Gaseous Loss of Nitrogen from Boreal Soils

Abstract:

Decades of research in high-latitude ecosystems has characterized a closed terrestrial nitrogen (N) cycle, defined by nitrogen limitation of primary production, small pools of inorganic N, rapid rates of internal cycling, and minimal losses of gaseous or dissolved inorganic forms of N. Low rates of N inputs from atmospheric deposition and N fixation result in rapid assimilation by N-limited plants and microbes. Strong competition for N between plants and microorganisms is thought to limit flux of N through dissimilatory pathways including nitrification and denitrification. However, several recent studies have reported large emissions of nitrous oxide (N₂O) in situ, or strong potential to produce N₂O. Rapid changes in the climate and disturbance regimes of high-latitude ecosystems, including warming soils, an intensifying fire regime, or release of N from thawing permafrost might accelerate gaseous emissions of N. We measured production of N₂O and nitrogen gas (N₂) from boreal soils in the laboratory using the nitrogen-free air recirculation method (N-FARM). Soils were collected from uplands, riparian zones, and the margins of a thermokarst lake in the region of spatially discontinuous permafrost in boreal Alaska. Rates of N₂ production by denitrification ranged approximately 10-200 mg N m^–2 d^–1, and were 1000-fold greater than rates of N₂O production. N₂O:N₂ was greatest for soils adjacent to the thermokarst lake, where permafrost is actively thawing. Observed rates of N₂ production were typically several-fold greater than available estimates of N fixation rates in this ecosystem. Significant loss of N₂ that outpaces inputs from N fixation suggests that nutrient limitation maintains a strong constraint on sequestration of carbon by plants, which might otherwise compensate for carbon lost from thawing permafrost.