The results of two European field-scale manipulation projects (NITREX (nitrogen saturation experiments) and EXMAN (experimental manipulation of forest ecosystems)) were used to evaluate the effect of ecosystem disturbance on nitrate leaching in coniferous forest ecosystems. The first principle component (PC1) of a principle component analysis explained 85% of the variation in nitrate leaching between the 12 sites. This PC1 consisted of nitrogen concentrations and fluxes in the ecosystem and was interpreted as an indicator of N status. Nitrate leaching responded rapidly to manipulation of nitrogen deposition, especially in sites with ambient high nitrate leaching. This rapid response could be explained partly by an immediate hydrological response of increased drainage. However, results of field-scale 15N tracer experiments indicated that microbial processes in the organic layer had changed after a few years of changed N deposition. In sites with already significant nitrate leaching, irrigation caused a large increase in nitrate leaching due to enhanced mineralization. Combined fertilization and irrigation had only a limited effect on nitrate leaching in nitrogen-limited sites, whereas in nitrogen-saturated sites, nitrate leaching was significantly increased. The hypothesized nitrate pulse as a result of rewetting after drought did not occur in any of the sites. We conclude that the effect of disturbance on nitrate leaching depends on the N status of the ecosystem: in sites that are nitrogen-saturated, nitrate leaching is very sensitive to disturbance.
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