Nitrate leaching from agricultural soils: ecological modelling under different economic constraints

Excess use of fertilizer-N results in leaching of nitrate, which threatens the environment in various ways. Taxes on nitrogen fertilizers have been suggested as a measure to reduce the nitrogen load to the environment. An economic model for predicting the farmers' decisions regarding fertilizer application, and an ecological model which predicts the nitrate leaching given these decisions is presented. From economic theory it is known that the economic optimal amount of fertilizer is where the value of the marginal product equals the input factor price. Marginal yields are derived from the estimated yield curves and fertilizer-N levels which maximize farmers' profits. Based on the economic models' predictions of the profit-maximizing fertilization practice, nitrate losses were predicted by the SOIL-SOILN model. High N-tax levels (> 200%) are needed to affect the nitrate leaching significantly. However, lower taxes may have an indirect effect on the ecology of farmlands by improving the economics of other measures against nitrogen losses.