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

The level of nitrogen leaching from organic compared to conventional farmin g was evaluated by using a systems modelling approach. Two different method s were used for estimating and evaluating nitrate leaching. A simple functi on was used in which nitrate leaching is dependent on percolation, soil cla y content, average nitrogen input and crop sequence. A nitrogen balance mod el was used to estimate the long-term potential for nitrate leaching. These methods were applied to models of both current conventional farming system s in Denmark in 1996 and of well-managed organic fanning systems. On averag e, 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 (146-31 1 kg N ha(-1) year(-1)). The N-balances in the organic fields showed a surp lus 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 s ystems 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 agri cultural systems due to: (I) the lower total input of nitrogen to the organ ic systems; and (II) the composition of the organic crop rotations includin g 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 r esults from two methods: (i) modelling of nitrogen leaching; and (ii) N-bal ances for the root zones, showed that organic arable crop production and da iry/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 o rganic arable crop production systems on loamy soil and organic pig product ion on loamy and sandy soil than from the same conventional systems in Denm ark. The results point to the need for future research in the following are as: (i) the ability to build up soil organic nitrogen in organic farming sy stems and the consequences for both the level of crop production and nitrog en leaching in the long term; (ii) the effects of catch crops in organic cr op rotations; and (iii) a better operational understanding of nitrogen leac hing from different types of organically managed grass and grass-clover fie lds. (C) 2000 Elsevier Science B.V. All rights reserved.