Effects Of Green Manure On Carbon Cycle And Microbial Community In Paddy Fields

The return of green manure to the field is a key initiative in the development of green and circular agriculture,which could provide nitrogen and retain or activates some of the nutrients for the subsequent crop to enhanc yields.While earlier studies have focused on the yield-enhancing and soil-amending effects of green manure,the role of green manure in the carbon cycle and its impact on soil microbial communities is becoming increasingly important in green agriculture and soil health.Therefore,in this paper,the carbon sequestration,release and effects on microbial community structure of four leguminous green manures（Broad bean,Pea,Hairy vetch and Chinese milk vetch）and two grass green manures（Barley grass and Ryegrass）during planting,field return and rice cultivation were systematically investigated using stable isotope labelling methods,combined with DNA high-throughput sequencing,DNA stable-isotope probing（DNA-SIP）and phospholipid fatty acid stable-isotope probing（PLFA-SIP）techniques.The main conclusions are as follows:（1）Broad beans accumulated significantly more biomass during cultivation than other crops and released significantly less CO₂during cultivation than other treatments.All green manure crops have a preference for nitrous,resulting in a constant soil NH₄⁺-N content during cultivation and a sharp drop in NO₃^--N content,slowing the loss of N from the soil.The planting of green manure increased the proportion of microorganisms using photosynthetic carbon in the soil and changed the microbial community structure.DNA-SIP combined with high-throughput sequencing results showed that soil microbial diversity was the richest after planting Chinese milk vetch,while the use of root secretions by soil microorganisms was significantly lower in Broad bean than in other green manure crops.The dominant bacteria using photosynthetic carbon were the phylum Actinomycetes（55-75%）Proteobacteria（14-28%）,and the dominant fungi were the phylum Ascomycota（26-51%）and Glomeromycota（0.1-10%）.（2）CO₂ emission rates were lowest with the addition of Chinese milk vetch at the same legume straw concentration returned to the field,but the priming effect of Broad bean and Chinese milk vetch had a hysteresis effect compared to Hairy vetch and Pea.If the green manure plants are removed after planting,the microbial community structure of the soil gradually returns to its pre-plant state,but the return of green manure to the soil leads to a continuous change in the microbial community structure,with the least effect in the case of Chinese milk vetch and the most in the case of Pea,where about 28-39%of the labelled PLFA belong to Gram-positive bacteria.（3）In this paper,the return of green manure to the field did not significantly contribute to rice biomass and yield.During rice growth,the carbon produced by the decomposition of green manure was mainly emitted into the air as CO₂,followed by microbial carbon and soil organic carbon,with only a small amount assimilated into the rice plant.The application of green manure changed the structure of the soil microbial community in the rice root system:in the tillering stage,the dominant bacteria in the rice root system was Gram-positive（11-15%）and in the maturing stage,the dominant bacteria in the rice root system was Gram-negative（20-25%）;the dominant genus using green manure was Gram-negative（17-43%）.In this thesis,the effects of green manure on carbon conversion and soil microbial community structure are measured in order to provide some basic data support and scientific research reference for the selection of eco-friendly green manure with high carbon accumulation,low carbon emission and yield enhancement.