Nitrogen Leaching in Small Agricultural Catchments

Kyllmar, K. 2004. Nitrogen Leaching in Small Agricultural Catchments Modelling and monitoring for assessing state, trends and effects of counter-measures. Doctoral thesis. ISSN 1401-6249, ISBN 91-576-6761-6. Nitrogen (N) leaching from arable land to the aquatic environment is considered a serious problem. Small agricultural monitoring catchments in Sweden were used for the application and testing of model-based methods for quantification of N leaching from arable fields, and for analysis of measured data. The physically-based modelling system SOILNDB was used in two different approaches for quantification of N leaching; by direct simulations using monitored field data and by producing field N leaching coefficients (field-NLCs) on the basis of agricultural statistics, etc. The field-NLCs were then used to calculate N losses from the fields. Measured N loads at the stream outlets varied widely between the 27 catchments, from 2 to 41 kg ha yr. They were correlated to climate, soil texture, proportion of arable land and crops grown in the catchments. Significant downward trends in N loads were revealed for seven catchments. In one monitoring catchment, direct simulations for individual fields resulted in a mean N leaching for the fields of 44 kg ha yr, whereas measured N load in the stream was 40 kg ha yr. The difference could be attributed to dilution by water from non-agricultural land with typically lower N concentration. Calculation of field N leaching using field-NLCs for nine monitoring catchments also showed a satisfactory agreement with measurements in the streams when contributions from other sources and uncertainties in groundwater flows were considered. The two applications also showed that N leaching varied greatly between individual fields. The potential effects of several counter-measures to reduce N leaching were estimated (using field-NLCs) to be between 34 and 54% for the nine individual monitoring catchments. The measures comprised changes in crop combinations (crop and following crop), application time of manure and adjusted mineral fertilizer dose, and introduction of catch crop. The potential to reduce N leaching was also estimated for a medium-sized catchment. This reduction was relatively low (21%), partly due to the restricted possibility of introducing catch crops into the crop rotations. In water quality management planning, methods for quantification of N leaching from arable land should cover these large spatial variations in N leaching so that areas with a large impact on the recipient can be identified and the most effective combinations of measures to reduce N leaching can be determined.