Nitrogen application, balances and their effect on water quality in small catchments in the Nordic–Baltic countries

• Statistical trends in N application (28 catchments) and N balance (27 catchments) were investigated in the Nordic–Baltic area.
• Decreasing trends in N application were found in six of the catchments.
• Increasing trends in N application were found in five of the catchments.
• 14 long-term time series of N balances and N concentrations were examined.
• Four of the 14 long-term time series showed significant positive relationship between annual N balances and N concentrations.

Various strategies to reduce loss of nitrogen (N) from agricultural land have been implemented in the Nordic–Baltic countries during the last two decades to combat nutrient pollution of surface waters. The mitigation methods have led to a significant reduction in the consumption of N fertilizers in some of the Nordic–Baltic countries. The objective of this study was to investigate trends in N application and N balance and their effect on N concentrations in runoff from agricultural areas. Nitrogen balances consisted of N input in fertilizer, manure, deposition and fixation minus N output in yield. The study was based on monitoring in small agricultural streams in the Nordic–Baltic region over the last two decades with corresponding information on average N application (28 catchments) and N balance (27 catchments) for a varying periods. Long-term time series (14–22 years) on annual N application was available for 17 catchments and for 14 of these catchments, annual information on N balances were also available for the long-term time series. Nitrogen balances were available for 23 catchments for 2007–2011 and our results showed that on average for this period the N balance varied from −12 N ha−1 yr−1 in Volbu to 132 kg N ha−1 yr−1 in Time, both located in Norway. For long-term time series, we found that the N application rates decreased significantly in six catchments, but increased in five catchments. Furthermore, the N balances decreased in four catchments but increased in two catchments. Hence the results describe the differences in agricultural development between countries, caused by e.g. changes in livestock production (Norway), strict regulations of fertilizer application (Denmark) and increased intensity in production (Latvia). The results showed that for 10 of the 14 catchments with long-term time series, there was no significant relationship between annual N balances and N concentrations. The effect of changes in N balances on N concentrations were significant for four of the 14 long-term time series which showed positive relationships (p = 0.0002, 0.0013, 0.0087, 0.0893) between annual N balances for the agricultural area in the catchments and N concentrations in the streams. The significant relationship between N balances and N concentrations were detected where long-term trends in N balances were identified and where a large change in N balances from a very high N surplus occurred.