Variations in soil microbial community composition and enzymatic activities in response to increased N deposition and precipitation in Inner Mongolian grassland

It has been predicted that the precipitation and atmospheric nitrogen (N) deposition will increase in northern China. Though a variety of publications have documented the effects of increased precipitation and N deposition on aboveground plant community, the responses of soil microbial community composition and function to these possible global changes still remain largely unknown. In this study, we take advantage of a long-term (9-year) field experiment that was established in a typical steppe in Inner Mongolia, China. The responses of microbial community composition and soil enzymatic activities to simulated N deposition and increased precipitation were examined. It was found that the low level of N addition showed no influence on the relative abundance of bacteria, but the relatively high N deposition increased the relative abundance of bacteria. The increased precipitation in combination with N addition, estimated at all N levels, significantly decreased the relative abundance of fungi and fungi/bacteria ratio. Increased precipitation played important roles in enhancing microbial activity in this semi-arid region. It was observed that water supply increased the activities of the five of the enzymes determined, including peroxidase (PER), polyphenol oxidase (PPO), beta-Glucosidase (BG), protease (PRO) and alkaline phosphomonoesterase (alkaline PME). The activity of cellulase was reduced by long-term increased precipitation treatment, but was stimulated by simulated N deposition, which may due to the changes in litter components under projected climate change. The influences of N deposition on the microbial enzymatic activities might be alleviated (such as PER), strengthened (acid PME), or not affected by the projected precipitation increment. We also found that the overall changes in soil enzymatic activity patterns are more sensitive to environmental changes than the variations in soil microbial community composition, which may be explained by the microbial function redundancy and the limitations of research method. By all accounts, in future scenarios of increased precipitation in combination with N deposition, the proportion of soil fungi would be reduced, and the microbial community composition might be shifted to be bacteria-dominant. We also predicted that with the increasing of N deposition level, the activities of cellulolase would be enhanced, whereas the activities of ligninolytic enzymes (PER and PPO) would be reduced sharply.