Modelling of excess nitrogen in small rural catchments

The objective of this research was to assess the potential excess nitrogen (PEN) under different land use categories based on a GIS model and compare modelling results with observed data in Porijõgi River catchment (258 km2) in southern Estonia. Three subcatchments of Porijõgi River (Vända, Sipe and Porijõgi upper course) were used to calibrate the GIS model. PEN was calculated as the difference between inputs to and outputs from an inorganic nitrogen pool. Negative values of PEN denote land cover categories as potential N sinks and positive values as potential N sources. Due to the significant decrease in agriculture from 1987 to 1997, the average PEN in Porijõgi River basin has decreased several times. The source areas embraced 50.1% of the catchment in 1987, while in 2001 this area dropped to 17.8% of the total area. The PEN values in agricultural Vända catchment decreased from 1.5 kg ha−1 in 1987 to −14 kg ha−1 in 2001. In agricultural, but with excessive buffer strips Sipe catchment average PEN values lowered from −8.5 to −18.5 kg ha−1. Mainly in wooded upper course catchment the sink areas were dominating and the PEN values remained between −40.4 kg ha−1 in 1987 and −28.4 kg ha−1 in 2001. This coincides with the measured data of nitrogen losses from this catchment (15.6 and 3.4 kg ha−1 year−1 in 1987 and 1997, respectively). In 1998–2001 the percentage of arable lands had slightly increased compared to 1997 but nitrogen loss has remained at the same level because of the minimal use of fertilizers and intensive denitrification of the former nitrogen pool.