Nitrogen legacies in agricultural catchments : an analysis of 170 years of nitrogen legacies data in four Swedish agricultural catchments

Sammanfattning: Despite the enforcement of legislation and application of measures, nitrogen (N) levels continue to be high in streams. One of the reasons for the low N reduction, is the legacy accumulation of N. Here we evaluate 170 years of N legacies in four Swedish agricultural catchments, by reconstructing land use trajectories, N mass balances, and N surplus over time. It was expected that a high percentage of agricultural land and high N input would lead to a higher N surplus and consequently higher total nitrogen loadings (TN) in the stream. N surplus was expected to create a delayed impact on total nitrogen loading, leading to a hysteresis effect. The long-term analysis showed that in catchments with a high percentage of agricultural land, mineral fertilizer was the main driver of N surplus. A higher N surplus led to high TN loadings in the stream. A positive correlation between N surplus and TN loadings was seen when a one year-shift of TN loadings was applied. No hysteresis effect was seen in any of the catchments. These results suggest that N surplus continues to be high in all catchments, therefore measures focusing on reducing the main drivers of N accumulation are needed. To identify legacies further research considering a longer period of measured TN loadings and variables such as tile drainage density and groundwater travel time is needed. Keywords: Nitrogen legacies, nitrogen mass balance, nitrogen surplus, land use trajectories, hysteresis effect, water quality, agriculture