Composition and carbon utilization of soil microbial communities subjected to long-term nitrogen fertilization in a temperate grassland in northern 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 C-13 glucose g(-1) soil was conducted using a temperate grassland soil. Soil respiration, inorganic N, soil total C (TC) and total N (TN), and C-13-phospholipid fatty acids were measured. High N fertilization rate (16 g N m(-2) yr(-1)) increased soil inorganic nitrogen (ION) significantly and resulted in a significant drop of soil pH, which decreased from a neutral (pH 7) to pH 5.8. Long-term N fertilization caused an increased C-13 utilization of gram-positive bacteria and actinomycetes, but reduced C-13 utilization of gram negative bacteria and fungi. Low and high N levels had inconsistent impacts on the temporal patterns of C-13 distribution in saprophytic fungi and ratios of incorporated C-13 in cyclopropyl to its precursor during the course of the decomposition. Decomposition theories such as 'nutrient stoichiometry' and 'N mining' were both supported in this study, as N mining was least prominent in soil with high N fertilization rates, while optimal nutrient ratio existed when labile C was added in soil under low N level. N fertilization in the temperate grassland might regulate the shift in labile C and SOM between microbial C utilization. To further understand the coupling of soil C and N, future work should focus on the beginning of the decomposition process, and increase the sampling frequency.