Effects Of Warming And Precipitation Altering On Organic Carbon Turnover In Black Soil Under Residue Return

Residue return directly to the soil has become one of the effective measures for improving soil fertility,carbon sequestration and CO₂ emission reduction in the black soil of Northeast China.Climate（temperature and precipitation）changes directly affect the decomposition rate and fertilization effect of returned residue.Global warming has led to an increasing temperature year by year,accompanied by frequent occurrence of extreme precipitation events.How do the fluctuations in water and heat resources affect the decomposition of residue under different residue return systems（residue covered on the soil surface,RC and residue incorporated into the soil,RI）,thereby affecting soil carbon input,distribution and output?What regulatory role do microorganisms play in this process?However,the current studies on how warming and altering precipitation affect the whole process is limited.Therefore,based on an in-situ open-top chamber（OTC）combined with artificial precipitation,this study simulated the impacts of warming and precipitation changes on plant-derived carbon input（residue decomposition rate and belowground biomass）,soil carbon fractions（physical and chemical fractions）and soil carbon emission traits under different residue return systems.At the same time,the soil microbial metabolic limitation was explored using the enzyme stoichiometry model,and high-throughput sequencing technology was used to clarify the impact of warming and precipitation altering on microbial community dynamics and driving factors under different residue return systems.Finally,structural equation models（SEM）were used to establish the relationships between soil physicochemical properties,microbial metabolic limitation,community structure,soil carbon pool allocation,and carbon emissions,clarifying the mechanism of carbon turnover.The specific results are as follows:（1）The changes and influencing factors of carbon input in soil under different residue return systems.Warming significantly increased the average temperature of 0-10cm soil growth season under RC and RI by 2.18℃-2.41℃and 1.36℃-2.47℃,respectively.Compared with RC（42.69%-51.12%）,the mass remaining of residue under RI（2.94%-5.96%）was significantly reduced.It was found that the increase of warming significantly increased the decomposition rate constant of residue under RC,but had no significant effect on RI.The decomposition rate constant of residue under the two residue return systems was significantly reduced by decreased precipitation.In addition,under the two residue return systems,the belowground biomass of maize was significantly increased by 14.72%-33%and 18.6%-36.1%by warming and interaction of warming and increased precipitation,respectively.Soil temperature and moisture are the main factors controlling residue decomposition rate and belowground biomass.（2）The impact of warming and precipitation changes on different carbon pools and their influencing factors.Warming,altering precipitation and their interactive effect have significant impacts on soil organic carbon（SOC）and physical（particulate organic carbon POC and mineral-associated carbon MAOC）,chemical fractions（labile carbon pool I LPI-C,labile carbon pool II LPII-C and recalcitrant carbon pool RP-C）.The contents of SOC（33.1%-97.96%）,POC（33.73%-68.66%）and MAOC（38.41%-41.42%）in 0-5cm soil layer were significantly increased by interaction of warming and increased precipitation,which was independent of the way of residue returned to the soil.However,the formation mechanisms of POC and MAOC under RC and RI were different.Under RC,total nitrogen（TN）and gene abundance of C-degrading fungi（bgl and cbh I）were the main factors affecting the formation of POC,while dissolved organic carbon（DOC）:SOC（DOC/SOC）directly participated in the formation of soil MAOC;Under RI,the gene abundance of C-degrading bacteria（glu and cel）and DOC/SOC were the driving factor for the formation of POC,while DOC/SOC,TN and C/N promoted the formation of MAOC by affecting the gene abundance of C-degrading bacteria.The response of labile carbon pool I and labile carbon pool II in different soil layers to the change of warming and precipitation mainly depended on residue returning systems,and the influence factors were also different.The RC and RI had no significant effects on the recalcitrant carbon pool,and DOC/SOC and TN were main driving factors.（3）The impact of warming and precipitation changes on microbial metabolic limitation and community structure.The effects of warming and precipitation on soil microbial metabolism limitation and the factors were affected by different residue return systems.Under RC,warming and altering precipitation significantly increased the microbial phosphorus limitation of surface soil,and the available phosphorus（AP）and DOC were the main factors affecting its phosphorus limitation;Under RI,warming significantly reduced the carbon limitation of surface soil,and warming and increased precipitation significantly reduced the carbon and phosphorus limitation of surface soil,among which DOC and available nitrogen（ATN）were the main influencing factors.The trait of microbial metabolic limitation drives the variation of soil microbial communities,but their contribution is smaller than that of soil physicochemical properties.Warming significantly changed theαdiversity and community structure of bacteria and fungi under the two residue return systems.However,there were significant differences in the impact of precipitation change according to different residue return systems,and there is no consistent rule.The dominant factors affecting the community structure of bacteria and fungi under different return systems were significantly different.Under RC,TP content was the main factor affecting the composition of bacterial and fungal communities;Under RI,p H and SOC content were the main factors of the changes bacterial and fungal communities,respectively.（4）The microbial mechanism of soil carbon turnover.Compared to RC,the accumulative CO₂ emissions of RI was significantly increased 23.55%.Warming significantly increased soil carbon emissions under RC and RI,but altering precipitation had no significant effect.There were two ways to control soil carbon emission under RC.The first is that warming promoted the effective use of DOC by microorganisms,significantly increased the gene abundance of carbon-degradation fungi,and soil carbon emission;the second is that the increase of extracellular enzyme activity and microbial carbon limitation were promoted,and then the abundance of carbon-degradation bacteria and soil carbon emissions were increased.Under RI,more P was released into the soil by affecting the effective use of DOC by microorganisms,reducing the microbial phosphorus limitation,increasing the abundance of carbon-degradation bacteria,and increasing soil carbon emissions.Under RC,the turnover of active carbon pools（LPI-C and LPII-C）is mainly controlled by changes in bacterial and fungal community structure,where fungal community structure indirectly affects carbon pool changes by influencing hydrolase and oxidase,but the interpretation rate for inert carbon pools is low.Under RI,the turnover pathway of each carbon pool is mainly dominated by changes in bacterial community structure,β-Glucosidase and peroxidase also have a positive impact on carbon pool changes.Overall,there are differences in the carbon input-distribution-output and microbial regulation mechanisms between the two residue return systems,resulting in different responses to warming and precipitation changes.Among them,the interaction of warming and increased precipitation accelerated the carbon turnover process under the two residue return systems,and promoted the accumulation of soil organic carbon.Compared to RI,RC treatment may be more conductive to soil carbon sequestration,which has a lower decomposition rate,carbon emission reduced and the similar SOC content.