De-eutrophication of a nitrogen-saturated Scots pine forest by prescribed litter-raking

Short-term (<7 years) effects of prescribed litter-raking on forest-floor nutrient pools, stand nutrition, and seepage water chemistry were studied in an N-saturated Scots pine (Pinus sylvestris L.) forest in Southern Germany subject to high atmospheric-nitrogen deposition. The study was based on a comparison of plots with and without annual prescribed litter raking at three sites with different N-deposition levels. Prescribed litter-raking resulted in a considerable reduction of forest-floor thickness and mass, as well as of forest-floor C, N, P, K, Mg, and Ca pools. Furthermore, it induced a significant decrease of the foliar N content in current-year needles of the pines and a more balanced nutritional status of the stand. Particularly on the site subject to the highest N deposition, but to a lesser degree also at the other sites, the mean NO₃^- concentration in the subsoil seepage water and the N export into the groundwater were substantially reduced on the litter-raked plots. The results show that in N-saturated Scots pine ecosystems prescribed litter-raking on areas of limited size, which are used as sources of groundwater-derived drinking water and/or serve as habitat for endangered plant species, is a quick and effective method to achieve a more balanced nutritional status of the trees and to reduce seepage-water NO₃^- concentrations and N export into the groundwater. In terms of sustainable ecosystem nutrient management, the conversion of conifer monocultures into broadleaf-rich mixed stands is the better, yet less immediately effective method to reduce the seepage-water N export from conifer forests subject to high atmospheric-N deposition.