VARIATION OF NITROGEN CONCENTRATION IN FOREST STREAMS INFLUENCES OF FLOW, SEASONALITY AND CATCHMENT CHARACTERISTICS

Short-term variation of nitrogen concentrations must be considered whe n assessing longterm changes in forest ecosystems, and calculating acc urate estimates of nitrogen export. In this study, 10 years of monthly concentration and daily flow measurements from 20 small forested catc hments in Sweden and Finland were used to link concentrations to flow dynamics and/or seasonal variations, and to relate the detected links to catchment characteristics. The methods used included regression ana lyses and the non-parametric Wilcoxon test. Significant correlations b etween different nitrogen fractions and flow volume were found for mos t of the catchments, but in general flow volume only explained 20% of the variation in NO3-N and organic-N, and 10% of the variation in NH4- N concentrations. Both positive and negative correlations between conc entration and flow were detected for the inorganic-N fractions, wherea s for organic-N only positive correlations were recorded. Increasing f low conditions were often found to be associated with twofold median i norganic-N concentrations, especially during the spring. However, the most extreme seasonal variation was associated with the influence of t he growing season, showing up to 20 times lower inorganic-N concentrat ions. Regarding NO3-N concentrations, high atmospheric nitrogen deposi tion appeared to moderate this seasonal influence whereas it was highe st for catchments including lakes. Intensively drained catchments show ed the quantitatively largest difference between growing and dormant s easons for NH4-N concentrations. In general, links between nitrogen co ncentrations and flow/seasons could be related to annual mean temperat ure, annual mean concentrations and the water regime of the catchments . The geographical heterogeneity of detected links and the still rathe r low explanation level for the concentration variation, show the impo rtance of a more dynamic approach to nitrogen leakage where also catch ment characteristics are included.