Spatial heterogeneity of ammonia fluxes in a deciduous forest and adjacent grassland

Gas-phase ammonia (NH₃), emitted primarily from agriculture, contributes significantly to reactive nitrogen (N_r) deposition. Excess deposition of N_r to the environment causes acidification, eutrophication, and loss of biodiversity. The exchange of NH₃ between land and atmosphere is bidirectional and can be highly heterogenous when underlying vegetation and soil characteristics differ. Direct measurements that assess the spatial heterogeneity of NH₃ fluxes are lacking. To this end, we developed and deployed two fast-response, quantum cascade laser-based open-path NH₃ sensors to quantify NH₃ fluxes at a deciduous forest and an adjacent grassland separated by 700 m in North Carolina, United States from August to November, 2017. The sensors achieved 10 Hz precisions of 0.17 ppbv and 0.23 ppbv in the field, respectively. Eddy covariance calculations showed net deposition of NH₃ (-7.3 ng NH₃-N m^-2 s^-1) to the forest canopy and emission (3.2 ng NH₃-N m^-2 s^-1) from the grassland. NH₃ fluxes at both locations displayed diurnal patterns with midday peaks and smaller peaks in the afternoons. Concurrent biogeochemistry data showed over an order of magnitude higher NH₃ emission potentials from green vegetation at the grassland compared to the forest, suggesting a possible explanation for the observed flux differences. Back trajectories originating from the site identified the upwind urban area as the main source region of NH₃. Our work highlights that adjacent natural ecosystems sharing the same airshed but different vegetation and biogeochemical conditions may differ remarkably in NH₃ exchange. Such heterogeneities should be considered when upscaling point measurements, downscaling modeled fluxes, and evaluating N_r deposition for different natural land use types in the same landscape. Additional in-situ flux measurements accompanied by comprehensive biogeochemical and micrometeorological records over longer periods are needed to fully characterize the temporal variabilities and trends of NH₃ fluxes and identify the underlying driving factors.