Effect of Long-Term Nitorgen Fertilizaiton on Composition and Carbon Utilization of Soil Microbial Communities in a Northern Grassland, China.

Carbon (C) and nitrogen (N) cycling in soil during microbial decomposition is well studied, yet the mechanism underlying the response of microbial C utilization to the presence of N still remains an open question. This study was designed to determine the effect of long-term N fertilization on grassland microbial communities, and to explore if the alteration of labile C utilization of microbial communities was affected by N. A 35-day multi-factorial incubation experiment with three N fertilization rates 0, 4, or 16 g N m-2 yr-1 (applied as urea) and one C substrate application, 0.4 mg 13C glucose g-1 soil was conducted using a temperate grassland soil. A treatment without glucose addition served as a control. Long-term N addition was found to significantly reduce fungal biomass but increased the biomass of gram-positive bacteria. N addition showed to strengthen the role of gram-positive bacteria and actinomycetes, but reduced the contribution of gram-negative bacteria and fungi in 13C utilization. In addition, the dynamics of gram-negative bacteria and fungi in13C utilization during the decomposition process were also regulated by N addition. The functioning of saprophytic fungi in this temperate grassland indicated the functional diversity in this community. Decomposition theories such as nutrient stoichiometry and N mining were both confirmed in this study, as N mining was least prominent in soil with high N enrichment, while optimal nutrient ratio existed when labile C was added in soil under low N enrichment. There was no significant difference of soil C among N treatments in the temperate grassland. It might be due to the tradeoff between microbial C utilization in labile C and SOM.