Effects of short- and long-term nutrient addition on microbial carbon use efficiency and carbon accumulation efficiency in the Tibetan alpine grassland

Microbial activities and physiological performances can directly affect the use and sequestration of organic carbon (C) in soil. Microbial carbon use efficiency (CUE) and carbon accumulation efficiency (CAE) are critical parameters hypothesized to control C transformation and sequestration in terrestrial ecosystem, and may be influenced by nutrient availability. However, knowledge regarding the influence of short-and long-term nutrient addition on microbial CUE and CAE is lacking, particularly in microbial CAE. This study aimed to investigate the effects of nutrient addition on microbial CUE and CAE and the underlying mechanisms. Here, based on a long-term nutrient addition field experiment on the Qinghai-Tibetan Plateau (QTP), we conducted an incubation experiment with and without the 13C-labeled glucose approach to assess microbial CUE and CAE in the top -(0-10 cm) and subsoil (20-30 cm) collected in the second (short-term) versus tenth (long-term) fertilized year. According to the microbial economic theory, short-term nitrogen (N) addition suppressed oxidase activities in the topsoil, whereas long-term nutrient addition significantly changed hydrolase activities. Furthermore, the short -and long-term nutrient additions had no significant effect on microbial CUE, which may be attributed to the fact that the slightly increased content of C allocated to microbial biomass production was not sufficient to signifi-cantly change the CUE. Microbial CAE only increased with 10-year of continuous N addition, which was mainly due to increasing N availability. We found that the C-acquisition enzymes and clay content dominantly regulated microbial CUE, whereas N availability played an important role in regulating CAE. In conclusion, our results elucidated the differential responses of microbial CUE and CAE to N and phosphorus (P) addition, providing empirical evidence for understanding microbially governed C and their feedback to nutrient interactions in the context of increasing anthropogenic N and P input.