Long-term sulphate and inorganic nitrogen mass balance budgets in European ICP Integrated Monitoring catchments (1990–2012)

Abstract

In general, TIN was strongly retainedin the catchments not affected by natural disturbances. The long-term annual variation in net releases forSO4was explained by variations in runoff and SO4concentrations in deposition, while a variation in TINconcentrations in runoff was mostly associated with a variation of the TIN retention rate in catchments.The net release of SO4from forest soils may delay the recovery from acidification for surface watersand the continued enrichment of nitrogen in catchment soils poses a threat to terrestrial biodiversityand may ultimately lead to a higher TIN runoff through N-saturation. Continued monitoring and furtherevaluations of mass balance budgets are thus needed. Empirical evidence based on integrated environmental monitoring including physical, chemical and bio-logical variables is essential for evaluating the ecosystem benefits of costly emission reduction policies.The international multidisciplinary ICP IM (International Cooperative Programme on Integrated Moni-toring of Air Pollution Effects on Ecosystems) programme studies the integrated effects of air pollutionand climate change on ecosystems in unmanaged and calibrated forested catchments. We calculatedsite-specific annual input-output budgets for sulphate (SO4) and total inorganic nitrogen (TIN NO3-N + NH4-N) for 17 European ICP IM sites in 1990–2012. Temporal trends for input (deposition) and output(runoff water) fluxes and the net retention/net release of SO4and TIN were also analysed. Large differ-ences in the input and output fluxes of SO4and TIN reflect important gradients of air pollution effectsin Europe, with the highest deposition and runoff water fluxes at IM sites located in southern Scandi-navia and in parts of Central and Eastern Europe and the lowest fluxes at more remote sites in northernEuropean regions. A significant decrease in the total (wet + dry) deposition of non-marine SO4and bulkdeposition of TIN was found at 90% and 65% of the sites, respectively. Output fluxes of non-marine SO4inrunoff decreased significantly at 65% of the sites, indicating positive effects of the international emissionabatement actions in Europe during the last 20 years. Catchments retained SO4in the early and mid-1990s, but this shifted towards a net release in the late 1990s, which may be due to the mobilization oflegacy S pools accumulated during times of high atmospheric SO4deposition. Despite decreased deposi-tion, TIN output fluxes and retention rates showed a mixed response with both decreasing (9 sites) andincreasing (8 sites) trend slopes, and trends were rarely significant.