Soil nitrogen transformations differ under sedge and bryophyte communities in a subalpine forest on the eastern Tibetan Plateau

Forest floor vegetative communities are shifting in composition under the pressures created by N deposition, invasive species, climate change, and intensive land management; however, our understanding of how different communities affect soil nitrogen (N) cycling remains limited. In this study, we compared soil N availability and N mineralization (Nmin), as well as the underlying microbial community structure and potential extracellular N–degrading enzyme activities under dominant floor vegetative communities (vascular sedges (Carex spp.) and nonvascular bryophytes (Pleurozium schreberi, Hylocomium pyrenaicum and Rhizomnium tuomikoskii)). Soil total N (TN), dissolved N (TDN), exchangeable NH4+ - N concentration and N mineralization (Nmin) rates significantly differed among vegetative communities. Soils under P. schreberi and Carex spp. had similar N transformation, microbial community structure and N – degrading enzymatic activities, and the effects of both were more prominent than those of H. pyrenaicum and R. tuomikoskii. Variation of Nmin among soils with different forest floor plant covers was most related to soil TDN concentration and extracellular N–transforming enzyme activities. Our results suggest that aggregating bryophytes according to their species specific character instead of considering them as a whole is appropriate when evaluating the influence of bryophytes on soil N cycling.