CH₄-and N-cycling Related Microbial Communities In Paddy Soils With Different Paddy Ages

Paddy soil, characteristic of unique profile and physical-chemical properties, can be developed from different parent materials after long-term rice cultivation. Previous studies showed that the effects of paddy age on soil properties were mainly on the accumulation of soil organic matter in plough layer and the differentiation of vertical layers. Paddy field is one of the most important anthropogenic methane sources and the growth of rice is mainly controlled by the content of nitrogen. Both methane and nitrogen cycles are mediated by soil microorganisms. However, relationships between these key functional microbes and paddy age are still unclear.Therefore, three kinds of paddy soil with different rice cultivation ages (15year,30year and100year) from the same geological site were chosen and the succession of methane-and N-cycling related microbes were explored.Firstly, we systematically investigated the methane production process and the response of related microbial community to paddy age by applying laboratory anaerobic incubation. Methanogenic community structures were established in the early stage of paddy soil development and remained relatively stable in further soil development and maturation. Increasing of soil organic carbon and substrate availability with prolonged use of paddy soils enhanced the activity and increased total methanogen population size, but had little effects on the community composition. For bacterial populations, Acidobacteria Gp1and Anaeromyxobacter decreased with the increase of paddy age, which were probably caused by the increase of soil nutrient level and the loss of available iron during soil maturation. Nonetheless, succession patterns of bacterial communities in the three soils were similar during the incubation, e.g., Firmicutes (Clostridium sensu stricto), the dominant group in original soils were replaced by Acidobacteria (Gpl and Gp3), Proteobacteria(Anaeromyxobacter and Geobacter), Cloroflexi (Bellilinea and Longilinea) and Clorobi (Ignavibacterium) in the later phase. Abundances of microbes capable of decomposing resistant compounds like propionate and aromatic compound increased at the end of incubation.In addition, fresh soils from different vertical layers of the three paddy soils and one upland soil located in the same geological site were collected to determine methanogen and methanotroph communities in situ. Results showed that the community structures of these microorganisms had been established and certain scale was formed at early stage of the development of paddy soil. The vertical distribution region extended deeper with the increase of paddy age. The unclassified Methanosarcinales (include ZC-I) might be indigenous methanogens of paddy soil and mainly inhabit below the plough layers. Type Ib methanotroph were better adapted to long term paddy management. While dynamics of N-cycling related microorganisms were more complicated. Ammonia oxidizing archaea rather than bacteria were more abundant in acid red soil and paddy soil. Nitrogen fixation populations were diverse and distinctive among different soil samples. nirK-type denitrificans were mainly distributed in upper layers within30cm, whereas nosZ-type denitrificans were commonly detected in all soil layers. Succession patterns of microbial populations based on the analysis of functional genes (e.g. amoA, nifH, nirK and nosZ) indicated that the relationship between paddy age and related microorganisms was rather complicated and further investigations are needed.In summary, methane related microbial populations had been established and formed to a certain scale at the early stage of paddy soil development, and remained relatively consistent in the further maturation of soil. To the contrary, responses of N cycle related functional microorganisms turn out to be much more complicated and deserve further investigations. And the effects of land use change were more significant than paddy age on N cycle related microbial populations. These results extend our understanding of paddy soil methane and N cycle and would facilitate the management of rice soils and methane emission.