Effects Of Corn Residue Addition On Microbial Community Structure In Brown Earth Aggregates

Soil organic carbon mainly comes from vegetation residues,root exudates and organic fertilizers,and its transformation processes are regulated by soil microorganisms,which is affected by many environmental factors,such as substrate quality and soil fertility.The quality of exogenous plant residue affects their microbial community structure.Soil fertility influences the nutrient environment of soil microorganisms and will lead to differences of the soil microbial community structure.Therefore,it is of great significance to study how the microbial community structure and its C sequestration capacity in soils with different fertility levels respond to the addition of different quality residues.The study will contribute to clarify the microbiological mechanism of soil C sequestration in farmland.In addition,the soil aggregates provide living environment conditions for soil microorganisms,resulting in differences of the microbial community structure in aggregates.Clarifying the community structure of microorganisms in different soil aggregates is conducive to study the microbial mechanism of soil C sequestration.In this paper,the microbial marker,phospholipid fatty acid（PLFA）,combined with isotopic labeling technology（PLFA-SIP）is used to study the microbial assimilation characteristics of corn residue-derived C in soil aggregates.Mainly,Maize residues（root,stem and leaf）labeled with ¹³C were mixed into brown earth of different fertility levels（high and low fertility level）and cultured in laboratory.Samples were collected on the 1^st,30^th,60^thand 180^thdays of cultivation.After sieving the soil samples according to>250μm macroaggregates and<250μm microaggregates.The total organic C,PLFA content and their ¹³C abundance in the aggregates were determined.The main results obtained were as follows:（1）The response of total soil microorganisms to the addition of plant residues mainly occurred in macroaggregates,while the responses of its community composition were present in both large and small aggregates.For the total amount of PLFA,the addition of corn residues could significantly increase the total amount of macroaggregate PLFA,while the effect on microaggregates was limited.The total amount of macroaggregate microorganisms in high-fertility soil only responded to corn root residues.However,the total amount of microorganisms in low-fertility soil was sensitive to the addition of exogenous organic matter,and has a significant response to the addition of root,stem and leaf residues.For the microbial community composition,in the early stage of the culture experiment（before 30 days）,the addition of maize residues had a limited effect on the composition of soil microbial community structure,while the addition of maize residues increased the proportion of fungal PLFA in the later stage of the culture experiment（after 60 days）.It increased by about 2%,but decreased the proportion of Actinobacterial PLFA by about 5%.（2）The C contribution of maize root residues to PLFA in aggregates was higher than that of stem and leaf residues,but the C content of maize stem and leaf residues assimilated by microorganism PLFAs was higher in soil aggregates.The C in maize root residues was more assimilated by fungi and actinomycetes PLFA,while the C in maize stem and leaf residues was more assimilated by bacteria,because fungi were more likely to assimilate and utilize root residues with high content of refractory substances.While bacteria were more likely to use stem and leaf residues with high content of easily decomposable substances.（3）The ability of soil aggregate microorganisms to assimilate exogenous C under the addition of stem and leaf residues was significantly responsive to soil fertility levels.As demonstrated by low fertilizer promoting Gram-negative and Gram-positive bacteria and arbuscular mycorrhizal fungi PLFA to stem and leaf residue carbon assimilation,high fertility soil promoted the assimilation of stem and leaf residue C by fungi and actinomycetes.However,under the condition of adding root residues,there was little difference in the utilization of exogenous C by the microbial group PLFA between the high-fertility and low-fertility soil aggregates.It could be seen that the response of soil total microbial PLFA content to external environmental factors,such as soil fertility and exogenous C addition in large aggregates.The response of soil microbial PLFA composition was pronounced in both large and small aggregates.The addition of corn residues increased the total amount of PLFA in aggregates,and soil microorganisms were more likely to assimilate C from corn stem and leaf residues.The main group of assimilated stem and leaf residues was bacteria,and fungi were more likely to assimilate the refractory stubble residues.