[Responses of soil microbial carbon use efficiency to short-term nitrogen addition in Castanopsis fabri forest]

As an important parameter regulating soil carbon mineralization, microbial carbon use efficiency (CUE) is essential for the understanding of carbon (C) cycle in terrestrial ecosystems. Three nitrogen supplemental levels, including control (0 kg N·hm^-2·a^-1), low nitrogen (40 kg N·hm^-2·a^-1), and high nitrogen (80 kg N·hm^-2·a^-1), were set up in a Castanopsis fabri forest in the Daiyun Mountain. The basic physical and chemical properties, organic carbon fractions, microbial biomass, and enzyme activities of the soil surface layer (0-10 cm) were measured. To examine the effects of increasing N deposition on microbial CUE and its influencing factors, soil microbial CUE was measured by the ¹⁸O-labelled-water approach. The results showed that short-term N addition significantly reduced microbial respiration rate and the activities of C and N acquisition enzymes, but significantly increased soil microbial CUE. β-N-acetyl amino acid glucosidase (NAG)/microbial biomass carbon (MBC), microbial respiration rate, β-glucosidase (BG)/MBC, cellulose hydrolase (CBH)/MBC, and soil organic carbon content were the main factors affecting CUE. Moreover, CUE significantly and negatively correlated with NAG/MBC, microbial respiration rate, BG/MBC, and CBH/MBC, but significantly and positively correlated with soil organic carbon. In summary, short-term N addition reduced the cost of soil microbial acquisition of C and N and microbial respiration, and thus increased soil microbial CUE, which would increase soil carbon sequestration potential of the C. fabri forest.