ALBERT

Description: Subalpine grasslands are highly seasonal environments and likely subject to strong variability in nitrogen (N) dynamics. Plants and microbes typically compete for N acquisition during the growing season and particularly at plant peak biomass. During snowmelt, plants could potentially benefit from a decrease in competition by microbes because of greater plant N uptake associated with active growth and freeze-thaw cycles restricting microbial growth. In managed subalpine grasslands, we expect these interactions to be influenced by recent changes in agricultural land-use, and associated modifications in plant and microbial communities. At a subalpine grassland site in the Central French Alps, a pulse of 15N was added to the soil at the end of snowmelt, allowing us to compare the dynamics of inorganic N uptake in plants and microbes during this period with that previously reported at the peak biomass in July. In all grasslands, specific plant (per g of biomass) dissolved inorganic N (DIN) uptake was two to five times greater at snow-melt than at peak biomass, whereas the specific microbial DIN uptakes were similar between the two sampling dates. On an area basis, plant communities took more DIN than microbial communities at the end of snowmelt, and the intensity of this DIN uptake by plants differed across land use types. Consequently, N partitioning after snowmelt switches in favor of plant communities allowing them to support their growing capacities at this period of the year. Seasonal differences in microbial and plant N-related dynamics were also affected by past (terraced vs. unterraced) rather than current (mown vs. unmown) land use. In terraced grasslands, microbial biomass N remained similar across seasons, whereas in unterraced grasslands, microbial biomass N was higher and microbial C : N lower at the end of snowmelt as compared to peak biomass. Further investigations on microbial community composition and their organic N uptake dynamics are required to better understand the decrease in microbial DIN uptake.