Environmental conditions rather than nitrogen availability limit nitrous oxide (N ₂ O) 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.