for residuals using Ordinary Kriging(OK). The prediction effect and prediction accuracy of the model under different types of variable combinations were compared.ResultThe results show that chronological variables, phenological variables, and extreme climate variables can improve the prediction performance of the model, and the residual error as an error item can further improve the accuracy of the model. The combination of chronological variables, phenological variables, extreme climate variables, topographic variables, and residuals had the highest prediction accuracy, improving R², MAE, and RMSE by 90.00%, 58.95%, and 57.14% compared to the combination of topographic variables, remote sensing variables, and climate variables. The analysis of variable contribution rates shows that SU, a3 and TXx were important variables affecting the distribution of cultivated land SOC in the study area.ConclusionPhenological variables and extreme climate variables have good application prospects. In the future, it is necessary to verify the validity of extreme climate variables as environmental variables in predicting soil properties under different land use and large-scale study areas.

Abstract:ObjectiveSoil organic matter(SOM) content and its stability are important indexes to evaluate soil quality. Thermogravimetric analysis has gained attention due to its good performance in reflecting SOM content and SOM stability. The objective of this paper is to explore the vertical variation patterns of SOM thermal stability in black soils under different pedogenetic modes. The causes for these variations are also explored. This study will provide a theoretical reference for the protection of black soil resources and carbon sequestration.MethodBlack soils of two major pedogenetic modes, i.e., naturally developed mode and depositional mode were selected for comparison. The former one included three typical black soils under stable topographic conditions (flat terrain) and the latter one included two black soils affected by erosional-depositional processes under unstable topographic conditions (with surface erosion and sedimentation) in the typical black soil area of Northeast China. Thermogravimetric analysis was used as a tool to evaluate SOM thermal stability. Two SOM fractions were recognized based on mass loss in responding to different temperature intervals, with Exo1(mass loss during 200~350℃) representing thermally labile SOM and Exo2 (mass loss during 350~550℃) representing thermally stable SOM. In addition, derivative thermogravimetry curves and two thermogravimetric parameters including Exo1/Exo2 and TG-50 were adopted to characterize the variations of SOM thermal stability in different soil profiles. Fourier transform infrared spectroscopy was used to assess SOM chemical stability.ResultFor naturally developed black soils from the stable land surface, the content of thermally labile SOM (Exo1) decreased at a higher rate with depth than that of thermally stable SOM (Exo2). With the increase of depth, Exo1/Exo2 decreased, TG-T50 increased, and aliphatic C/aromatic C decreased, indicating that the SOM of naturally developed black soils tended to be thermally stable with depth. Under unstable geomorphic settings, on the contrary, the thermal stability of SOM did not show a regular decrease trend with depth, and the content of SOM and Exo1/Exo2 in deeper layers could be higher than those in the surface layer. This was mainly due to the different land surface histories that caused variations in sources of soil parent material and SOM.ConclusionThis study confirmed the usefulness of thermogravimetric analysis in reflecting SOM stability. Spatial variations in SOM stability in black soils were largely conditioned by a pedogenetic mode which was largely related to geomorphic stability. We found that depositional landscape positions tended to stack a large amount of labile SOM, which was carried by erosional processes. This occurs both at hillslope and watershed scales. These thermally unstable SOMs could be preserved in the deep soil for a long time, due to the blocking effect of burial. Once eroded, however, these blocked labile SOMs are easily decomposed, which may also lead to the release of a large amount of buried 'old carbon' and become the 'hot spot' of carbon emissions in the black soil area.

Abstract:ObjectiveThe ecological environment of Loess Plateau is fragile and once the soil quality of cultivated land changes, it will directly affect the level of soil productivity. A long-term cultivated land in Tianzhen County, a typical county in the north of the Loess Plateau, was selected to study the spatiotemporal evolution of soil organic matter and total nitrogen and their influencing factors.MethodBased on the cultivated land quality data in 1983, 2008 and 2019, the temporal and spatial evolution characteristics and distribution patterns of organic matter and total nitrogen in cultivated soil of Tianzhen County were analyzed using GIS and geostatistics methods.Result(1) In the past 36 years (1983—2019), soil organic matter and total nitrogen content in Tianzhen County showed a significant change characteristic, which increased as a whole and rose rapidly in the later period. The average annual increase of organic matter and total nitrogen content was lower in the early period (1983—2008), showing 0.20 g·kg^-1 and 0.01 g·kg^-1, respectively, while the number was higher in the late period (2008—2019), showing 0.29 g·kg^-1 and 0.03 g·kg^-1. In the early stage of the study period, the ratio of carbon to nitrogen showed no significant change, but showed a significant decrease in the late stage, from 9.32 in 1983 to 8.43 in 2019. (2) The semi-analysis of variance (ANOVA) results revealed that over the past 30 years, the block-base ratio of soil organic matter increased from < 25% to 49.8% and the block-base ratio of soil total nitrogen increased from < 25% to 38.5%. This indicated that the influence of natural factors such as topography was gradually diminished, while the influence of human factors such as fertilizer application was enhanced. (3) The spatial distribution characteristics of soil organic matter and total nitrogen had similar characteristics and showed a pattern of low in the west and high in the east. From 1983 to 2008, the content of soil organic matter showed an overall increasing trend, with a significant increase in the northwest and a slow increase in other regions; total nitrogen increased significantly in the northwest and decreased in the east. Also, from 2008 to 2019, soil organic matter decreased slightly in the south while it increased in other regions; total nitrogen increased significantly in the north but more slowly in other regions.ConclusionOver the past 36 years (1983—2019), the soil organic matter and total nitrogen content in Tianzhen County showed an overall upward trend, and factors such as fertilization and straw returning to the field were the main factors leading to the changes in organic matter and total nitrogen content. Combined with the local conditions to increase the application of organic fertilizer, straw returning and rational application of nitrogen fertilizer can quickly improve the soil fertility of cultivated land. Therefore, this study can provide a theoretical basis for improving the quality of cultivated land on the Loess Plateau and protecting the agricultural ecological environment.

Abstract:ObjectivePredicting soil nutrients by visible-near infrared (vis-NIR) and mid-infrared (MIR) spectroscopy has the advantages of being fast, cost-effective and environmental friendly. Soil spectra contain abundant information of soil properties, and can be combined with machine learning methods to effectively and accurately predict soil nutrients, which can provide support and guidance for timely fertilization management. The objective of this study was to compare the predictive ability of vis-NIR (350-2500 nm) and MIR spectroscopy (4000-650 cm^-1) for predicting both the total and available contents of soil nitrogen (N), phosphorus (P) and potassium (K), in order to construct an optimal model for estimation of different nutrient contents.MethodIn this study, 500 samples were collected from the surface layers (0-20 cm) of the dryland in Guizhou Province for determination of soil N, P and K contents and spectral analysis. The vis-NIR spectra were measured by Cary 5000 and the MIR spectra by Thermo Scientifit Nicolet iS50. Soil spectra were pre-processed by Savitzky-Golay (SG) smoothing for denoising and standard normal variate (SNV) transformation for baseline correction. Partial least squares regression (PLSR) and support vector machine (SVM) were used to predict the contents of total nitrogen (TN), total phosphorus (TP), total potassium (TK), alkali-hydrolyzable nitrogen (AN), available phosphorus (AP) and available potassium (AK).ResultThe results showed that: (1) Whether using the vis-NIR spectroscopy or the MIR spectroscopy, the prediction accuracy of PLSR model was better than that of SVM model. (2) The accuracy of MIR spectroscopy for prediction of TN, TK and AN was significantly higher than that of vis-NIR spectroscopy. Vis-NIR and MIR spectroscopy could reliably predict TN and TK(ratio of performance to interquartile distance (RPIQ) > 2.10), while MIR spectroscopy could predict AN with moderate accuracy (RPIQ = 1.87). However, both types of spectra had poor ability to predict TP, AP and AK (RPIQ < 1.34). (3) When the variable in the projection (VIP) score was > 1.5, there were more important bands selected by PLSR models in the MIR region than the vis-NIR region. The important bands selected for estimation of TN were mainly concentrated near 1910 and 2207 nm in the vis-NIR region, and centered around 1 120, 1 000, 960, 910, 770, and 668 cm^-1 in the MIR region. The important bands of TK were mainly distributed around 540, 2 176, 2 225, and 2 268 nm in the vis-NIR region, and around 1 040, 960, 910, 776, 720, and 668 cm^-1 in the MIR region.ConclusionTherefore, MIR spectroscopy combined with PLSR model proved to be promising for accurate prediction of soil nutrients, especially for the estimation of TN and TK, and can provide technical support for guiding timely fertilization.

Abstract:ObjectiveIn order to comprehensively study the effect of desulfurized gypsum in improving saline-alkali soils, this paper statistically analyzed the change characteristics of soil pH value, exchangeable sodium percentage (ESP), soil electrical conductivity (EC), and crop yield after applying 0~60 t·hm^-2 desulfurized gypsum in the northwest, north, northeast, and coastal areas of China through literature search. To achieve this, a comprehensive improvement evaluation model of saline-alkali soil was used in which the desulfurized gypsum amount was designed as an independent variable based on the grey relational analysis and entropy weight method.MethodSoil pH value and ESP are often used to evaluate the alkalization property of saline-alkali soil while EC is used to evaluate the salinization property of saline-alkali soil. This paper used the data of literature to explore the relative change rates of soil pH, ESP, and EC in 0~20 cm soil layer with and without desulfurized gypsum within 1 year after the crop harvest in order to quantitatively reveal its improvement effect. Grey relational analysis and entropy weight method were used to synthesize three soil indexes to evaluate the improvement effect of desulfurized gypsum on saline-alkali soil. Due to the few literature data on light saline-alkali soil and moderate saline-alkali soil, the application of desulphurized gypsum in severe saline-alkali soil was mostly studied. This paper combined light saline-alkali soil and moderate saline-alkali soil into light and moderate saline-alkali soil for analysis.ResultApplication of desulfurized gypsum reduced soil pH and ESP and increased crop yield, but increased EC. As the application rate of desulfurized gypsum increased, the decrease rate of pH and ESP and the increase rate of crop yield increased first and then decreased whereas the increase rate of EC showed an increasing trend. When the amount of desulfurized gypsum applied to moderate saline-alkali soil and severe saline-alkali soil exceeded 30 t·hm^-2, the increase rate of crop yield(Oil sunflower, alfalfa, corn, rice, wheat, Lycium barbarum, and sweet sorghum) showed a decreasing trend. To further analyze the effect of desulfurized gypsum on the improved saline-alkali soil, an entropy weight grey correlation evaluation model was established, and the application of desulfurized gypsum was optimized.ConclusionThrough the study on the improvement effect of desulfurized gypsum applied in the range of 0~60 t·hm^-2 in saline-alkali soil, using statistical analysis and mathematical modeling, it was concluded that the amount of 18~22 t·hm^-2 desulfurized gypsum in light and moderate saline-alkali soil; the amount of 23~29 t·hm^-2 desulfurized gypsum in severe saline-alkali soil could achieve good soil improvement effect and increase crop yield. The research results provide a reference for improving saline-alkali soil with desulfurized gypsum.

Abstract:ObjectiveSoil moisture movement is the main driving force of the material and energy transport in soil. However, the paths of soil moisture in soils are complex and difficult to comprehend. This is especially true in the red soil region with abundant rainfall and diverse land use types.MethodWith the help of an electrical resistivity tomography(ERT), soil moisture infiltration processes in the vadose zone were captured for the forest land and farmland of the red soil from Quaternary red clay.ResultThe results showed that in the red soil region, ERT could realize the visualization of the soil moisture infiltration process in the soil profile of the red soil. There was a highly significant correlation (P < 0.01) between the resistivity measured by ERT and the soil moisture measured by the oven drying method. The linear coefficients of determination between them were 0.72 for forest land and 0.53 for farmland. Land use types affected the mode and depth of soil moisture infiltration. With a 2 cm water head height and 100 L water injection volume, the infiltration depth of forest land was more than 80 cm, much deeper than that of farmland (50 cm). With the continued injection of water for 200 min, the infiltration depth of the two land uses exceeded 150 cm and a preferential flow phenomenon was observed during the infiltration process. However, the preferential flow of forest land was more developed than that of farmland. The preferential flow of forest land moves down rapidly through macropores formed by plant roots and soil animal activities while the preferential flow observed in farmland moved to the deep layer intermittently mostly in the way of the finger flow. In addition, the forest land was dominated by vertical infiltration with weak horizontal flow while the horizontal movement in farmland was stronger.ConclusionUnder the influences of disadvantageous factors such as complex soil layers, high background water content and high soil clay content, ERT technology still could successfully capture the occurrence and development processes of preferential flow in red soil area.

Abstract:ObjectiveRare earth mining excessively increased the content of leaching agents(e.g. ammonium sulfate) in the soil. The high concentration of ammonium nitrogen(NH₄⁺-N) may be converted into nitrate nitrogen(NO₃^--N) under active biochemical action, resulting in potential environmental risks, especially nitrate pollution of water bodies around tailings. Therefore, it is necessary to evaluate the content of soil NO₃^--N, explore the influencing factors and understand the nitrate pollution degree of the rare earth tailings.MethodWe chose an ionic rare earth tailing after in-situ mining in southern Jiangxi province, which used ammonium sulfate as a leaching agent. Up to sampling, this mine had been closed for 4 years. We set three sampling points regularly from the top to bottom of this mine and collected soil profile samples in different layers from the topsoil to the bedrock. Soil samples were divided into two parts. One part was stored at a low temperature to analyze soil nitrate nitrogen and ammonium nitrogen. The other one was used for analyzing relevant physical and chemical properties after air drying.ResultThe results showed that the variation range of soil NO₃^--N content in the tailing area was large(2.80 to 193.99 mg·kg^-1), with a mean of 46.30±55.16 mg·kg^-1. The average content of topsoil NO₃^--N was 5.16 mg·kg^-1, which was similar to that of natural soil. Also, the average content of soil NO₃^--N in ore-bearing layers was 48.64 mg·kg^-1, which was nearly 10 times that in the natural soil. The soil NO₃^--N of the ore-bearing layer in the deep profile was higher than that of the top layer. Moreover, the distribution of NO₃^--N with depth was different from that of the natural soil and was mainly caused by a large number of leaching agents remaining in the ore body. NH₄⁺-N content dominated the generation of NO₃^--N and determined the upper limit of soil NO₃^--N accumulation. The accumulation degree of NO₃^--N in different soil layers and different parts of mountains was controlled by rainfall leaching and the NO₃^-N migration process. However, the soil's physical and chemical properties, including water content, cation exchange capacity and particle composition, had no significant correlation with NO₃^--N content, and were not the determining factors of NO₃^--N content.ConclusionThe soil NO₃^--N in the tailing mainly originated from nitrification. Four years after mining, a large amount of NH₄⁺-N remained in the tailing, and the NO₃^--N generated by nitrification was continuously released into the environment. In the long term, the soil NH₄⁺-N enriched in tailing will be transformed into NO₃^--N and the NO₃^--N will migrate with water, threatening the ecological environment and human health. This study can provide a theoretical basis and scientific reference for the assessment and treatment of soil and downstream water pollution in rare earth in-situ leaching sites.

Abstract:ObjectiveAs one of the typical forces in the process of erosion and migration, sediment abrasion can accelerate the fragmentation of microplastics, thereby affecting microplastic migration and redistribution within the watershed. However, how sediment abrasion affects the fragmentation behavior of microplastics is still unclear. Therefore, the objective of this study was to investigate the effect of sediment abrasion on the fragmentation of microplastics.MethodIn this study, topsoil was collected from a mulched cornfield in the Wangdonggou small watershed of the Loess Plateau and the abundance, type, and morphology of microplastics were analyzed using a laser infrared imaging system after different treatments. The different treatments include air-dried original soil, sediment standstill, and sediment abrasion with three concentrations of 560 kg·m^-3, 800 kg·m^-3 and 930 kg·m^-3.ResultThe results showed that: (1) PU, Polytetrafluoroethylene (PTFE), and Rubber (RB) were the main microplastics in the study area, mostly in the form of fragments with a diameter of 10~50 μm.(2) The average area of microplastic was largest in the air-dried soil treatment (5, 234 μm²) and the smallest (2, 067 n·kg^-1) was in the sediment standstill treatment. The microplastics after sediment abrasion treatments had the greatest average abundance (14, 400 n·kg^-1) and the smallest average area (2, 868 μm²). (3) The average abundance and area of microplastic under the three sediment abrasion treatments were significantly different. The average abundance of microplastic followed the pattern: moderate sediment concentration (18, 300 n·kg^-1) > low sediment concentration (13, 730 n·kg^-1) > high sediment concentration (8, 667 n·kg^-1), whilst the average area of microplastic showed: low sediment concentration (3, 932 μm²) > moderate sediment concentration (2, 472 μm²) > high sediment concentration (2, 099 μm²).ConclusionOverall, this study demonstrates that sediment abrasion significantly increased the microplastic abundance and reduced their areas, but the average abundance of microplastic reached the maximum at the moderate sediment concentration abrasion intensity. The sensitivity of different microplastic types to sediment abrasion was different, providing guiding information for the risk assessment of soil microplastic fragmentation and migration in eroding settings.

Abstract:ObjectiveRare earth elements(REE) are one of the most important tracers in soil erosion study. Understanding the content variation of REEs alongside particle size ranges is significant to accurately quantify soil erosion rates.MethodBased on the REE contents in different particle size ranges(< 1, 1-10, 10-50, 50-250 and 250-1000 μm) of soil/sediment samples measured by Samonova et al.(2020), this study analyzed the effects of valley morphology, landforms, erosional and depositional areas on the contents and enrichment coefficient of REE in soil/sediment at the left bank of the Protva River in Russia. Landform types including hillslope, gully slope/wall, valley bottom and alluvial fan, were separately selected from a "U" and a "V" shape valley, respectively. These four landforms were further classified into erosional (i.e., hillslope and gully slope/wall) and depositional areas (i.e., valley bottom and alluvial fan).ResultThe results showed that: (1) The contents of light rare earth elements (LREE), heavy rare earth elements (HREE) and total rare earth elements (ΣREE) were all decreased with the increase of particle size ranges under the four landform types of "U" and "V" shape valleys. Under the condition of the same landform types, the contents of LREE, HREE and ΣREE in fine particles (< 50 μm) of the "U" shaped valley were 14.6%-24.7%, 10.0%-33.5% and 14.2%-21.1% lower than those of the "V" shaped valley, respectively. (2) Both LREE and HREE were enriched in fine particles at all landform types for both "U" and "V" shape valleys. Particularly for the "V" shape valley, LREE and HREE were significantly enriched in < 10 μm particles. (3) Comparing the erosional and depositional areas, the enrichment degree of LREE and HREE in the fine particles of the depositional areas in both "U" and "V" shape valleys was significantly (P < 0.05) higher than that of the erosional area.ConclusionAccordingly, REE were enriched in fine particles regardless of river valley shapes, landforms, and erosional or depositional areas. However, both valley shape and erosion-deposition processes had important impacts on the REE content variation in different particle size ranges.

Abstract:ObjectivePolycyclic aromatic hydrocarbons (PAHs) have carcinogenic, teratogenic, and mutagenic effects, and soil PAH pollution has become a world-wide problem. However, ecological soil screening and controlling levels (Eco-SSCLs) for PAHs in China have not yet been established. Thus, soil ecological security risk assessment is still unsubstantiated.MethodHere, we systematically investigated the research outcomes in related fields at home and aboard, and screened out 248 toxicity data (Effect concentration₁₀, EC₁₀ and No observed effect concentration, NOEC) of 16 pri-control PAHs listed in USEPA. The species sensitivity distributions were developed by a series of cumulative distribution functions, and were successfully used to derive soil environmental criteria of each PAH (PAH-SEC) for ecological security under different land use types.ResultThe PAH-SEC values under different land use types were 1.00-10.60 mg·kg^-1 (natural reserve and agricultural land), 1.03-25.44 mg·kg^-1 (parkland), 1.12-51.00 mg·kg^-1(residential land), 1.20-68.41 mg·kg^-1 (commercial service and industrial land).ConclusionThe results can provide data support for the formulation of soil environmental quality standards for ecological security, and offer bases for ecological risk assessment of PAHs polluted soil.

Abstract:ObjectiveDiphenylarsinic acid (DPAA) is one of the main degradation products of arsenic-containing chemical weapons in the environment. The objective of this study was to investigate the effects of iron and sulfate reduction on the mobilization and thionation of DPAA in soil.MethodAcrisol from a peanut field and woodland was selected for soil incubation experiment, and both were spiked with DPAA and different concentrations of sodium lactate (C source) and sodium sulfate (S source) under flooded condition. Four treatments were established in the current study, treatment one (S-C-) was created without the addition of S or C sources, treatment two (S+C-) only received 426 μg·g^-1 S, treatment three (S+C+) received 426 μg·g^-1 S and 1 300 μg·g^-1 C, and treatment four (S+C++) received 426 μg·g^-1 S and 2 170 μg·g^-1 C. Samples of the soil and supernatant were collected for the analysis of DPAA, Fe²⁺, HCl-extractable Fe²⁺, SO₄^2-, sulfide, pH, Eh and bacterial community structure.ResultResults for the peanut field indicated that the mobilization of DPAA was significantly enhanced in the S+C+ and S+C++ treatments due to the addition of carbon, as compared to S-C- and S+C- treatments without carbon. This difference was likely caused by the elevated Fe reduction (> 45%) due to lactate addition, which then promoted DPAA mobilization. In comparison to other treatments, sulfate reduction was most significant in the S+C+ treatment of the peanut field (with the concentration of dissolved sulfide reaching 11.28 mg·L^-1 after 13 weeks of incubation), where the removal rate (59.6%) of DPAA was the highest and DPAA was primarily thionated. However, the increased Fe reduction and decreased sulfate reduction in the S+C++ treatment were not conducive to the thionation of DPAA. Unlike peanut land, the mobilization and thionation of DPAA were not observed in woodland regardless of the addition of sodium lactate and sulfate. This difference can be explained by the lower degree of Fe reduction (< 25%) and the absence of sulfate reduction (without the detection of dissolved sulfide) in woodland. The relative abundances of Clostridium, Bacillus and Desulfosporosinus were significantly higher in the peanut field soil than those in woodland. This probably promoted the occurrence of iron and sulfate reduction, respectively, and both processes ultimately had a stronger influence on the mobilization and thionation of DPAA.ConclusionGenerally, instead of the stimulating effect of Fe reduction on DPAA mobilization, the degree of sulfate reduction and the concentration of sulfide in the liquid phase is a more significant factor in determining DPAA thionaiton in Acrisol under flooded condition. The findings will serve as a theoretical foundation for the in-depth understanding of the environmental behavior of DPAA as well as the development of biostimulation remediation solutions for DPAA-contaminated Acrisol.

Abstract:ObjectiveMicrobial arsenic(As) methylation is a vital As resistance mechanism that alters the toxicity and mobility of As, and can be used for the control of As contamination in soils. As-resistant plant growth-promoting bacteria use this mechanism to positively affect the growth of rice in As-contaminated soil. However, the arsenic methylation efficiency of rice rhizosphere bacteria and its effect on rice growth under arsenic stress remain unclear.MethodIn this study, an As-methylating functional Bacillus sp. LH14 was isolated from rice rhizosphere soil contaminated with As. The As methylation efficiency, As resistance and plant growth-promoting traits of the strain were analyzed. Additionally, the effect of strain inoculation on rice growth, rhizosphere As species and microbial interactions in As-contaminated soil were explored.ResultThe results show that strain LH14 promoted As methylation and volatilization and produced dimethylarsenate and trimethylarsenic oxide, which accounted for 54.9% of the initial As(Ⅲ) in the medium. LH14 inoculation significantly increased the copy numbers of arsenic methyltransferase gene(arsM) and methylated As in rhizosphere soil, indicating that LH14 was involved in soil As transformation. LH14 produced IAA under As stress and significantly increased seed germination rate, root and shoot length and biomass in the presence of high As concentration. Also rice growth in the soil was significantly promoted by LH14 inoculation, and this was associated with the enrichment of beneficial microorganisms(e.g., Burkholderiaceae and Gemmatimonadaceae) in the rhizosphere.ConclusionInoculation with As-methylating plant growth-promoting bacteria altered As speciation in rice rhizosphere and directly or indirectly promoted rice growth by producing plant hormones, enriching beneficial bacteria, and alleviating As stress. These findings provide theoretical support for the application of As-methylating bacteria in the remediation of As-contaminated soil and alleviation of As stress in plants.

Abstract:ObjectiveThe goal of this study was to evaluate the degradation of fenpropathrin in soil and its effects on earthworms, as well as to establish a foundation for a full environmental risk assessment of fenpropathrin.MethodHigh performance liquid chromatography was used to identify the bioaccumulation of fenpropathrin and the major metabolite, 3-phenoxybenzoic acid, then soil exposure tests were used to assess the toxicity of fenpropathrin to earthworms.Result(1) Fenpropathrin decomposed four times more quickly in unsterilized soil than in sterilized soil and more quickly in alkaline soil than in acidic soil, suggesting that microorganisms and pH were the main factors affecting the degradation rate of fenpropathrin in soil. Moreover, during the decomposition process, the synthesis of its primary metabolite, 3-phenoxybenzoic acid, was discovered. (2) Fenpropathrin content in earthworms increased, then dropped, with a maximum bioconcentration factor of 0.3.(3) The subacute toxicity data showed that after 14 d exposure to high doses (5 mg·kg^-1) of fenpropathrin, the protein content in earthworms was significantly reduced (P < 0.05). Also, the cytochrome P450 (CYP450), carboxylesterase (CarE), glutathione-S-transferase (GST), superoxide dismutase (SOD) and catalase (CAT) activities and malondialdehyde (MDA) content were significantly increased (P < 0.05) after 14 d of fenpropathrin exposure, and there was a dosage effect.ConclusionThe rate of fenpropathrin degradation in soil was strongly linked with soil characteristics, and fenpropathrin residues in soil could cause subacute toxic effects in earthworms. These findings help in a thorough evaluation of the ecotoxicity and environmental behavior of fenpropathrin in soil, as well as a more solid foundation for pyrethroid pesticide risk assessment.

Abstract:ObjectiveThis study aimed to reveal the characteristics of soil nitrogen (N) transformation in different clonal Chinese fir plantations in Yangkou National Forest Farm of Fujian. This study provided theoretical basis for artificial nitrogen management and improved seed breeding of different clones of Chinese fir plantations.MethodAn incubation experiment was carried out with 7 different kinds of 15-year old third-generation excellent culture materials and seedlings (Y003, Y008, Y020, Y061, Y062, Ysec and Ymix) as the research objects, and the basic physical and chemical properties, net N mineralization and nitrification rate of soil of different clones were evaluated.ResultThe results showed that the net rates of N mineralization and nitrification were significantly affected by different clonal Chinese fir plantations. Specifically, the net mineralization rate and net nitrification rate were -0.09-0.118 mg·kg^-1·d^-1 and -0.021-0.051 mg·kg^-1·d^-1, respectively. During the whole incubation period, the average net soil N mineralization rate of Y061 was 0.117 mg kg^-1 d^-1, which was significantly higher than that of other clones and followed by Ymix (0.046 mg·kg^-1·d^-1) and Y062(0.033 mg·kg^-1·d^-1). In contrast, the average net N mineralization rates of the other four clones were negative, indicating the occurrence of net N immobilization. The average net soil nitrification rate of the Y008 clone was the highest, which was 0.051 mg kg^-1 d^-1, followed by Ymix (0.003 mg·kg^-1·d^-1) and Y020 clone (0.007 mg·kg^-1·d^-1). There were no significant differences in soil pH, ammonium nitrogen, C/N and the composition of silt and sand, but there were significant differences in soil nitrate nitrogen, organic matter, total nitrogen and clay composition. The results showed that ammonium nitrogen, nitrate nitrogen, pH and total nitrogen in soil were the main factors affecting the net nitrification rate and were all positively correlated, In contrast, the average net nitrification rate of the other four clones was negative, indicating the occurrence of net immobilization of nitrate. There were no significant differences in soil pH and carbon (C)/N among different clones, but significant differences in soil particle size composition, organic matter and total N content. Soil pH and total N were positively correlated with net mineralization and net nitrification rate, while soil C/N was negatively correlated with sand content.ConclusionThe results showed that the soil N supply capacity and N retention capacity of Y061 and Y062 clones were significantly higher than those of other clones, and the risk of N loss such as leaching in Y008 clones was higher than that of other clones. Therefore, the clone species should be rationally selected to ensure the soil fertility supply in actual planting. This study provides a theoretical basis for artificial nitrogen management and improved seed breeding of different clones of Chinese fir plantations.

Abstract:ObjectiveSoils and their functions are under threat from acidification. Phosphorus(P) and silicon(Si) mobility and their interactions in soils are sensitive to soil pH. Although Si fertilizer application has been reported to efficiently increase P availability in soils, the responses of Si mobility and availability to soil P enrichment are not well understood, especially in areas of intensive agriculture where soil P accumulation is significant.MethodThis study investigated the effects of P addition and artificial soil acidification on Si sorption and mobility in two typical farmland soils with different available silicon levels(the lower available silicon level, LASi and the higher available silicon level, HASi) using P and Si competitive adsorption experiments and soil incubation experiments.ResultWhen Si and P were simultaneously added at equimolar concentrations between pH 3.5 and 8.0, the presence of P decreased Si adsorption by 26%-74% and 31%-84%, respectively, in LASi and HASi soils at the corresponding pH, suggesting that P had a greater affinity to soil solids than did Si. Si sorption by the soil generally decreased with decreasing pH in the pH range of 3.5 to 8.0, and the reduction of Si sorption due to P addition was more significant under higher pH conditions.ConclusionSoil acidification and P addition decreased Si sorption and the soil HOAc-NaOAc-extractable Si content. The responses of soil CaCl₂-extractable Si content to soil acidification and P addition were different between the two farmland soils, and further studies are needed to understand the mechanisms.

Abstract:ObjectiveThe aggregate distribution, the composition of the fungal community, and the production of biological binding agents can be significantly changed by different nitrogen (N) application rates. However, whether there is a correlation between these properties remains unclear.MethodA 16-year field experiment located in Fengqiu Agro-ecological National Experimental Station was used as the research platform, and this included five levels of N application rates, i.e. (1) F0: 0, (2) F1: 150 kg·hm^-2, (3) F2: 190 kg·hm^-2, (4) F3: 230 kg·hm^-2, and (5) F4: 270 kg·hm^-2. The effects of different N application rates on water-stable aggregate distribution (> 2 000 μm, 2 000-250 μm, 250-53 μm and < 53 μm) were explored, and the correlation between aggregate distribution and the major biological binding agents(glomalin-related soil proteins(GRSP) and microbial biomass carbon (MBC)) and soil fungal community was studied.ResultThe soil aggregate distribution and fungal community composition were divided into three significantly different groups: F0, F1 and F2, F3 and F4, respectively. The results showed that: (1) F1 and F2 treatments had the highest mean weight diameter of soil aggregates and significantly increased the proportion of > 2 000 μm aggregates and this was mainly related to the enrichment of Pyrenochaetopsis; (2) F1 and F2, F3 and F4 treatments all increased the proportion of 2 000-250 μm aggregates, while they significantly decreased the proportion of < 53 μm aggregates. The increase of the proportion of 2 000-250 μm aggregates was significantly positively correlated with the ratio of easily extractable glomalin-related soil proteins to total glomalin-related soil proteins (EE-GRSP/T-GRSP) and easily extractable glomalin-related soil proteins (EE-GRSP). However, it was significantly negatively correlated with the relative abundance of Didymella and Mortierella. The decrease of the proportion of < 53 μm aggregates was significantly positively correlated with the relative abundance of Mortierella, Phlebia, Melanospora, Fusicolla, Podospora and Didymella, but significantly negatively correlated with EE-GRSP and/or the relative abundance of Acremonium, Scytalidium and Exophiala, EE-GRSP/T-GRSP and MBC.ConclusionThe variation of soil aggregate stability was affected by the level of N application. Also, the stability of soil aggregates under N application rates of 150 kg·hm^-2 and 190 kg·hm^-2 was higher than that under N application rates of 230 kg·hm^-2 and 270 kg·hm^-2, which was significantly correlated with the changes in fungal community composition and biological binding agents under different N application rates.

Abstract:ObjectiveGreen manuring is considered to be an effective strategy to achieve sustainable development in agriculture. It plays key roles in ameliorating soil fertility, improving fruit quality, and enhancing resource use efficiency. China is the world's largest strawberry (Fragaria×ananassa Duch.) producer, with strawberry cultivation taking place mainly in greenhouses. However, many field management practices, such as long-term monoculture and irrational chemical fertilization, reduce the fruit yield and quality of strawberries. Therefore, it is important to investigate the effects and mechanisms of green manuring on strawberry yield and quality in greenhouses.MethodThree gramineous green manures, i.e., maize (Zea mays L.), sorghum [Sorghum bicolor (L.) Moench], and sorghum-sudangrass (Sorghum×sudangrass), were selected to carry out a field experiment from 2017 to 2021. There were four treatments: fallow farmland-strawberry (control), maize-strawberry, sorghum-strawberry, and sorghum-sudangrass-strawberry. Samples were collected in the full fruit stage of strawberries to determine plant growth and physiological parameters, in addition to fruit yield and quality attributes. Rhizosphere soil chemical properties, bacterial community structure, and metabolome were also analyzed.ResultThe different green manure treatments promoted plant growth and yield formation, and improved the fruit quality of subsequent strawberry crops. The most prominent effects were observed for the corn green manure treatment, which increased strawberry plant dry weight, root growth, and leaf chlorophyll content (SPAD value) by 53.4%, 21.0% - 94.7%, and 7.8%, respectively, compared with the control treatment. Also, fruit yield per plant was markedly improved by 44.6% in the corn green manure treatment, along with the increase of nutrient contents (total sugar: by 13.9%; vitamin C: by 14.4%; and soluble solids: by 12.8%). Soil pH, cation exchange capacity, organic matter content, and nutrient availability (N, P and K) were also strongly increased under the corn green manure treatment, while sucrase, phosphatase, and urease activities were enhanced simultaneously. Green manuring mediated an increase in the diversity and abundance of rhizosphere soil bacterial communities, coupled with a distinct enrichment of potentially beneficial bacteria, such as Flavobacterium, Variovorax, and Pedobacter. The increase in the abundance of potentially beneficial bacteria might be related to a considerable increase in the relative abundance of carbohydrate metabolites (e.g., sorbose, mannose, and fructose) associated with green manuring. A remarkable decrease in the relative abundance of lipid metabolites (e.g., palmitic acid, stearic acid, and oleic acid) alleviated the autotoxicity of strawberries.ConclusionGramineous green manure-mediated shifts in specific metabolites in the rhizosphere soil improved strawberry yield and quality by recruiting potentially beneficial bacteria and alleviating allelopathic autotoxicity. Maize green manure is a better option compared to sorghum and sorghum-sudangrass green manures for greenhouse strawberry production.

Abstract:ObjectiveBt toxins released from Bt plants and Bt biopesticides are potential exogenous pollutants in the environment with biocidal activity. The environmental behavior and ecological effects of Bt toxins are the focus of safety risk assessment of transgenic plants and transgenic microorganisms. Fungus is an important component of soil microbes and plays a key role in maintaining soil ecosystem stability, but the dynamic response of soil fungal communities and potential functions to exogenous Bt toxins remains unclear.MethodIn this study, the retention dynamics of Bt toxins in soils incorporated with different concentrations of Bt toxins were analyzed, and high-throughput sequencing technology of fungal 18S rRNA gene was used to analyze the effects of Bt toxins application on the soil fungal community and functional diversity.ResultThe results showed that the concentration of water-dissolved Bt toxins in soil decreased significantly with the prolongation of soil incubation time, and the amounts of water-dissolved Bt toxins in soil with initial Bt toxins concentrations of 50, 100 and 500 ng·g^-1 decreased to those of control soil on the 100th day. Both the application of Bt toxins and incubation time could significantly affect the composition of the soil fungal community, and with an increase of initial Bt toxins concentration and prolongation of soil incubation time, the difference in soil fungal community gradually widened. The application of Bt toxins increased the Shannon index of the soil fungal community, the negative correlation and modules of the association network, and thus it did not adversely affect the diversity and stability of the soil fungal community.ConclusionThe results indicate that the initial concentration of Bt toxins and its long-term effects should be of concern when assessing the environmental behavior and micro-ecological effects of Bt toxins. With the increase of Bt toxins concentration, the relative abundances of Phymatotrichopsis, Homalogastra, Geosmithia and Apiotrichum increased significantly, as well as functional genes encoding enzymes involved in protein degradation, carbon metabolism and phosphorus metabolism. It is speculated that the above-mentioned fungal taxa and potential functions were involved in the degradation and transformation process of Bt toxins in the soil. This study provides a scientific reference and theoretical basis for the ecological safety risk assessment of Bt plants, Bt recombinant biopesticides and Bt toxins.

Abstract:ObjectiveIn the past decades, the surface area of land covered by bamboo (Phyllostachys edulis) in China has rapidly increased. Many studies on bamboo forests have highlighted that bamboo invasion is associated with an increase in the soil pH. The objective of this study was to verify the mechanism by which bamboo invasion increased soil pH as well as to explore the accompanying soil nutrients and microbial properties change trend.MethodA meta-analysis was conducted to test whether the invasions of bamboo into the adjacent forests lead to an increase in soil pH. The changes in soil nutrients and microbial community after the bamboo invasion were also analyzed. In this meta-analysis, 101 sets of data from 42 studies were collected. Besides data from references, 18 sets of data from 12 plots of bamboo invasion zones from experimental plots were also used to analyze soil pH and nutrient change.ResultOverall, the results showed that in all the data sets, 84.9% of the soil pH increased by different amplitude after the bamboo invasion. The magnitude of soil pH increased with invasion time but decreased with soil depth. Also, the increase of soil pH in a coniferous forest(CF)was higher than that in a broadleaf forest (BL). The invasive pure bamboo forest (BB) decreased soil total nitrogen (-15.9%, P < 0.05), nitrate nitrogen (-21.7%, P < 0.05), total organic carbon (-2.0%, P < 0.05), but increased soil available phosphorus (+54.9%, P < 0.05), ammonia nitrogen (+14.7%, P < 0.05) and alkali-hydrolyzable nitrogen (+8.2%, P < 0.05)compared with the original forest. In addition, the bamboo invasion also changed soil microbial community structure and the relative abundance of Actinomycetes was increased (+25.86%, P < 0.05), while that of Acidobacteria (-15.49%, P < 0.05), planctomycetes (-26.66%, P < 0.05) and Bacteroidetes (-22.58%, P < 0.05)was decreased. Based on the meta-analysis results, the ammonia nitrogen was increased while nitrate nitrogen decreased after the bamboo invasion. It could be inferred that the process of ammonification (NH₃+H⁺→NH₄⁺) was improved while nitrification (NH₄⁺+2O₂→NO₃^-+H₂O+2H⁺) was suppressed. Therefore, the possible mechanism of soil pH increase was attributed to the reduction of the accumulation of soil protons released during nitirifcation.ConclusionBamboo invasion into adjacent forests increased soil pH, altered soil chemical properties and microbial community. The effect of bamboo invasion on soil pH is a common phenomenon and the possible mechanism for increasing pH may be related to changes in soil ammonia nitrogen and nitrate nitrogen concentrations.