Analysis Of The Microbe-mediated Transformation Of Maize Residue Carbon Into Soil Organic Carbon

Strengthening the research on the dynamics of soil organic carbon(SOC)is of great significance for understanding the carbon(C)cycle in terrestrial ecosystem and estimating the global C balance accurately.In the agricultural ecosystem,long-term intensive farming and traditional agricultural production methods for increasing crop yields have significantly reduced soil fertility and C storage,which has become an urgent problem worldwide.Crop residue retention is an effective measure to replenish SOC and soil microorganisms are the key to transforming plant-derived organic C into stable SOC.Besides,there are complex interactions between soil microorganisms and soil factors(such as SOC content,aggregate composition,etc.)and the input of exogenous organic materials.Thus,the rapid response of the microbial communities to external environments is bound to produce"cascade reaction"to plant residue decomposition.The microbe-mediated plant residue C transformation and SOC formation processes are still unclear.In this study,a 500-day in-situ fled experiment was conducted at the site of an on-going 29-year old fertilizer management.The ¹³C-labeled maize roots and/or shoots were applied into soils with low or high fertility level,and abundances of ¹³C were analyzed in SOC and phospholipid fatty acids(PLFAs)to trace C flow from the decomposing residues into soil by different microbial communities.Meanwhile,the amino sugar biomarkers were analyzed in order to better elucidate the accumulation of microbial residues during maize residue decomposition.In addition,the aggregate classification technology was used to explore the impact of aggregate composition on the microbe-mediated maize residue C distribution process,and clarify the physical protection of soil aggregates on microbial residues(fungal residue/bacterial residue).The main results of this study are as follows:(1)Different groups of microorganisms had different abilities to assimilate maize residue C.Maize residue addition increased the relative proportion of fungal PLFA.Meanwhile,maize residue C contributed the most to the fungal PLFA-C pool(on average of 23.4%)among all microbial communities,indicating that fungi were more sensitive to plant residue input and preferred to assimilate maize residue C than other microbial groups.However,due to the high relative proportions of Gram-positive bacterial and Gram-negative bacterial PLFAs(30.0%and 22.5%,respectively),?50%of the assimilated maize residue C was allocated to bacteria.(2)The process of microbial utilization of exogenous C was affected by the maize residue quality.The contribution of maize root C to microbial PLFA-C pool and the amount of assimilated root C were both higher than that of shoot C.Besides,maize root addition promoted the formation and accumulation of microbial residues more effectively than shoot addition.Meanwhile,maize root addition increased the contribution of microbial residues to SOC pool to a greater extent than shoot addition.Therefore,plant root played a more significant role in enhancing the accumulation of microbial derived organic C than the aerial parts of plants.However,at the end of the experiment,the accumulative residue C mineralization in the maize residue amended treatments were similar(?78%),and the microbial community structure and microbial residue composition were not affected by the maize residue quality.(3)Maize residue addition increased the contribution of microbial residues to soil organic carbon and changed the composition of microbial residues.At the end of the experiment,the contribution of microbial residue to SOC increased in all maize residue amended treatments,indicating that microbial anabolism could produce positive feedback on the input of maize residue.Moreover,the responses of microbial residue composition(characterized by the ratio of glucosamine to muramic acid)to the addition of maize residues varied with different fertility soils:maize residue addition was more helpful to increase fungal residues in the low fertility soil,while it was more conducive to the accumulation of bacterial residues in the high fertility soil.(4)Soils with low initial fertility were more conducive to the accumulation of microbial residues after the maize residue input.Microorganisms in the low fertility soil preferred to utilize maize residue C for anabolism.In addition,the contribution of microbial residue to SOC in the low fertility soil was 28.4%higher than that in the high fertility soil.Meanwhile,the increment of contribution of microbial residue to SOC in the low fertility soil(21.7%)was significantly higher than that in the high fertility soil(13.8%)after maize residue addition,indicating that plant residue retention was an effective measure to improve soil C sequestration potential and organic C quality and stability in the low fertility soil.(5)Microaggregates were the main site for microorganisms to assimilate and sequestrate maize residue C.After 150 days of in-situ experiment,compared with macroaggregates(>2mm and 1?2 mm),microaggregates(0.25?1 mm and<0.25 mm)contained higher maize residue C content,PLFAs content,the ratio of maize residue C in PLFA-C pool and the amount of assimilated residue C,indicating that microaggregates have become the main site for microbial assimilation and residue C sequestration,which was of great significance to the accumulation of stable organic C in microaggregates.(6)Maize residue addition could change the distribution of microbial residues in different aggregates.The addition of maize root was more conducive to the accumulation of fungal residues in macroaggregates,while the addition of maize shoot was more conducive to the accumulation of fungal residues in microaggregates,indicating that the type of organic matter input could greatly affect the role of microorganisms in regulating the distribution of organic C in aggregates.Based on the results of the ratio of glucosamine to muramic acid,the addition of maize root or shoot promoted the enrichment of bacterial residues in microaggregates,indicating that the retention of bacterial residues was more dependent on their combination with clay particles.Totally,this study can provide new evidence for the in-depth understanding of the formation of SOC mediated by the iterative process of microbial metabolism and reproduction-growth-death,and provide new theoretical basis for the regulation mechanism of plant residue quality and soil fertility on plant residue decomposition process,and help to reveal the mechanism of microbial-organic C interaction at the aggregate scale.