The Relationship Of Microbial Community Structure And Activity To Carbon And Nitrogen Transformations In Paddy Soil

As a unique artificial wetland, paddy soil experiences frequent drying-rewetting cycle throughout the growth of rice. To understand the relationship of microbial community structure and activity to carbon and nitrogen transformations in paddy soil is very important to assess the contribution of microbial carbon fixation and improve rice production. Based on phospholipid fatty acid (PLFA)-SIP, MicroRespTM and high-throughput sequencing etc. techniques, we investigated the effect of storage condition on microbial activities and community structures in flooded and drained paddy soil, then determined the impacts of straw, black carbon, urea-C, CO2 and root rhizodepostion on soil microbes.One paddy soil was pre-incubated to maintain two statuses:flooded and drained (60% water holding capacity). Various biochemistry indices were determined after soils were stored at different conditions. Results showed that nitrate concentration increased significantly in all treatments while total PLFA amount did not changed significantly across all treatments except for the flooded soil stored at 4℃. For drained soil, ammonium concentration and basal respiration increased significantly after storage but recovered after 7-day re-incubation. However, no significant difference of ammonium concentration and basal respiration after the treatment of storage was detected for flooded soil. Storage condition significantly affected soil microbial functional structures in both flooded and drained soil but no significant difference on microbial community structures. In particular, the microbial a-diversity in flooded paddy soil was higher than that in drained soil, and species richness of flooded soil and evenness of drained soil were enhanced in all storage treatments. According to phylum and class analysis, a larger shifts of microbial population composition occurred in flooded soil than drained soil.Two different carbon sources (0.2% straw,1% straw,0.5% black carbon or 2.5% black carbon) were added to paddy soil and planted rice in pots. Soil was sampled to determine the transformation of C&N in paddy soil and their microbial metabolism profiles. Results indicated that both straw and black carbon affected C&N transformation in paddy soil and enhanced the production of rice. Microbial metabolism profiles, which were identified by MicrorespTM, suggested that the utilization of alanine, glucose, fructose and protocatechuic acid by microbes increased significantly with the amount of straw and black carbon addition. In short, black carbon was more conducive to improve yield of rice. In contrast, the effect of straw on microbial biomass carbon and net carbon mineralization was significantly higher than that of black carbon. These suggest that a high concentration of black carbon was greater beneficial to enhance the rice yield and carbon sequestration in soil than applied straw to field directly.Four agriculture soils (pH 3.9～7.8) were selected and applied with 13C-labeled urea. Significant hydrolysis of urea occurred within 2 h and less than 2% of urea-C was retained in the soil except for the fluvo-aquic soil at pH 7.8 with high urea-N application after 3-day incubation. Results of principal component analysis (PCA) indicated that soil physicochemical properties rather than urea addition and incubation time determine soil microbial community composition. Moreover, the scores of the second principal component value were significantly correlated with pH values. The total enrichment of !3C-urea to PLFAs increased with soil pH, this may be related to increases in the speciation of inorganic C into bicarbonate at high pH. Labeled profiles suggest the microbial urea-13C utilization not only be affected by soil pH, but also land use types.Different microbial inorganic carbon fixation pathways assimilate different formation of carbon. We selected six paddy soils to conduct a 13CO2 incubation experiment. The results showed that soil 13C retention amount was significantly correlated with soil pH and increased with incubation days, however, the total PLFA-C amount was not affected by incubation time and significantly correlated with soil organic carbon. Correlation analysis found the effect of soil pH on microbial community structure, while the profile of labeled 13C-PLFA was affected by soil organic carbon and ratio of labeled 13C-PLFA was significantly negatively correlated with soil organic carbon. In summary, microbe utilized inorganic carbon were affected by various soil properties, such as pH, organic carbon content and microbial communities etc..In order to investigate the effect of nitrogen fertilizer application on microbes utilized root exudate, we chose three nitrogen application levels (0,100,200 mg kg-1 N) in a rice planted system and labeled rice plant with continuous 13CO2. On the one hand, nitrogen application increased rice biomass and enhanced the δ13C value of rice plant stem, root and planted soil, on the other hand, nitrogen addition did not affect the microbial biomass and community structure while planted rice enhanced microbial biomass and significantly changed microbial community structure. The percentage of 13C labeled PLFA was very low and no significant difference between treatments with or without nitrogen fertilizer addition in unplanted soil. For planted treatments, nitrogen application significantly enhanced total 13C labeled amount and changed the labeled PLFA profiles. Our results suggested that nitrogen addition significantly increased rice biomass and their root rhizodepostion, which stimulated the growth of soil microbes. Fungi and Gram negative bacterium were the main microorganisms using rice root exudates.