Arctic ecosystems are nitrogen (N) limited, and quantifying N concentrations, inputs and outputs therefore improve our understanding of the tundra ecosystem structure and functioning. N mineralization rates in summer and winter, respectively, may further increase in a warmer future, and can impact tundra C sequestration and plant community composition.
In the early spring thaw season, snow melts and drains through the shallow thawed topsoil layer, where N mineralized during winter may be picked up and redistributed downslope in the landscape, thus causing an N loss upslope and an input downslope. The onset of this process, however, depends on snowmelt date, which varies in the sloping tundra landscape. The importance of the early season N loss, N input and the potential for redistribution in a heterogeneous landscape affected by varying snow conditions has not been quantified. It is also not known how it may be regulated by changes in environmental conditions such as higher surface temperatures or herbivory. Here, we studied soil nitrate (NO_3‾) release, movement and ecosystem importance as N input along an Arctic hillslope in W Greenland using a topsoil core incubation experiment, a tracer experiment with 15N in the field, and a Structural Equation Model explaining environmental temporal patterns in the slope NO_3‾ content and N₂O emissions across years. We further studied the effects of summer warming, winter warming and herbivory on the N turnover using field experiments in a tundra heath and a fen, and we compared the effects of lateral N input to the direct effects of near-surface warming using numerical modelling.
The results showed that a pulse of 244±81 g NO_3‾-N enters solution upon the very first thaw in the topsoil of the whole slope, which is less than 1 % of total N, but 200 % of annual average N deposition. Most of the NO_3‾ was rapidly immobilized and did not cause a pulse of denitrification-related N2O release, but N₂O emissions rather increased with time over the first month of thaw as NO_3‾ content and soil moisture decreased. This pattern was general across the whole slope, but only detectable when normalizing data based on timing of season onset across the slope (date of first soil thaw following snowmelt).
About half of NO_3‾-N input from lateral flow in the early thaw season was picked up by the downslope heath ecosystem, while the rest drained further downslope. Early season lateral N input is thus a part of the tundra heath ecosystem N budget, but was mainly utilized by soil and microbes, while plants obtained only 2 % of the lateral N input, with evergreen shrubs being the most adept in the early season. Simulated near-surface warming caused increased summer N mineralization rates and benefited plants at the expense of the soil N pool, especially deciduous shrubs, so that their biomass and C sequestration increased, causing the tundra heath to become a net C sink, while not increasing N₂O emissions. Direct warming and the N release related to that overprinted the small effects of increased lateral N input.Experimental and modelled summer warming dried the topsoil, which decreased NO_3‾ concentrations in the heath and the fen, except when herbivory was simulated by clipping. We illustrate the role of plants in regulating soil biogeochemistry via uptake and soil moisture now and under warmer surface conditions.
Our results illustrate the complicated nature of tundra ecosystems and how spatial variability and lateral N fluxes interact with environmental conditions of the Arctic tundra. All these factors must be taken into account in order to understand N availability and fluxes now and in the future.