Distribution And Transformation Of Straw Carbon In Soil Aggregates Under Plastic Film Mulching And Fertilization

Many studies have carried on the effect of plastic film mulching?PFM?and fertilization on the dynamic changes of soil organic carbon?SOC?,and have made great progress on the SOC turnover and transformation.However,few studies have reported how both agricultural management techniques affect SOC components and how newly-added maize straw carbon?C?is stored and transformed in total soil and different aggregate size fractions.Therefore,clarifying the distribution of fresh C sources and the mechanisms of heir transformation and agglomeration in soil with straw returning in the northeastern region is of great significance for restoring and improving the organic matter and fertility of soil.In this study,an in situ¹³C-tracing technique was used to identify the dynamic distribution and accumulation patterns of crop residue-derived C in soil aggregates over a 360-day incubation period as affected by a long-term?i.e.27-year?application of PFM and four fertilization treatments?control,CK;nitrogen,N;organic manure,M;nitrogen and organic manure,MN?.Soil samples were separated into four particle-size fractions:large aggregates?>2 mm?,medium aggregates?1-2mm?,small aggregates?0.25-1 mm?and microaggregates?<0.25 mm?by dry-sieving.The contents of total SOC,water-soluble organic C?WSOC?,particulated organic C?POC?,microbial biomass C?MBC?and straw-derived C in soil and aggregates were determined,and the dynamical changes of added straw-derived C components and its transformation in different aggregates of soil were discussed.The main findings of this study are as follows:?1?PFM promoted the accumulation of added straw-derived C in larger soil aggregates.The results showed that dynamic changes of added straw-derived C components were greatly in two months after incubation,and the distribution of them was more in the large aggregates while as was less in the microaggregate.Under PFM conditions,the higher content of added straw-deried C in large soil aggregates indicates that PFM accelerates the decomposition of exogenous C and dereases its accumulation in soil microaggregates,which means that PFM is beneficial to the accumulation of organic C in large aggregates of soil.?2?PFM increased the proportion of straw-derived C in the water-soluble fraction and improved the quality of SOC.The proportion of straw-derived C in the water-soluble fraction(WSOC_straw)to total WSOC(WSOC_total)was higher at the initial incubation period under PFM conditions?6.7%?compared to no PFM conditions?5.1%?and specifically higher in microaggregates?8.4%?,which also indicates that microaggregates are more conducive to decomposing microbial activities and accelerating the decomposition and transformation of exogenous C.?3?Soil microaggregates were beneficial to the accumulation of POC and could promote the formation of POC from added organic C.The results showed that the contents of straw-derived C in POC(POC_straw)and total POC(POC_total)in total soil samples showed a decreasing trend over time.After 360 days of in situ ¹³C-straw incubation in the field,the proportion of POC_straw in POC_total was consistently higher in microaggregates than that of other particle-size fraction,indicating that the internal conditions of soil are beneficial to the decomposition of organic C and the formation of POC in microaggregates.?4?Organic manure increased the contents of soil MBC,and PFM promoted the transformation of exogenous C into the microbial biomass.Under M and MN treatments,the contents of MBC were significantly higher?P<0.05?compared with the CK and N treatments in 60 days after incubation.The contents of straw-derived C in the microbial biomass(MBC_straw)increased by 93%and 28.6%in the M and MN treatments,respectively.These results suggested that PFM and organic fertilizers could promote the transformation of fresh C sources into the microbial biomass through biotic and abiotic actions,and finally improve soil fertility.