Nitrogen leaching from conventional versus organic farming systems - a systems modelling approach

The level of nitrogen leaching from organic compared to conventional farming was evaluated by using a systems modelling approach. Two different methods were used for estimating and evaluating nitrate leaching. A simple function was used in which nitrate leaching is dependent on percolation, soil clay content, average nitrogen input and crop sequence. A nitrogen balance model was used to estimate the long-term potential for nitrate leaching. These methods were applied to models of both current conventional farming systems in Denmark in 1996 and of well-managed organic farming systems. On average, the total estimated nitrogen input to the organic systems was lower (104-216 kg N ha^-1 year^-1 than to the conventional farming systems (146-311 kg N ha^-1 year^-1). The N-balances in the organic fields showed a surplus of nitrogen (net input of nitrogen) in to the root zone of 60-143 kg N ha^-1 year^-1. In the conventional systems the surplus varied from 25 to 155 kg N ha^-1 year^-1. The modelled nitrogen leaching from the organic systems varied from 19 to 30 kg N ha^-1 year^-1 on loamy soils to 36-65 kg N ha^-1 year^-1 on sandy soils. The modelled nitrogen leaching from the organic systems was always lower than from the comparable conventional agricultural systems due to: (I) the lower total input of nitrogen to the organic systems; and (II) the composition of the organic crop rotations including extensive use of catch crops. However, the modelling of nitrogen leaching has many uncertainties, principally due to difficulties in predicting the nitrogen leaching from different types of grass fields. Comparison of the results from two methods: (i) modelling of nitrogen leaching; and (ii) N-balances for the root zones, showed that organic arable crop production and dairy/beef farming on sandy soils are farming systems with a clear potential for lower nitrogen leaching than from the selected conventional systems. It is still uncertain whether the nitrogen leaching is lower or higher from organic arable crop production systems on loamy soil and organic pig production on loamy and sandy soil than from the same conventional systems in Denmark. The results point to the need for future research in the following areas: (i) the ability to build up soil organic nitrogen in organic farming systems and the consequences for both the level of crop production and nitrogen leaching in the long term; (ii) the effects of catch crops in organic crop rotations; and (iii) a better operational understanding of nitrogen leaching from different types of organically managed grass and grass-clover fields. (C) 2000 Elsevier Science B.V.