Dinitrogen emissions: an overlooked key component of the N balance of montane grasslands

Abstract

While emissions of nitric oxide (NO), ammonia (NH 3 ) and nitrous oxide (N 2 O) from grassland soils have been increasingly well constrained, soil dinitrogen (N 2 ) emissions are poorly understood. However, N 2 losses might dominate total gaseous nitrogen (N) losses. Knowledge on N losses is key for the development of climate-adapted management that balances agronomic and environmental needs. Hence, we quantified all gaseous N losses from a montane grassland in Southern Germany both for ambient climatic conditions and for a climate change treatment (+ 2 °C MAT, − 300 mm MAP). Monthly measurements of soil N 2 emissions of intact soil cores revealed that those exceeded by far soil N 2 O emissions and averaged at 350 ± 101 (ambient climate) and 738 ± 197 µg N m −2 h −1 (climate change). Because these measurements did not allow to quantify emission peaks after fertilization, an additional laboratory experiment was deployed to quantify the response of NH 3 , NO, N 2 O, and N 2 emissions in sub daily temporal resolution to a typical slurry fertilization event (51 kg N ha −1 ). Our results revealed that total N gas losses amounted to roughly half of applied slurry-N. Surprisingly, N 2 but not NH 3 dominated fertilizer N losses, with N 2 emissions accounting for 16–21 kg or 31–42% of the applied slurry-N, while NH 3 volatilization (3.5 kg), N 2 O (0.2–0.5 kg) and NO losses (0–0.2 kg) were of minor importance. Though constraining annual N 2 loss remained uncertain due to high spatiotemporal variability of fluxes, we show that N 2 losses are a so far overlooked key component of the N balance in montane grasslands, which needs to be considered for developing improved grassland management strategies targeted at increasing N use efficiency.