The Major Drivers,Mineralization And Sequestration Of Organic Carbon In Cultivable Black Soil Of Northeast China

As the largest organic carbon（C）pool in terrestrial ecosystems,minor changes in soil organic carbon（SOC）can affect global soil C stocks and atmospheric CO₂concentration,which in turn will affect soil C cycle and its feedback to climate warming.Furthermore,the dynamics of SOC is the key factor affecting soil fertility and food production in cropland ecosystems.Therefore,knowledge concerning the dominant drivers of SOC mineralization,sequestration and storage is thus crucial for model developments to accurately predict soil C dynamics and its feedback to climate warming,but also contributes to providing scientific basis for sustainable agriculture.As one of the four black soil zones in the world,the northeast black soil zone with a wide region and climate range owns black soils with different physical and chemical properties and SOC contents.In addition,the northeast black soil zone is also an important commercial grain base in China.Therefore,the SOC dynamics of cropland black soil not only affects global climate changes,but also affects soil fertility and health,which plays a crucial role in grain yield.Environmental factors,soil and microbial properties,soil organic matter（SOM）stability,and microbial necromass affect SOC content,but their relative importance to SOC content remains unclear.In addition,as the main source of SOC storage,plant C input also affects SOC mineralization and sequestration in cropland black soil,but the effect of exogenous C input on SOC mineralization and sequestration is still uncertain.In this study,we investigated the key factors of SOC content in cropland black soil,and the effect of exogenous C input amount on SOC mineralization and sequestration and its main microbial mechanisms through a combination of large-scale field sample collection and laboratory incubation experiments.The main conclusions are as follows:（1）At the regional scale,soil microbial residue C（MRC）was the most important factor affecting SOC content,and the contribution of MRC to SOC increased with the increasing of temperature.Temperature is not conducive to the storage of soil MRC and SOC,so that soil MRC and SOC decrease with the increasing of temperature.In addition,soil with lower SOM chemical recalcitrance in cold regions at high latitudes will accelerate the turnover of MRC,thus reducing the contribution of MRC to SOC.Therefore,although cropland soils in cold regions at high latitudes have high MRC and SOC at the regional scale,their SOM chemical recalcitrance is relatively low and easier to be decomposed by soil microorganisms,which is not conducive to the long-term stability of SOC.（2）At the regional scale,soil microbial properties were the main factors driving soil priming effect（PE）.Higher microbial activity and bacterial diversity will accelerate SOM mineralization,but in the case of sufficient exogenous C input,microorganisms will preferentially decompose labile exogenous C and inhibit SOM mineralization,thus reducing soil PE.In addition,SOM stability was also one of the influencing factors of PE in cropland soils along the Mollisol transect,which was mainly showed the inhibitory effect of mineral and aggregate protection on soil PE.（3）Straw addition amount and initial SOC content affect soil microbial biomass and community structure,and then affecting SOC mineralization.Soil fungal biomass,F/B ratio,and percentage of fungi utilizing ¹³C-labeled straw increased with the increasing of straw addition amount,which in turn promoted soil CO₂ emissions and PE.Soil with low initial SOC content promoted PE due to its relatively high F/B ratio,and the increase of F/B ratio was more obvious with the increase of straw addition amount,thus making soil PE more sensitive to straw addition amount.Therefore,straw returning will promote SOC mineralization by increasing soil microorganisms with the increasing of straw addition amount,and this phenomenon is more obvious in soils with lower initial SOC content.（4）Straw addition amount and initial SOC content affect soil microbial activity and metabolism,and then affecting SOC sequestration.The increase of straw addition amount increased soil F/B ratio,and then promoted straw decomposition and the amount of straw C residue,but increased the proportion of straw C loss caused by respiration,thereby reducing soil microbial C use efficiency and SOC formation efficiency.The lower the initial SOC content,the higher the F/B ratio and the amount of straw decomposition,which would increase the residue of straw C in different SOC fractions.Therefore,although straw returning will promote SOC mineralization with the increasing of straw addition amount,it will also promote the sequestration of straw C in different SOC fractions through microbial metabolism to compensate the native SOC mineralization,which is conducive to the increase of SOC storage.In the present study,the research on the major drivers,mineralization and sequestration of SOC reveals the key factors affecting SOC content in cropland of black soil transect,and clarifies the effects of exogenous C input amount on SOC mineralization and sequestration and its microbial mechanisms,thus providing a scientific basis for sustainable agriculture.