Longer thaw seasons increase nitrogen availability for leaching during fall in tundra soils

Abstract

Climate change has resulted in warmer soil temperatures, earlier spring thaw and later fall freeze-up, resulting in warmer soil temperatures and thawing of permafrost in tundra regions. While these changes in temperature metrics tend to lengthen the growing season for plants, light levels, especially in the fall, will continue to limit plant growth and nutrient uptake. We conducted a laboratory experiment using intact soil cores with and without vegetation from a tundra peatland to measure the effects of late freeze and early spring thaw on carbon dioxide (CO₂) exchange, methane (CH₄) emissions, dissolved organic carbon (DOC) and nitrogen (N) leaching from soils. We compared soil C exchange and N production with a 30 day longer seasonal thaw during a simulated annual cycle from spring thaw through freeze-up and thaw. Across all cores, fall N leaching accounted for ∼33% of total annual N loss despite significant increases in microbial biomass during this period. Nitrate \(({{{\rm{NO}}}_{3}}^{-})\) leaching was highest during the fall (5.33 ± 1.45 mg N m⁻² d⁻¹) following plant senescence and lowest during the summer (0.43 ± 0.22 mg N m⁻² d⁻¹). In the late freeze and early thaw treatment, we found 25% higher total annual ecosystem respiration but no significant change in CH₄ emissions or DOC loss due to high variability among samples. The late freeze period magnified N leaching and likely was derived from root turnover and microbial mineralization of soil organic matter coupled with little demand from plants or microbes. Large N leaching during the fall will affect N cycling in low-lying areas and streams and may alter terrestrial and aquatic ecosystem nitrogen budgets in the arctic.