Proton budgets for a monitoring network of European forested catchments: impacts of nitrogen and sulphur deposition

Proton (H+ ion) budgets were calculated for 17 forested sites in Europe using open field (bulk) and throughfall deposition and runoff or soil leachate data. Proton budgets integrate information about the complex chemical and biological processes that govern the generation or consumption of acidity in the ecosystem into a single parameter. The sites belong to the multidisciplinary ICP integrated monitoring network, set up to assess the environmental impacts of transboundary air pollution. Mean annual H+ budgets were estimated to quantify the relative importance of different biogeochemical processes, with special emphasis on the N deposition. N deposition exceeded S deposition on an equivalent basis at the studied sites. Model based estimates for quantifying the impact of agreed international emission reduction measures showed that the relative importance of N deposition still is likely to increase in the future. Base cation weathering and ion exchange were the main processes for proton consumption. Sites on base poor soil material showed low base cation fluxes, export of acidity and high external/internal H+ source ratios. At these sites the dissociation of organic acids was commonly a significant internal H+ source. Depending on deposition inputs, the importance of N processes on the H+ budget varied between −4.5 mequiv. m−2 a−1 (small H+ consumption) and 46.2 mequiv. m−2 a−1 (H+ production). A relationship between the deposition inputs and the H+ production from N transformations was also observed, with higher H+ production at higher deposition levels. Net release of sulphate (and associated H+ production) was observed at many sites, being consistent with observations from other recent European budget studies.