Ecosystem properties of acid grasslands along a gradient of nitrogen deposition
Sixty-eight randomly selected grasslands belonging to the National Vegetation
Classification group U4 (Festuca ovina-Agrostis capillaris-Galium saxatile grassland)
were studied during the summers of 2002 and 2003 along a gradient of atmospheric N
deposition ranging from 6 to 36 kg N ha-1 yr-1 to investigate potential damage caused
by acidification and eutrophication. At each site vegetation was surveyed and
samples were taken from the topsoil and subsoil. Above-ground plant material was
collected from three species: Agrostis capillaris, Galium saxatile and Rhytidiadelphus
Species richness showed a negative linear trend with N deposition, with a reduction of
1 species for every 2.5 kg N ha -1 yr-1. At the current mean N deposition in Europe this
gave a reduction in species richness of 23% from a projected pristine condition.
Forbs accounted for the majority of this decline, showing reductions in both cover and
abundance along the N-deposition gradient. Grasses showed a decrease in species
richness, but an increase in cover, indicating increased dominance of a few species.
Using canonical correspondence analysis, several individual species were identified as
being positively and negatively associated with N deposition.
Soil pH declined along the N-deposition gradient. This was also associated with
increased mobilisation of aluminium, arsenic and lead. Nitrate concentration in the
soil did not increase with increasing N inputs, possibly due to rapid plant uptake and
its mobility in the soil profile. Soil extractable ammonium concentration showed a
significant positive correlation with N inputs. A weak relationship was also identified
between the C:N ratio in the soil and N inputs. Potential ammonium mineralization
showed considerable variation across the sites.
There was no correlation between aboveground tissue Nand N deposition for any of
the three species examined. A weak correlation was identified between tissue Nand
soil C:N for G. saxatile. Tissue N:P ratios gave no clear indication of nutrient
The relative merits of different indicators ofN-deposition are discussed.