Warming may cause substantial nitrogen losses from subarctic grasslands

High-latitude soils are particularly vulnerable to temperature-driven C losses and may contribute substantially to the increasing atmospheric CO₂ concentrations. The magnitude of their contribution is, however, uncertain, and largely dependent on the interactions between C and nitrogen (N) biogeochemical cycles, soil microbial activities and the feedbacks between plants and soil microbes. Warming may cause a particularly pronounced acceleration of soil N transformation in N-poor cold regions. The consequent alleviation of plant N limitations in cold ecosystems may increase plant productivity and C inputs to the soil, compensating the expected soil C loss, at least partially. Alternatively, warming may desynchronize or unbalance the intimate coupling between microbial N mineralization and vegetation N uptake, leading to potential soil N loss, but also higher soil C losses. We aimed to elucidate potential mechanisms of ecosystem N losses in subarctic grasslands by determining the effects of soil warming on the seasonal patterns of plant N acquisition and microbial net N immobilization. For this, we performed a seasonal isotope tracing experiment using a mix of ¹⁵N-labelled amino-acids along soil temperature gradients in geothermal systems in Iceland.

Soil microbial biomass acted as a temporal reservoir of N by increasing N immobilization particularly during unfavorable winter periods for vegetation, likely due to the alleviated microbial C limitation. However, soil warming exacerbated microbial C limitation and decreased the N storage capacity of soil microbes during snowmelt periods. As a result, a higher proportion of N remained in the extractable soil fraction susceptible to leaching losses.  , however, this increased plant N uptake did not compensate for the lower microbial biomass N storage, leading to ecosystem N losses. Our results highlight the relevant role of soil microbes to safely store and immobilize N when plants do not need it and to release N when plants require it. Warming can weaken this particularly important soil microbial function in cold regions, leading to substantial ecosystem N and fertility losses, which may also promote irreversible soil C losses in these ecosystems.