Soil Organic Carbon Preservation And Enzyme Activity Under Different Straw Returning Practices

With the reduce of organic manure applied in farmland and the increasingpromotion of straw-returning to field, the organic matter, as the basis of soil fertility, isdependent on crop straw inputs. An incubation experiment and a four-year (2008-2012)field experimen were conducted to evaluate the effect of different straw-returning wayson soil organic carbon and seek the optimized straw management under wheat-maizedouble cropping system. The primary results are as follows:The laboratory incubation experiment included two soils (S1: Straw-amended soilwith straw and root residues returned to soil; S2: unamended soil with root residuesreturned to soil) sampled from a seven-season in summer maize/winter wheat rotationsystem. Equal amounts of wheat or maize straws were incorporated into soil and placedon the soil surface to study changes of soil total organic carbon (TOC), labile organiccarbon (LOC), microbial biomass carbon (MBC) and protease (PROT), dehydrogenase(DHA) activities over a60-day incubation. Compared with the treatments those withoutcrop straw unamended, contents of SOC fractions and ratios of LOC/TOC, MBC/TOCand activities of enzymes were enormously increased by adding crop straw regardless oftheir placements in S1or S2; SOC fractions’ contents and enzyme activities in S1weretremendously higher than those in S2; soils with wheat straw amended had highercontents of TOC and LOC than those with maize straw amended; SOC fractions’contents and enzyme activities with crop straw incorporated were higher than those insoil with crop straw placed on soil surface, but the net increase of LOC/TOC wascontrary.The field experiment included3wheat straw-returning ways and3maizestraw-returning ways. The former included straw chopping and direct returning to field(WM), high stubble mulching and returning to field (WH) and no straw returning (WN);The latter included straw chopping and direct returning to field (MC), chopped strawmulching and returning to field (MM) and no straw returning (MN), hence, there were9treatment combinations of wheat/maize straw-returning ways. The results indicated thatTOC and LOC contents and SCS were increased when wheat or maize straw wasreturned to field, the increase extent showed the tendency as follows: double straw-returning> single straw-returning> no straw-returning, of which the WM-MMhad the largest increase of TOC and LOC contents and SCS; the CPMI of WM-MM wassignificantly higher than other treatments in0-20cm．Correlation analysis showed that,it was most effective for CPMI to reflect the sequestration of soil organic carbon in0-30cm.From the above results, we could make the conclusions that method of crop strawincorporated into soil is beneficial to improve contents of each fraction of soil organiccarbon and enzyme activities, and expected to become the best straw-returning waysmeanwhile, the WM-MM is the best straw-returning way which could promote tomaximum extent the organic carbon sequestration. However, the above studies wereconducted under ideal conditions, in which the field experiment study has not yetconsidered the loss of carbon such as carbon dioxide and methane, which is generatedby farm machinery and labor, the change of soil quality and additional agriculturalinputs. Therefore, it’s pending further study to explore the best straw-returning modeconducive to agricultural sustainable development.