Modelling impacts of lateral N flows and seasonal warming on an arctic footslope ecosystem N budget and N2O emissions based on species-level responses

Laura H. Rasmussen,Barbara Kitzler,Per-Erik Jansson,Wenxin Zhang,Per Ambus,Bo Elberling

doi:10.1007/s10533-022-00894-z

Abstract

Future Arctic tundra primary productivity and vegetation community composition will partly be determined by nitrogen (N) availability in a warmer climate. N mineralization rates are predicted to increase in both winter and summer, but because N demand and –mobility varies across seasons, the fate of mineralized N remains uncertain. N mineralized in winter is released in a “pulse” upon snowmelt and soil thaw, with the potential for lateral redistribution in the landscape. In summer, the release is into an active rhizosphere with high local biological N demand. In this study, we investigated the ecosystem sensitivity to increased lateral N input and near-surface warming, respectively and in combination, with a numerical ecosystem model (CoupModel) parameterized to simulate ecosystem biogeochemistry for a tundra heath ecosystem in West Greenland. Both measurements and model results indicated that plants were poor utilizers of increased early-season lateral N input, indicating that higher winter N mineralization rates may have limited impact on plant growth and carbon (C) sequestration for a hillslope ecosystem. The model further suggested that, although deciduous shrubs were the plant type with overall most lateral N gain, evergreen shrubs appear to have a comparative advantage utilizing early-season N. In contrast, near-surface summer warming increased plant biomass and N uptake, moving N from soil to plant N pools, and offered an advantage to deciduous plants. Neither simulated high lateral N fluxes nor near-surface soil warming suggests that mesic tundra heaths will be important sources of N2O under warmer conditions. Our work highlights how winter and summer warming may play different roles in tundra ecosystem N and C budgets depending on plant community composition.

Highlights

Lateral nitrogen (N) redistribution and uptake in Arctic ecosystems have been documented in mesic tundra heath (Rasmussen et al 2021), and tussock tundra (Giblin et al 1991; Yano et al 2010)
N mineralization rates are predicted to increase in winter and summer, but because N demand and –mobility varies across seasons, the fate of mineralized N remains uncertain
2019 had significantly higher Total dissolved Organic Carbon (TOC) concentrations compared to 2018, which could be caused by a warmer summer and earlier snowmelt in 2019, which can result in increasing rates of root exudation and higher mineralization rates (Rasmussen et al 2020)

Summary

Introduction

Lateral nitrogen (N) redistribution and uptake in Arctic ecosystems have been documented in mesic tundra heath (Rasmussen et al 2021), and tussock tundra (Giblin et al 1991; Yano et al 2010). It moves with meltwater flux or precipitation on the frozen surface found in seasonally and permanently frozen soils, at shallow depths in the early season and deeper in the soil in the later growing season (Treat et al 2016). Later in the growing season, precipitation events can cause osmotic release of small organic compounds and movement of water and N in solution around the landscape (e.g. Rastetter et al 2004; Rasmussen et al 2020)

Methods

Results

Conclusion