The effect of precipitation chemistry upon anion and cation fluxes from the surface layer of ombrotrophic mires in the UK

1. Replicate cores and monoliths (bearing intact vegetation) were sampled from the acrotelm and surface litter layer of ombrotrophic mires across a gradient in precipitation chemistry. Leaves from Calluna vulgaris shrubs growing on the surface of these mires were also sampled. The chemical characteristics of the cores as initially sampled and incident Calluna vulgaris leaves were determined. During a 1-year study the monoliths were subjected to 1200 mm of simulated rainfall of the same chemical composition as they received in the field. Drainage water was analysed periodically for component anions and cations. 2. The results show that changes in the chemical quality of precipitation in the UK has induced changes in the N cycling and base status within the acrotelm, and surface litter layer of ombrotrophic mires. The processes affecting NO3- and NH4+ concentrations in this layer were not the same. Ammonium concentrations were largely dependent upon ion exchange reactions whereas NO3- concentrations were more dependent upon biologically mediated processes. 3. The results of the study of total element chemistry in Calluna vulgaris stands showed that leaves contained higher N and lower P and Mg concentrations in areas receiving high concentrations of chemical components, delivered in precipitation associated with industrial and domestic atmospheric emissions. 4. Fluxes of all of the ions from monoliths were highly seasonal, and generally reflected fluctuations in nutrient uptake by the vegetation and the activity of soil microbes and fauna. Unlike NH4+, NO3- and P were rapidly immobilized in the acrotelm and surface litter layer of mires in areas receiving high H, N and S atmospheric inputs. Total P concentrations in this layer were highest in areas receiving high concentrations of H, N and S in precipitation. This probably reflected the rate of litter decomposition and hence the mineralization of organic-P; and may be linked to the chemical composition of Calluna vulgaris leaves.