Abstract
Understanding N budgets of tundra ecosystems is crucial for projecting future changes in plant community composition, greenhouse gas balances and soil N stocks. Winter warming can lead to higher tundra winter nitrogen (N) mineralization rates, while summer warming may increase both growing season N mineralization and plant N demand. The undulating tundra landscape is inter-connected through water and solute movement on top of and within near-surface soil, but the importance of lateral N fluxes for tundra N budgets is not well known. We studied the size of lateral N fluxes and the fate of lateral N input in the snowmelt period with a shallow thaw layer, and in the late growing season with a deeper thaw layer. We used 15N to trace inorganic lateral N movement in a Low-arctic mesic tundra heath slope in West Greenland and to quantify the fate of N in the receiving area. We found that half of the early-season lateral N input was retained by the receiving ecosystem, whereas half was transported downslope. Plants appear as poor utilizers of early-season N, indicating that higher winter N mineralization may influence plant growth and carbon (C) sequestration less than expected. Still, evergreen plants were better at utilizing early-season N, highlighting how changes in N availability may impact plant community composition. In contrast, later growing season lateral N input was deeper and offered an advantage to deeper-rooted deciduous plants. The measurements suggest that N input driven by future warming at the study site will have no significant impact on the overall N2O emissions. Our work underlines how tundra ecosystem N allocation, C budgets and plant community composition vary in their response to lateral N inputs, which may help us understand future responses in a warmer Arctic.
Highlights
Arctic ecosystems are often limited by the availability of nutrients (Marion and Miller, 1982; Sistla et al 2012; Wild et al 2013) - especially nitrogen (N) and phosphorous (P)
While most N turnover occurs during summer, winter N mineralization may provide an important N source upon thaw in spring, where a pulse of water-soluble nitrate (NO3−) in soil solution has been observed in studies of Arctic ecosystems (Schimel et al 2004; Buckeridge et al 2013; Rasmussen et al 2020), resulting in plant N uptake (Semenchuk et al 2015)
The importance of lateral N inputs following early spring thaw and throughout the growing season for plant growth, soil N stocks or N2O/N2 fluxes is necessary to quantify in order to understand present and future downslope ecosystem N dynamics, but is yet unclear
Summary
Arctic ecosystems are often limited by the availability of nutrients (Marion and Miller, 1982; Sistla et al 2012; Wild et al 2013) - especially nitrogen (N) and phosphorous (P). Because the shallow thaw layer in spring, where the spring pulse of N in solution occurs, can be infiltrated by meltwater from snow melt (Slater et al 2007), the spring N pulse can be transported and utilized biologically downslope (Giblin et al 1991; Yano et al 2010). Such coupling of landscape hydrology and N cycling may lead to N-loss in one part of the landscape, but N-input to other parts, and may be relevant especially at the beginning of the snow-free season, when plant demand for N is limited and the internal turnover is relatively slow (e.g. Bilbrough et al 2000). The importance of lateral N inputs following early spring thaw and throughout the growing season for plant growth, soil N stocks or N2O/N2 fluxes is necessary to quantify in order to understand present and future downslope ecosystem N dynamics, but is yet unclear
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