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 13C glucose g−1 soil was conducted using a temperate grassland soil. Soil respiration, inorganic N, soil total C (TC) and total N (TN), and 13C-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 13C utilization of gram-positive bacteria and actinomycetes, but reduced 13C utilization of gram-negative bacteria and fungi. Low and high N levels had inconsistent impacts on the temporal patterns of 13C distribution in saprophytic fungi and ratios of incorporated 13C 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.