Effects of Long-term Exposure to Ammonia from Animal Farms on the Dry and Dune Heath Vegetation: Lessons from a Gradient Study

In this gradient study, we examined the effects of ammonia deposition on vegetation in two different heathland habitats, a dry and dune heath (Annex I code: 4030 & 2140, respectively). During the summer of 2020, we conducted a vegetation survey and soil sampling along a transect at each heathland with increasing distance to a farm unit(s). At the dry heath, the transect length was ~ 1420 m; at the dune heath, the transect length was ~ 580 m. The dry heath site was mainly in the downwind or crosswind of a pig—and a cattle farm, while the dune heath site was primarily upwind of the farm. The estimated average exceedance range of the upper end of the empirical critical nitrogen load was at dune heath within ~ 400 m. At the dry heath, the upper end of the empirical critical load was estimated to be exceeded along the entire transect. We documented a significant adverse effect of high nitrogen loads on cryptogams at the dune heath and did not observe them at estimated N deposition levels above 22 kg N ha⁻¹ year⁻¹, confirming results from other studies that bryophytes and lichens are sensitive to excess reactive nitrogen. Moreover, we documented a significantly increased graminoid/dwarf shrub ratio on the dune heath closer to the farm, however, nitrogen deposition did seemingly not affect the graminoid/dwarf shrub ratio on the dry heath. At the dry heath, we found a decline in forbs in areas grazed by sheep and horses.