Effect Of Straw Return On Soil Organic Carbon Pool In Cropland Of Purple Soil

Agricultural carbon cycle is an important part of global carbon cycle.Soil organic carbon（SOC）content and its compositions are important indexes to indicate soil fertility and carbon stock.How to increase soil carbon storage to realize carbon sequestration and CO₂ emission reduction is a current research hotspot.Straw returning is generally considered as an effective cropland management to improve soil fertility and increase soil carbon stock.However,the long-term different amounts of straw returning may have positive or negative effects on soil carbon stock,different soil carbon components,soil respiration,carbon temperature sensitivity(Q₁₀)and soil microbial community structure.Therefore,in order to explore the long-term response of soil organic carbon stock and its stability on straw returning to the field,this research taked purple soil,the most important agricultural soil resource in Southwest China,as the research object.There were four treatments,including none straw returning treatment and with nitrogen,phosphorus and potassium fertilization（NPK treatment）,NPK-100%straw returning treatment（MW1 treatment）,NPK-50%straw returning treatment（MW2 treatment）and NPK-30%straw returning treatment（MW3 treatment）.In order to find out the effect of different straw returning on soil organic carbon pool and stability we mainly investigated the SOC content and stock,different organic carbon compositions,the soil respiration in the field for one year,and the temperature sensitivity of soil organic carbon decompositions.In addition,the soil microbial community structure was investigated through laboratory test.The main results and conclusions were as follows:1.S traw returning reduced soil bulk density,and increased SOC content,stock and total nitrogen（TN）content of purple soil.Compared to NPK treatment,SOC content and stock in MW1 treatment were increased by 30%and 26%,and by 30%and 15%in MW3 treatment,respectively.MW3 and original sample had no significant difference,MW1 and MW2 treatment increased by 55%and 50%,MW3treatment and NPK treatment had no significant difference.2.Straw returning increased soil microbial carbon（MBC）and soil dissolved organic carbon（DOC）by 22%～52%and 35%～41%respectively,among which MW2 treatment had the largest soil MBC and DOC content,but MW2 treatment had the lowest soil POC content of,which was 22%lower than that in MW1 treatment.Straw returning treatment increased the aggregate content of>2mm and decreased the aggregate content of 0.25～2mm,among which MW2 treatment had the highest aggregate content of>2mm,which was 28%,and the average weight diameter（GWD）and average geometric diameter（MWD）of aggregate were the largest,which were 1.91 and 0.76 respectively,which significantly increased the stability of soil aggregate.Straw returning also increased soil aggregate organic carbon content,soil aggregate organic carbon contents of>2mm and 0.25～2mm in NPK and MW3treatmentwere of no significant difference,but they were significantly lower than those in MW1 and MW2 treatment.Compared with NPK treatment,organic carbon in MW2 treatment were increased by 20%,17%and 14%respectively in 0.25-2 mm,0.053-0.25 mm and<0.053 mm soil aggregates,which were higher than those in m MW1 and MW3 treatment.3.The amount,diversity and structure of soil bacterial and fungal communities were affected by straw returning.The highest Chao1 index of soil bacteria and fungi was in MW2 treatment.The community structure of bacteria in MW1 treatment was similar to that in MW2 treatment,but different from that in MW3 treatment.The fungal community structure of fungi in MW3 treatment was significantly different from that in other treatments.The fungal community structure in MW1 and MW2treatment were similar to that in NPK treatment.4.Results of field observation showed that the change trends of soil heterotrophic respiration were same,and the emission fluxes was small.The amount of straw returned to the field affected the heterotrophic respiratory fluxes of soil.The annual cumulative respiration fluxes in NPK treatment was 8185.79 kg C ha^-1,which was 28%and 54%higher than that in MW1 and MW2 treatment,respectively,and17%lower than that in MW3 treatment.The maximum Q₁₀ of heterotrophic respiration was in MW2 treatment,which was 2.33.5.Different amount of straw returned to the field affected the cumulative emission fluxes of soil respiration and the response of different organic carbon compositions to incubation temperature.Except for 25℃,NPK treatment had the lowest cumulative emission fluxes of soil respiration,MW2 treatment had the largest emission fluxes of 325.90 kg C ha^-1 at 10℃in incubation experiment,MW1treatment had the largest emission fluxes of 392.94,229.95 and 392.95 kg C ha^-1 at20,25 and 30℃in incubation experiment,respectively.In incubation experiment,the sensitivity of soil respiration temperature Q₁₀ value in MW3 treatment was the highest,which was 1.08,and that in MW2 treatment and NPK treatment were the lowest,which were both 1.01.The content of soil POC in NPK treatment soil did not decrease after incubation at all temperatures.The content of soil POC in MW1 and MW2 treatment decreased both by 16%at 10℃and 25℃respectively.Except for MW3 treatment,the content of soil POC in other treatments increased after incubation at 30℃.The content of soil DOC in MW2 treatment was decreased by13%,21%and 59%at 5℃,10℃and 30℃respectively The soil MBC content in MW1 and MW2 treatment were increased by 280%and 94%respectively at 15℃,while that in NPK treatment was increased by 100%at 25℃,and there was no significant difference at 15℃.6.There were significant positive correlation between the amount of straw returning and the contents of DOC,POC and fungi quantity.The Q_10cvalue of soil heterotrophic respiration and the stability of soil aggregate were mainly affected by the content of soil MBC and DOC.In conclusion,MW2 treatment was the best management to stabilize SOC.Long-term 50%straw returning to the field could not only increase the SOC content and stock,improve the stability of soil aggregate structure and soil aggregate organic carbon content,but also improve the number and diversity of microbial community and reduce the CO₂ emission of soil.In addition,SOC was easier to decompose in the soil under high temperature,so as to provide better carbon source for plants and play a better role in maintaining the stability of soil carbon pool.