Environmental conditions rather than nitrogen availability limit nitrous oxide (N₂O) fluxes from a temperate birch forest

Abstract. Forest ecosystems play an important role in the terrestrial nitrogen (N) cycle, accounting for over a quarter of the land area of the Earth. However, our understanding of nitrogen dynamics in forest systems is limited. The consequences of N deposition to forest ecosystems are often overlooked. In this study, dry deposition of NH₃ was replicated over a two-year period in a temperate semi-natural birch forest via a unique custom-built automated NH₃ release system to investigate the impact on emissions of the greenhouse gas nitrous oxide (N₂O). This study provides evidence that in both natural forest soils (in-situ) and soils under controlled laboratory conditions (ex-situ), the substantial addition of reduced N compounds (NH₃/NH₄⁺) had no direct impact on N₂O emissions. Emissions of N₂O from these soils were dependant on the meeting of several additional thresholds, below which N₂O producing activity was constrained. When environmental conditions in-situ were considered warm and wet (soil temperature >12 °C and volumetric water content >20 %), emissions of N₂O were an order of magnitude higher than when either of these thresholds was not met, regardless of exposure to NH₃ deposition. Ex-situ experiments indicated that microbial activity in the soils was highly constrained by the availability of labile carbon. The addition of glucose to these soils resulted in a considerable increase in N₂O emissions after N application. While cumulative NH₃ deposition to the in-situ soils was relatively large over the measurement period, there was no accumulation of mineral N observed in the soil, suggesting plant-uptake of N was able to mitigate N loading. The implication of these results is that forest ecosystems may be able to mitigate localised NH₃ pollution plumes, in the short-term at least, without incurring an N₂O penalty. However, the long-term impacts of N enhancement remain unclear and further long-term field experiments are required to examine the impact of prolonged exposure to high quantities of N deposition to forest soils.