Upslope release—Downslope receipt? Multi‐year plant uptake of permafrost‐released nitrogen along an arctic hillslope

Abstract

As arctic permafrost continues to thaw, previously inaccessible nitrogen (N) becomes available to N‐limited arctic plants. Increased N availability could enhance plant growth and thereby potentially offset climate‐induced carbon release. Arctic plants can take up newly available permafrost‐N locally upon release. However, in a topographically diverse arctic landscape, permafrost‐N may be transported along hillslopes, away from the point‐of‐release. The extent to which topographical N transport can impact arctic vegetation change depends on whether N is retained locally, captured by downslope recipient plant communities, or transported away. We used stable isotope labelling (15N) to simulate upslope release of ammonium (NH4+) and nitrate (NO3−) from thawing permafrost on an arctic hillslope, western Greenland. We tracked the plant species‐specific uptake of simulated permafrost‐released N from the upslope point‐of‐release to the bottom of the slope through 4 years. We found that arctic tundra plants successfully acquired locally released permafrost‐N, even in sloping terrain, and that N was strongly retained in the plant–soil system through multiple years. At the same time, we also importantly demonstrate that permafrost‐N can be transported and taken up by plants up to 30 m downslope from the point‐of‐release. Especially NO3− was more easily redistributed vertically within the soil column compared to NH4+ and therefore potentially more accessible to plants. Specifically, plant species with fast N uptake capacity and deep‐soil foraging strategies may have competitive advantages for capitalising on deep‐soil released and topographically transported permafrost‐N (here exemplified by Equisetum arvense and Salix glauca). Nevertheless, even mosses gained access to permafrost‐N via vertical and lateral redistribution on the slope. Ultimately, the intricate balance between strong local N retention, downslope transport and plant species‐specific uptake strategies may contribute to shaping arctic vegetation change. Synthesis. Across spatially complex arctic ecosystems, arctic plants can take up permafrost‐released N both at the local point‐of‐release and at a considerable distance downslope. The potential for arctic plants to take advantage of topographically transported permafrost‐N could lead to long‐term and landscape‐scale changes in species composition, plant productivity, and ultimately carbon and climate feedbacks across the Arctic.

Spatial Distribution of Bioavailable Inorganic Nitrogen From Thawing Permafrost

“…There may be several explanations for a relation between the ice content and the NH 4 + (Figure 4a): (a) the higher water/ice content will limit the oxygen availability and thereby everything else equal reduce the oxidation of NH 4 + through nitrification prior to permafrost formation in the case of syngenetic permafrost, (b) the transport of inorganic N from well‐drained uplands to wet, lowland areas (Pedersen et al., 2022), (c) N mineralization (Schimel et al., 2004, (d) a combination of higher N‐fixation rate in wetter areas (Rousk et al., 2017)) and corresponding higher N‐input to the ecosystem, and (e) an abiotic reason for higher contents of NH 4 + in the wetter areas may be ion exclusion (Wren & Donaldson, 2011). The freezing of the water, associated with syngenetic permafrost growth in wetlands, will exclude most water‐soluble compounds being incorporated in the ice and may consequently lead to higher ion concentrations (which may include NH 4 + ‐ions) at specific depth intervals.…”

“…This indicates that the release of inorganic N from the permafrost may not be critical for the two ecosystems in question. However, as plants may have very different patterns in terms of where N is utilized, long‐rooted plants may benefit from N release from permafrost thawing (Pedersen et al., 2020), which consequently may trigger changes in plant diversity and abundance (Pedersen et al., 2022).…”

“…For N‐limited high arctic plant systems, increasing N availability may enhance growth and alter the community composition if plants can take advantage of this newly available N (Hewitt et al., 2019; Keuper et al., 2017; Salmon et al., 2018). This requires that plant roots can reach the zone of permafrost thaw (Pedersen et al., 2020) or that released N can move across the landscape to areas with shallow depth to the permafrost (Abbott et al., 2015; Finger et al., 2016; Pedersen et al., 2022). Changes in precipitation patterns are expected over the Arctic region, including a higher proportion of the precipitation falling as liquid water instead of snow (Meredith et al., 2019).…”

“…that plant roots can reach the zone of permafrost thaw (Pedersen et al, 2020) or that released N can move across the landscape to areas with shallow depth to the permafrost (Abbott et al, 2015;Finger et al, 2016;Pedersen et al, 2022). Changes in precipitation patterns are expected over the Arctic region, including a higher proportion of the precipitation falling as liquid water instead of snow (Meredith et al, 2019).…”

Arctic coastal nutrient limitation linked to tundra greening

“…These changes in terrestrial greening may result in lower terrestrial nutrient inputs from land to rivers and further to coastal waters 13,14 .…”