The Effects Of Soil Erosion Degree And Straw Returning Amount On Microbial Carbon Use Efficiency In Farmland

Soil is the largest reservoir of organic carbon(SOC)in terrestrial ecosystems and an important source of carbon dioxide(CO₂)emissions to the atmosphere.Microbes,as the main drivers of soil carbon,play a key role in the mineralization and fixation of soil SOC,especially the microbial carbon use efficiency(CUE),which largely determines the storage and emission of soil carbon.Therefore,CUE has become the frontier and hot spot of soil carbon cycle research.Although the black soil in Northeast China is fertile and rich in organic matter,due to unreasonable exploitation and use of human beings,serious soil erosion has occurred.In order to control soil erosion,relevant departments introduced some measures such as straw return to the field.So far,although some studies have reported the effects of soil erosion and straw returning on the carbon pool of black soil in Northeast China,the understanding of its internal microbiological mechanism is not thorough enough.Based on this,this study has reported the influence of soil erosion degree and straw returning amount on CUE from the perspective of CUE,and provides an important theoretical basis for effectively curbing the loss of farmland SOC reservoir and promoting soil carbon sequestration.This study collected farmland soil with different erosion degree(ER1:mild erosion;ER2:moderate erosion;ER3:severe erosion;ER4:severe erosion)and different straw returning amount(SR0:control;SR33:33%straw returning;SR67:67%straw returning;SR100:100%straw returning)in Jilin Province,H₂¹⁸O labeling method was used to determine the soil microbial carbon use efficiency,and the methods of variance analysis,pearson's correlation analysis,random forest and structural equation model were used to reveal how different soil erosion degree and different straw return amount affect soil microbial carbon use efficiency.The main findings and conclusions are as follows:(1)The increase of soil erosion leads to the decrease of microbial CUE.High soil erosion mainly reduces microbial CUE by increasing microbial carbon absorption and microbial respiration,thus speeding up soil SOC mineralization,which is not conducive to soil SOC storage.The results of random forest showed that soil erosion significantly affected soil microbial CUE through the following factors:qGrowth>qCO₂>microbial biomass carbon(MBC)>MBC/MBN.Structural equation model(SEM)showed that ER,TC,TN,DON,pH,MBC and?-1,4-glucosidase enzyme activity(?G)could explain 24%variation of CUE.ER has a direct negative impact on CUE and indirectly affects CUE by affecting soil physical and chemical properties.These factors have a comprehensive impact on CUE.(2)The straw returning amount significantly affected the microbial CUE,and the high straw returning amount(SR67 and SR100)decreased the microbial CUE,but the internal mechanism of the influence on the microbial CUE was different in different soil layers.The 0-5 cm soil layer mainly reduced CUE by decreasing the microbial carbon uptake and microbial growth,while the 5-20 cm soil layer decreased CUE by increasing the microbial carbon uptake and microbial respiration.Although high straw returning leads to a decrease in CUE,it does not promote SOC storage.One possible reason is that the contribution ratio of microbial source carbon to SOC is much lower than that of input source carbon of straw returning,so the reduction of microbial CUE under the condition of straw returning will not lead to the decrease of SOC.The results of random forest showed that the straw returning amount significantly affected the soil microbial CUE through the following factors,and the effect intensity was as follows:qGrowth>qCO₂>qUptake>Clay.The results of structural equation model showed that the straw returning had a direct negative effect on CUE and an indirect effect on CUE by affecting soil physical and chemical properties.These factors have a comprehensive impact on CUE.In conclusion,soil erosion degree and the increase of straw returning amount significantly reduce microbial CUE,but there are significant differences in the action mechanism of microbial CUE and the effects on soil organic carbon storage.The degree of soil erosion mainly affects MBC and enzyme activity by affecting physical and chemical properties TC,TN,DON and pH,and then affects CUE.The amount of straw returning mainly affects MBC and enzyme activity through influencing soil physical and chemical properties SOC,SOC/TN and water content,and then affects CUE.However,both treatments are influenced by physical,chemical and biological factors.Although both treatments reduced microbial CUE,the increase of soil erosion accelerated the mineralization of soil SOC,which was not conducive to the storage of soil SOC,while the high amount of straw returning did not affect the storage of soil SOC.This study systematically elaborated the action mechanism of soil erosion degree and straw returning amount on microbial CUE in farmland ecosystem,which provided an important theoretical basis for in-depth understanding of SOC changes in farmland ecosystem.