Comparison of inorganic nitrogen uptake dynamics following snowmelt and at peak biomass in subalpine grasslands

[摘要] Subalpine grasslands are highly seasonal environments and likely subject tostrong variability in nitrogen (N) dynamics. Plants and microbes typicallycompete for N acquisition during the growing season and particularly atplant peak biomass. During snowmelt, plants could potentially benefit from adecrease in competition by microbes, leading to greater plant N uptakeassociated with active growth and freeze-thaw cycles restricting microbialgrowth. In managed subalpine grasslands, we expect these interactions to beinfluenced by recent changes in agricultural land use, and associatedmodifications in plant and microbial communities. At several subalpinegrasslands in the French Alps, we added pulses of ¹⁵N to the soil atthe end of snowmelt, allowing us to compare the dynamics of inorganic Nuptake in plants and microbes during this period with that previouslyreported at the peak biomass in July. In all grasslands, while specificshoot N translocation (per g of biomass) of dissolved inorganic nitrogen(DIN) was two to five times greater at snowmelt than at peak biomass,specific microbial DIN uptakes were similar between the two sampling dates.On an area basis, plant communities took more DIN than microbial communitiesat the end of snowmelt when aboveground plant biomasses were at least twotimes lower than at peak biomass. Consequently, inorganic N partitioningafter snowmelt switches in favor of plant communities, allowing them tosupport their growing capacities at this period of the year. Seasonaldifferences in microbial and plant inorganic N-related dynamics were alsoaffected by past (terraced vs. unterraced) rather than current (mown vs. unmown)land use. In terraced grasslands, microbial biomass N remained similaracross seasons, whereas in unterraced grasslands, microbial biomass N washigher and microbial C : N lower at the end of snowmelt as compared to peakbiomass. Further investigations on microbial community composition and theirorganic N uptake dynamics are required to better understand the decrease inmicrobial DIN uptake.

[学科分类] 地球化学与岩石