Effects Of Different Fertilizer Application On Soil Organic Carbon Stability And Microbial Community Structure In Paddy Soils

Soil organic carbon pool is the largest carbon pool in terrestrial ecosystem and plays an important role in global soil carbon cycle.Soil organic carbon（SOC）regulates soil processes by controlling soil structure and function,and provides ecosystem services such as biomass production,biodiversity maintenance,water storage,carbon sequestration and emission reduction.Paddy soil is a type of anthropogenic soil with Chinese characteristics,which is of great significance to food security in China.Fertilization is not only an important farmland management measure to improve soil fertility and crop yield,but also has an impact on soil organic carbon content and stability.Because of the different clay content and clay mineral types,different fertilization measures may have different effects on soil aggregate composition,organic carbon protection and microbial community structure,thus affecting the stability of organic carbon in paddy soil.Soil samples of albic paddy soil and sandy loam paddy soil under four fertilizationmanagements were collected from Liyang（4-year experiment）and Rugao（11-year experiment）of Jiangsu Province respectively.The fertilization treatments included:no fertilization（CK）,local farmer’s fertilizer practice（N 300 kg/hm²,FFP）,optimal fertilization（N 240 kg/hm²,OPT）and the N rate substituted by 20%N with cattle manure based on OPT treatment（CMSN）.Bulk soil of 0-15 cm in surface layer was collected,and soil samples were divided into four particle size aggregates by wet sieving method.The mineral composition and organic carbon spectral characteristics were analyzed by X-ray diffraction and Fourier transform infrared spectroscopy.The stable carbon isotope natural abundance method was used to study the effects of exogenous corn straw（C4）on soil organic carbon mineralization in rice-wheat rotation（C3）paddy soil.High throughput sequencing technology was used to study the distribution characteristics of bacteria and Fungi Community Structure in bulk soil and four aggregates of sandy loam paddy soil,and to understand the effect of Fertilization on soil microbial population and flora.The purpose of this study is to analyze the relationship between the microstructure-organic carbon stability and biological activity of paddy soil aggregates,and to explore the best fertilization scheme,so as to provide scientific basis for the sustainable management of soil organic matter in paddy soil.The results are as follows:（1）Compared with the control,the three fertilization treatments had no significant effect on the stability of aggregates in albic paddy soil,but significantly improved the stability of aggregates in sandy loam paddy soil.The content of silt in albic paddy soil is the most（61.94%-71.79%）,and the content of macroaggregates is the lowest（4.26%-6.32%）.The aggregates of sandy loam paddy soil are mainly microaggregates and silt particles,while the mass fraction of macroaggregates and clay particles is low.CMSN significantly increased the proportion of silt fraction and macroaggregates of sandy loam paddy soil in total soil,and decreased the proportion of microaggregates of albic paddy soil.CMSN increased the content of SOC in macroaggregates and silt components of albic paddy soil and sandy loam paddy soil,FFP and OPT also increased the content of SOC in macroaggregates of albic paddy soil.CMSN increased the total nitrogen content in the microaggregates of albic paddy soil and clay particles of sandy loam paddy soil.The mineral composition of the two paddy soils is similar.The main components of macroaggregates,microaggregates and silt are primary minerals（>50%）,and the main components of clay are secondary minerals（>90%）,The contents of polysaccharides and lignin were higher than that of carbohydrates and alkanes,and the content of aliphatic compounds was the lowest.（2）During the 60-day incubation,the carbon dioxide release rate of corn straw treatmentincreased firstly and then decreased with the increase of culture time,and showed obvious stage characteristics.After 60 days of incubation,the mineralizable carbon content of albic paddy soil and sandy loam paddy soil with exogenous corn straw addition by OPT and CMSN was higher than that of FFP,and the increase of mineralizable carbon content in albic paddy soil was higher than that of control without fertilization.The content of SOC,DOC,POC in albic paddy soil and the content of SOC in sandy loam paddy soil were significantly increased in CMSN.The content of SOC,MBC,POC in albic paddy soil and the content of POC in sandy loam paddy soil were significantly increased in OPT.At the 1st,4th and 15th days of incubation（excluding the 1st day of FFP on albic paddy soil）,the CO₂ released from the soil under different fertilization treatments was mainly from the new carbon.The priming effect of CMSN treatment was low in both soils,especially the negative priming effect in sandy loam paddy soil.（3）Fertilization treatments had no significant effect on theα-diversity of bacterial community structure in the bulk soil of sandy loam paddy soil,but had different effects on theα-diversity of fungal community.Compared with CK,FFP and OPT could improve the diversity of fungal community structure in bulk soil,but CMSN had no significant effect on the diversity of fungal community.FFP,OPT and CMSN significantly increased the Chao1index by 48.80%,44.20%and 46.04%,respectively.The Simpson index of bacteria in the CMSN treatment group was significantly higher than that in the other three treatments.FFP increased Shannon index by 65.10%,while FFP and OPT significantly decreased Simpson index by 70.28%and 59.30%,respectively in bulk soil.At the phylum level,Proteobacteria（27.86%-37.63%）were the dominant bacteria and Ascomycetes（57.92%-95.96%）were the dominant fungi.Compared with CK,the relative abundance of species at each treatment phylum level changed as follows.Under FFP,Patescibacteria increased by 83.94%and Ascomycetes decreased by 4.66%in bulk soil,Mortierellomycota increased by 40.16%in macro-aggregate,Actinobacteria decreased by 37.00%in micro-aggregate.Under OPT,Basidiomycota decreased by 68.62%in macro-aggregate,Ciliophora increased by 99.83%and Ascomycetes decreased by 9.11%in micro-aggregate,Nitrospirae increased by 58.92%in silt.Under CMSN,Basidiomycota increased by 4.53 times,Acidobacteria and Ascomycetes decreased by 20.78%and 3.27%,respectively in bulk soil.Ascomycetes increased by 8.97%,Actinobacteria and Ciliophora decreased by 33.36%and 52.10%,respectively in macro-aggregate.Mortierellomycota increased by 99.83%in micro-aggregate,Mortierellomycota increased by 99.83%and Ciliophora decreased by 33.20%in silt.In conclusion,CMSN treatment can promote the formation of macroaggregates in sandy loam paddy soil,increase the content of organic carbon in macroaggregates in albic paddy soil and sandy loam paddy soil,and increase the content of total nitrogen in microaggregates in albic paddy soil,and help to improve the stability of soil organic carbon.The priming effect of CMSN in albic paddy soil and sandy loam paddy soil is low,especially the negative priming effect in sandy loam paddy soil,which is beneficial to the storage of soil organic carbon.However,the change of soil organic carbon composition and the mechanism of microbial action under long-term fertilization condition still need to be further studied.