Atmospheric deposition of nitrogen, runoff of organic nitrogen, and critical loads for soils and waters

Abstract

Here we test the hypothesis that increased deposition of inorganic N compounds leads to increased leachingand runoff of organic N and thus a higher critical load. We use mainly Norwegian data from input-output fluxes at small catchments, national lake surveys, and large-scale experiments with N deposition to whole catchments. Concentrations of organic-N are not significantly related to N deposition. Much of the variance in organic-N levels are explained by TOC concentrations. For the small catchments, there is a significant relationship between C/N ratio in dissolved organic matter and the N deposition. The sites with high N deposition have low C/N ratio. Chronically high N deposition and long-term accumulation of N in soils and biomass may have led to organic matter more enriched in N relative to prestine sites. Time trend data from manipulated catchments do not show changes in organic-N leaching over 4-10 years. Although organic N levels may have increased as a result of N deposition, the resultant effect on estimate of critical load for nitrogen for freshwaters is minor. For practical purposes organic N outputs can be neglected in estimating and mapping critical loads for nitrogen in Norway.