Ecosystem properties of acid grasslands along a gradient of nitrogen deposition

Stevens, Carly (2004). Ecosystem properties of acid grasslands along a gradient of nitrogen deposition. PhD thesis The Open University.



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 squarrosus.

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 N and soil C:N for G. saxatile. Tissue N:P ratios gave no clear indication of nutrient limitation.

The relative merits of different indicators of N-deposition are discussed.

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