Data for "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 (Nr) deposition. Excess deposition of
Nr 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 absolute magnitudes largest
midday and with 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 the fact 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 Nr 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.