TY - JOUR

T1 - Normalizing time in terms of space

T2 - What drives the fate of spring thaw-released nitrogen in a sloping Arctic landscape?

AU - Rasmussen, Laura Helene

AU - Mortensen, Louise H.

AU - Ambus, Per

AU - Michelsen, Anders

AU - Elberling, Bo

N1 - CENPERM[2022] Publisher Copyright: © 2022 Elsevier Ltd

PY - 2022

Y1 - 2022

N2 - 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 NO3− 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 NO3− concentrations and N2O 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 NO3− release and N2O 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 NO3− concentrations and N2O 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 NO3− content over the season did not control the increasing N2O emissions. Rather, nitrification replaced denitrification as the main N2O -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 NO3− pulse released into solution after 24 h of thaw, when meltwater moves along the slope and connects upslope with downslope ecosystems, thus constituted a “hot moment” for interaction between landscape N pools, but the NO3− was immobilized by microorganisms or taken up by plants rather than denitrified and did thus not constitute a hot moment for N2O emissions. Thus, our results regarding what drives the fate of spring-thaw released N in the sloping Arctic landscape highlight the importance of snowmelt timing and the following number of Day After Soil Thaw as a normalizing factor for biogeochemical processes. This provides an analytical concept for reducing spatial and inter-annual variability to understand general seasonal patterns otherwise hidden.

AB - 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 NO3− 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 NO3− concentrations and N2O 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 NO3− release and N2O 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 NO3− concentrations and N2O 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 NO3− content over the season did not control the increasing N2O emissions. Rather, nitrification replaced denitrification as the main N2O -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 NO3− pulse released into solution after 24 h of thaw, when meltwater moves along the slope and connects upslope with downslope ecosystems, thus constituted a “hot moment” for interaction between landscape N pools, but the NO3− was immobilized by microorganisms or taken up by plants rather than denitrified and did thus not constitute a hot moment for N2O emissions. Thus, our results regarding what drives the fate of spring-thaw released N in the sloping Arctic landscape highlight the importance of snowmelt timing and the following number of Day After Soil Thaw as a normalizing factor for biogeochemical processes. This provides an analytical concept for reducing spatial and inter-annual variability to understand general seasonal patterns otherwise hidden.

KW - Arctic tundra

KW - NO emission

KW - Nitrogen cycling

KW - Snowmelt

KW - Structural equation model

KW - Topography

U2 - 10.1016/j.soilbio.2022.108840

DO - 10.1016/j.soilbio.2022.108840

M3 - Journal article

AN - SCOPUS:85139416903

VL - 175

JO - Soil Biology & Biochemistry

JF - Soil Biology & Biochemistry

SN - 0038-0717

M1 - 108840

ER -