Normalizing time in terms of space:What drives the fate of spring thaw-released nitrogen in a sloping Arctic landscape?

• Published in: Soil Biology and Biochemistry
• Main Authors: Rasmussen, Laura Helene, Mortensen, Louise H., Ambus, Per, Michelsen, Anders, Elberling, Bo
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2022
• Subjects: Arctic tundra; NO emission; Nitrogen cycling; Snowmelt; Structural equation model; Topography; Arctic; Greenland; Tundra
• Online Access: https://curis.ku.dk/portal/da/publications/normalizing-time-in-terms-of-space(61788210-92fc-48db-b58f-2f0ac34a7198).html; https://doi.org/10.1016/j.soilbio.2022.108840; https://curis.ku.dk/ws/files/358630759/1_s2.0_S0038071722002978_main.pdf

In the Arctic tundra, snowmelt is followed by soil thaw allowing water and dissolved nutrients to move downslope. However, the fate of the released nitrogen (N) remains unclear, which includes the fraction of N that is lost to downslope transport or converted to N gasses. We have quantified the release of NO 3 − into the soil solution and the loss of gaseous N upon thaw and up to a month after first thaw in an Arctic hillslope in W Greenland. We further investigated which factors of the slope ecosystem that influence the NO 3 − concentrations and N 2 O fluxes throughout two snowmelt and growing seasons using a Structural Equation Model (SEM) linking physical, biological and biogeochemical characteristics across the slope. Snowmelt controls growing season onset, but varies in the landscape. To account for this, we normalized the spatiotemporal variation in snowmelt and soil thaw by measuring NO 3 − release and N 2 O loss in a controlled laboratory thaw experiment with topsoil cores from along the slope. We furthermore normalized seasonal progression of ecosystem variables in space based on the first day of soil thaw in the field. We tested the variable Day After Soil Thaw (DAST) as the temporal driver in our SEM, and found that season progression is the most important factor to describe patterns in NO 3 − concentrations and N 2 O fluxes. We conclude that DAST is a useful tool for analysing seasonal patterns in a spatially heterogeneous snowmelt landscape and between different snowmelt years. When normalizing based on first day of soil thaw, we saw that the decreasing NO 3 − content over the season did not control the increasing N 2 O emissions. Rather, nitrification replaced denitrification as the main N 2 O -source during the growing season, where soil temperatures increased and soil moisture decreased. The gaseous N loss from the slope during the first month of thaw was minor and amounted to 1% of the annual N deposition. A NO 3 − pulse released into solution after 24 h of thaw, when meltwater moves along the ...