Response Of Soil Microbial Community Structure And Diversity To Nitrogen Addition And Increased Precipitation On Meadow Steppe

Increasing nitrogen(N) deposition and varied precipitation regimes, is impacting the ecosystem function and structure, especially in the semi-arid grasslands, where N and water availability are limiting factors. However, the general responses of the soil microbial community composition to an increase in N and precipitation and, especially, to their combined effects are highly variable and less synthesized. The meadow steppe as an arid and semiarid ecosystem in northeastern China is a typical vegetation type in the Eurasian grassland zone. Both N and water are important limiting resources in this area. Therefore, this region is particularly sensitive to altered precipitation and N deposition. Moreover, Plant-microbe interactions in soil are an integral component of the influence of plant diversity on ecosystem function. Different plant community type could cause a significant change in microbial communities, and there were different respond strategy of microbes under different plant community type. Therefore, to investigate the mechanism of N and water additions affected the microbial communities, and the relationships among plant-soil-microbes may enhance our understanding on the potential effects of global changes on grassland ecosystem.We used the denaturing gradient gel electrophoresis(DGGE) and phosphor lipid fatty acid(PLFA) methods to evaluate the impacts of increased precipitation and nitrogen addition on soil microbial community structure during two hydrologically contrasting years on Stipa baicalensis meadow steppe, and the importance of precipitation regime in regulating microbial community structure. In addition, a three-year simulated N deposition experiment was conducted in Chloris virgata and forbs communities on L. chinensis meadow steppe. The mainly results are as follow:(1) The dominant bacterial communities were belong to Proteobacterium and Firmicutes on L. chinensis meadow steppe and Stipa baicalensis meadow steppe. Soil microbial biomass carbon, total PLFAs, the ratio of fungi to bacteria and bacterial community diversity changed with the variations of environment conditions. However, the dominant bacterial communities had no obvious change under N and water additions. These results suggested that soil bacterial communities had the capacity of maintain the community stability. Meanwhile, there were strongly correlations between soil bacterial community and plant community, which maintain the grassland ecosystem productivity.(2) On the L. chinensis meadow steppe, N addition increased the soil microbial biomass carbon(SMBC) and soil bacterial diversity. Moderate N(23, 46, 69 kg ha-1 yr-1) addition increased the soil bacterial diversity, whereas excess N(92 kg ha-1 yr-1) addition inhibited it. On the Stipa baicalensis meadow steppe, N and water additions had significant interactive effects on soil microbial community structure. In addition, N effects were covered by water effects.(3) On the L. chinensis meadow steppe, the soil microbial biomass carbon and soil bacterial diversity were related to richness of plant functional groups. In particular, soil microbial biomass carbon had positive correlation with biomass of annuals and biennials, suggesting that the effects of the plant community on the soil bacteria could be explained by a relationship between the soil bacterial community and a subset of plant species rather than all species.(4) In contrast to L. chinensis meadow steppe, the soil microbial community structure was mainly affected by soil characteristics, such as soil water content, available N and soil pH on the Stipa baicalensis meadow steppe. Indirect effects of water increasing and nitrogen enrichment in important soil properties for microbes(e.g. pH) may explain at least in part of the changes in microbial groups.(5) Soil microbial community structure is sensitive to interannual precipitation variation in dry year. The response of soil microbial community structure to the increased precipitation and nitrogen addition were dependent on the precipitation regimes in this meadow steppe.Overall, this study emphasized the importance of plant community type and precipitation regime on soil microbial community structure and bacterial diversity. To identify the effects of N and water additions on soil microbial community and bacterial diversity may enhance our understanding on the potential effects of N deposition and increased precipitation on grassland ecosystem.