Nitrogen limitation of microbial activity in alpine tundra soils along an environmental gradient: Intra-seasonal variations and effect of rising temperature

Abstract

Alpine tundra ecosystems exhibit one of the most limited availabilities of nitrogen. Furthermore, climate change may potentially stimulate not only soil organic matter decomposition, but also N mineralization; therefore, a better understanding of the response of microorganisms to increasing N availability is crucial for evaluating soil C cycling under warmer climate scenarios. In this study, we suggested that the degree of N limitation of microbial activity in the surface and subsurface soil horizons of a N-poor dwarf-shrub heath (DSH) and an N-rich graminoid meadow (GM) during a single active growing season. We conducted three series of laboratory incubations under natural (+10 °C) and high (+20 °C) temperatures with soil samples collected at the beginning, peak, and end of the growing season to examine the effects of rising temperature and intra-seasonal variations on the N limitation of microbial activity. We measured the contents of microbial C and N, microbial respiration, and the potential activity of peroxidase and β-1.4-glucosidase (enzymes that play a key role in the SOM turnover processes). It was found that the N limitation of microorganism activity could be dynamic and demonstrate temporal (depending on the season) and spatial (depending on the soil horizon) variations. The results disputed the universality of previous conclusions about the high degree of N limitation of microorganisms in alpine tundra soils, thereby suggesting that N is not always a limiting factor for microbial activity, even in the N-poor DSH. We found that warming could increase the N limitation of microorganisms, even in the N-rich GM soil, thereby limiting the effect of warming on the mineralization of soil organic matter. These data demonstrate the importance of further understanding the internal feedback mechanisms that control soil organic matter mineralization and microbial activity for accurately predicting the effects of global climate change in the Arctic.