Effects of Long-term Straw Returning on Organic Carbon and Extracellular Enzymes in Paddy Soil Aggregates
[Objective]Soil aggregate organic carbon and extracellular enzymes play an important role in improving soil structure and carbon sequestration,which are easily affected by agronomic management practices.In order to study the effects of long-term straw returning combined with chemical fertilization on organic carbon components and extracellular enzymes in soil aggregates,a 35-year field positioning experiment was carried out.[Method]The field experiment was designed to have three treatments in a rice-wheat rotation system:no chemical fertilizer(CK),chemical fertilizer only(NPK),and straw plus chemical fertilizer(NPKS).The contents of soil organic carbon(SOC)and its labile components(dissolved organic carbon(DOC),easily oxidizable organic carbon(EOC)and microbial biomass carbon(MBC))in soil aggregate were analyzed,as well as the activities of extracellular enzymes related to carbon cycle(β-1,4-Glucosidase(BG),β-1,4-Xylosidase(BX)and β-D-Cellobiohydrolase(CBH)).[Result]The contents of SOC,DOC and MBC in>0.25 mm aggregates were significantly higher than those in<0.25 mm aggregates,and their contents of NPKS treatment were the highest.This showed that NPKS promoted soil macro-aggregates organic carbon regeneration.The values of MBC/SOC and DOC/SOC in each particle size aggregate were relatively stable,which indicated that dynamic change trends of MBC and DOC were consistent with that of SOC.Thus,MBC and DOC could be used as sensitive indexes to evaluate soil organic carbon.The 2-0.25 mm aggregates were the main carriers of extracellular enzymes,and their activities in NPKS treatment were the highest.However,the enzymes activities in>2 mm aggregates had no significant difference among treatments.The soil organic carbon components and extracellular enzymes in soil aggregates promoted each other,in which the contents of SOC,DOC and MBC in aggregates were mainly affected by CBH,followed by BG;while EOC was only positively affected by CBH.These two extracellular enzymes(CBH and BG)could promote soil organic carbon turnover and the interaction was enhanced in 2-0.25 mm aggregate.[Conclusion]Long-term application of straw returning combined with chemical fertilizer could increase organic carbon regeneration and turnover rate,and increase soil organic carbon content,which is an important agronomic way for sustainable carbon sequestration in paddy soil.
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