Frozen soils, season and topography: N release, movement and slope ecosystem interactions in the permafrost landscape

Abstract:

Snowmelt dominates water supply in permafrost ecosystems, and spring, with its thin layer of thawed soil and snowmelt water moving downslope through that layer, is therefore a period of potential soluble N transport. As cold ecosystems tend to be N-limited, a seasonal redistribution could be a little known source of N for vegetation and soils downslope, as well as affect N₂O fluxes. However, the N-limited ecosystem may simply reincorporate the N released by thawing, making N movement negligible.
This study has a process-based approach to a large spatial dataset covering a low-arctic tundra heath slope in W Greenland. Overlain by a semi-permanent snowfan, the slope represents a gradient of growing season length with earlier snowmelt and soil thaw downslope and later snowmelt and soil thaw upslope.

In a field experiment covering two years on the slope, we combine spatially distributed soil water chemistry measurements of Total Organic Carbon, Total Dissolved N (TDN) and nitrate (NO₃^-) with monitoring of soil temperature and –moisture, N₂O flux measurements, vegetation analysis and soil properties. We study which factors that impact soil water chemistry, and how the effect of slope affects seasonality.

In an incubation study of frozen topsoil cores, we quantify how much N actually enters solution upon thaw compared to loss as N₂O and retained N.

Our results show that <1 % of soil N is released as N₂O upon thaw and, similarly, <1 % enters solution as NO₃^- with potential for downslope transport. The NO₃^- content is, however, highest 24 h after exposure to positive temperatures indicating a thaw ‘pulse’ of mineral N. Soil water chemistry in situ across the slope varied with season. However, the thaw pulse was delayed in the footslope, indicating hydrological contact with upslope.

Most important for the contents of TOC, TDN and NO₃^- were soil temperature, soil moisture, spatial cover of N fixers (mosses, lichen) and growing season length. Day after soil thaw independently of slope position was linked to NO₃^- contents and N₂O flux, with negative fluxes and higher NO₃^- concentration just after thaw, positive fluxes and less NO₃^- after 40 days of soil thaw. We conclude that spatial variability in snow melt, thus onset of thaw, confounds seasonal biogeochemical patterns, soil thaw is a more useful timescale than day of year for process analyses.