Variation of nitrogen concentration in forest streams — influences of flow, seasonality and catchment characteristics

Short-term variation of nitrogen concentrations must be considered when assessing longterm changes in forest ecosystems, and calculating accurate estimates of nitrogen export. In this study, 10 years of monthly concentration and daily flow measurements from 20 small forested catchments 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 analyses and the non-parametric Wilcoxon test. Significant correlations between different nitrogen fractions and flow volume were found for most of the catchments, but in general flow volume only explained 20% of the variation in N03N and organic-N, and 10% of the variation in NH4N concentrations. Both positive and negative correlations between concentration and flow were detected for the inorganic-N fractions, whereas for organic-N only positive correlations were recorded. Increasing flow conditions were often found to be associated with twofold median inorganic-N concentrations, especially during the spring. However, the most extreme seasonal variation was associated with the influence of the growing season, showing up to 20 times lower inorganic-N concentrations. Regarding NO3N concentrations, high atmospheric nitrogen deposition appeared to moderate this seasonal influence whereas it was highest for catchments including lakes. Intensively drained catchments showed the quantitatively largest difference between growing and dormant seasons for NH4N concentrations. In general, links between nitrogen concentrations and flow/seasons could be related to annual mean temperature, annual mean concentrations and the water regime of the catchments. The geographical heterogeneity of detected links and the still rather low explanation level for the concentration variation, show the importance of a more dynamic approach to nitrogen leakage where also catchment characteristics are included.