How intensive agriculture affects surface-atmosphere exchange of nitrogen and carbon compounds over peatland

Mid-latitude peatlands are often exposed to high atmospheric nitrogen deposition when located in close vicinity to agricultural land. As the impacts of altered deposition rates on nitrogen-limited ecosystems are poorly understood, we investigated the surface-atmosphere exchange of several nitrogen and carbon compounds using multiple high-resolution measurement techniques and modeling. Our study site was a protected semi-natural bog ecosystem. Local wind regime and land use in the adjacent area clearly regulated whether total reactive nitrogen (∑Nr) concentrations were ammonia (NH3) or NOx-dominated. Eddy-covariance measurements of NH3 and ∑Nr revealed concentration, temperature and surface wetness-dependent deposition rates. Intermittent periods of NH3 and ∑Nr emission likely attributed to surface water re-emission and soil efflux, respectively, were found, thereby indicating nitrogen oversaturation in this originally N-limited ecosystem. Annual dry plus wet deposition resulted in 20 to 25 kg N ha-1 depending on method and model used, which translated into a four- to fivefold exceedance of the ecosystem-specific critical load. As the bog site had likely been exposed to the observed atmospheric nitrogen burden over several decades, a shift in grass species' composition towards a higher number of nitrophilous plants was already visible. Three years of CO2 eddy flux measurements showed that the site was a small net sink in the range of 33 to 268 g CO2 m-2 yr-1. Methane emissions of 32 g CO2-eq were found to partly offset the sequestered carbon through CO2. Our study demonstrates the applicability of novel micrometeorological measurement techniques in biogeochemical sciences and stresses the importance of monitoring long-term changes in vulnerable ecosystems under anthropogenic pressure and climate change.