Microbial Regulatory Mechanism Of Organic Carbon Transformation Under Soil Redistribution In Loess Region

Globally,an estimated one to five billion tons of soil organic carbon（SOC）are redistributed annually due to natural（water and wind erosion）and anthropogenic（terrace building and engineering accumulation）factors.Soil redistribution,characterized by subsoil exposure and topsoil addition,strongly influences soil environmental factors（soil water,soil temperature and soil nutrients,etc.）and leads to soil microbial differentiation,ultimately impacting SOC conversion and accumulation.However,the microbial mechanisms governing SOC conversion in soil redistribution remain unclear.In this study,we conducted a five-year simulation experiment（2015-2019）in the Loess Plateau,addressing the following objectives.（1）Comprehensively evaluation was conducted to explore the impact of soil redistribution（subsoil exposure+topsoil addition）on soil-plant ecosystem carbon pool by analyzing the microbial necromass carbon content and the composition and temporal dynamics of organic carbon pool,and plant carbon pool.（2）In order to explore the effect of soil redistribution on SOC quality,comparatively analyses were conducted on the differentiations in soil organic matter functional groups composition between subsoil exposure and topsoil addition plots.（3）The couplings between SOC mineralization and microbial r-K trophic strategy in subsoil exposure and topsoil addition plots were revealed through analysed the shift of r-K strategy in soil bacterial/fungal communities and variations in C-acquiring enzymes（polyphenol oxidase andβ-D-xylosidase）activities.（4）To reveal the regulatory mechanisms of microbial trophic strategies on SOC priming effect,the response of SOC priming effect to root exudates(δ¹³C glucose)input were investigated in subsoil exposure and topsoil addition plots,respectively.Main results are as follows:（1）Soil redistribution exerted a significant influence on the dynamics of the soil-plant ecosystem carbon（C）pool.In comparison to undisturbed soil,the SOC pool and plant C pool in subsoil exposure plot decreased by 8%-11.2%and 15.8%-69.3%,respectively,but increased by 11.7%-14.1%and 10.1%-15.5%in topsoil addition plot.In 2015,the combined soil redistribution system（subsoil exposure+topsoil addition）displayed a 2.9%increase in the SOC pool and a 53.8%decrease in the plant C pool,resulting in a 10.3%reduction in the soil-plant ecosystem C pool（the sum of the SOC and plant C pools）.By the conclusion of the experiment in 2019,the soil redistribution system’s SOC pool and plant C pool had increased by 3.8%and decreased by 5.8%,respectively,while the soil-plant ecosystem C pool showed a 1.9%increase.This shift suggests that the soil redistribution system transformed from a“carbon source”to a“carbon sink”.Key factors contributing to the rise in the soil-plant ecosystem C pool include the incorporation of fungal necromass carbon into the SOC pool and the restoration of the plant C pool in subsoil exposure plots.（2）Notable disparities in soil organic carbon（SOC）quality were observed between subsoil exposure and topsoil addition plots.The relative content of C=C in the subsoil exposure plot was prominently increased by 8.8%-14.6%compared with that in the undisturbed soil,while that in the topsoil addition plot decreased by 6.1%-9.7%.In contrast,the relative content of C=O,COO and C–H in the subsoil exposure plot was prominently decreased by 18.6%-21.2%,5.4%-20.4%and 33.9%-50.5%,respectively,while the relative content of COO increased by 15.6%-25.5%in the topsoil addition plot.Furthermore,the relative content of low quality SOC（C=C/COO and C=C/C=O）in the subsoil exposure plot increased by 14.8%-44.2%and 33.9%-45.4%compared with that in the undisturbed soil,while that in the topsoil addition plot decreased by 18.6%-28.1%and3.8%-11.2%.（3）Significant disparities were observed in soil microbial community composition and trophic strategy between subsoil exposure and topsoil addition plots.The relative abundance of Bacteroidetes（r-strategists）in subsoil exposure plot was significantly decreased by 26%compared with undisturbed soil,while that in topsoil addition plot was significantly increased by 49%.Conversely,Actinobacteria,Chloroflexi and Basidiomycota（K-strategists）in subsoil exposure plot was prominently increased by 21%,15%and 81%,respectively,whereas that in the topsoil addition plot decreased by 6%,15%and 28%,respectively.The ratio of K-strategists to r-strategists in the subsoil exposure plot increased by 4.3%-32.6%（Bacterial community）and 54.6%-162.0%（Fungal community）,however,the ratio of K-strategists to r-strategists in the topsoil addition plot markedly decreased.This suggests that bacterial and fungal communities in subsoil exposure plot tended towards a K-trophic strategy,while the bacterial community in topsoil addition plot leaned towards an r-trophic strategy.The interspecific relationship of bacterial and fungal network structures in topsoil addition plot is more complex,with a relatively stable community structure.Additionally,subsoil exposure diminished the metabolic function of carbohydrates,lipids,and amino acids in bacteria but increased the relative abundance of saprophytic and symbiotic fungi.The content of SOC and functional group composition may constitute the primary factors influencing the differences in bacterial and fungal community structure and metabolic function.（4）Subsoil exposure and topsoil addition exhibited contrasting effects on C-acquiring enzyme activity,soil respiration,and temperature sensitivity(Q₁₀).In comparison to undisturbed soil,the activities ofβ-D-xylosidase and cellobiohydrolase in subsoil exposure plot prominently decreased by 26.4%-29.1%and 24.3%-56.3%,and the activity of polyphenol oxidase markedly increased by 6.5%-12.2%.Enzyme activities in topsoil addition plot exhibited an opposing trend.Furthermore,the annual cumulative soil respiration in the subsoil exposure plot decreased by 10.2%-37.7%and increased by2.2%-19.4%in topsoil addition plot.However,Q₁₀ showed the opposite trend,with a significant increase in subsoil exposure plot and a marked decrease in topsoil addition plot.The low-quality SOC in subsoil exposure plot encouraged bacterial and fungal communities to adopt K-trophic strategies and secrete more active polyphenol oxidase to degrade stable SOC,thus significantly increasing Q₁₀.On the contrary,the high-quality SOC in topsoil addition plot promoted bacterial communities to lean towards an r-trophic strategy and secrete higher activityβ-D-xylosidase to mineralize high-quality SOC,consequently reducing Q₁₀.（5）Following the input of fresh organic matter,the soil in subsoil exposure plot displayed a more pronounced priming effect compared to that in topsoil addition plot.Cumulative apparent CO₂ emissions increased by 77%and 178%for low sugar and high sugar addition,respectively,in subsoil exposure plot,while in topsoil addition plot,these values only rose by 35%and 107%,respectively.Throughout the cultivation process,sugar addition in subsoil exposure plot exerted a positive priming effect on SOC mineralization,with CO₂ release from the priming effect intensifying as the quantity of glucose added increased.Under high sugar addition conditions,topsoil addition plot exhibited a negative priming effect during the initial 4 and 8 hours,later transitioning to a positive priming effect as glucose was consumed.The degradation of SOC by bacterial and fungal communities adopting a K-trophic strategy in subsoil exposure plot may be the critical factor contributing to the positive priming effect.However,the preferential utilization of fresh organic matter by the bacterial community employing an r-trophic strategy in topsoil addition plot may induce a negative priming effect.This study comprehensively analyzed the transition of the soil-plant ecosystem carbon pool in soil redistribution system from lower than undisturbed soil to higher than undisturbed soil on a time scale.This revealed the soil redistribution system transitioning from being a“carbon source”to a“carbon sink”.K-trophic strategy microbial community in subsoil exposure plot and r-trophic strategy microbial community in topsoil addition plot significantly affected the Q₁₀ by secreting different extracellular enzymes.Furthermore,the study revealed the SOC priming effect dominated by different microbial trophic strategies in subsoil exposure and topsoil addition plots during vegetation restoration.These results contribute to our understanding of the biogeochemical cycle of SOC under soil redistribution.This study preliminarily investigated the microbial mechanism of SOC conversion in anthropogenic soil redistribution.However,soil redistribution induced by natural factors（water erosion and wind erosion,etc.）should also be of concern.The coarsening of soil texture in natural erosion area and the discrepancy of soil particle size at different levels in deposition area may significantly alter soil environmental factors（soil water content,soil temperature,and soil nutrients,etc.）,thus affecting the organic matter input and the SOC conversion induced by soil microbes,which should be paid more attention in the future.