Synthesis of Nitrogen Pools and Fluxes from European Forest Ecosystems

Abstract

To investigate which ecosystem parameters determine the risk and magnitude of nitrate leaching we compiled data from published and unpublished sources on dissolved inorganic nitrogen (DIN: NO₃ ⁻ + NH₄ ⁺) in throughfall, DIN leaching loss in runoff or seepage water, and other ecosystem characteristics from 139 European forests. Not all data were available for all sites: 126 sites had at least one year’s data on DIN inputs and DIN leaching loss; 40–50 sites had some data on soil chemistry and/or vegetation pools of N. DIN inputs in throughfall range between < 1 and about 70 kg N ha⁻¹ yr⁻¹ and the losses with seepage or runoff range between < 1 and 50 kg N ha⁻¹ yr⁻¹. Retention of N within the ecosystem increases with increasing DIN deposition and increasing proportion of NH ⁺₄ in deposition. The amount of N in needles and litterfall shows a significant linear relationship with throughfall deposition of DIN, whereas the C:N ratio of the organic (OH) horizon is uncorrelated to the level of throughfall-DIN flux. About 50% of the variability in DIN leaching loss can be explained by the flux of DIN in throughfall. Alternatively, about 60% ofthe variability in DIN leaching loss can be explained in a two-variable multiple regression combining the C:N ratio of the organic soil and the pH of the mineral soil. The survey data suggest that leaching of DIN from forest ecosystems in Europe is related in part to current DIN deposition and in part to the longer-term internal ecosystem N status as reflected in the chemistry of the humus and acidification status of the soil.