Do organic farming practices reduce nitrate leaching?

Agriculture is a contributor of nitrate to natural waters and there is concern about the excess nitrogen burden loadings from agriculture on natural waters. Agricultural practices that reduce nitrate leaching from arable land are needed. It is postulated by certain groups that organic farming practices reduce nitrate leaching among other environmental benefits. The objectives of this paper are: (1) to compile, summarize and critically analyse information about NO3-N leaching from farming systems that were managed according to organic farming principles; (2) to compare NO3-N leaching from organic farming systems with that from conventional systems. This review consists of several parts. The available literature on leaching of NO3-N from organic farming and conventional farming systems was analysed. Leachable amounts of NO3-N in soils from two types of farming systems were compared. Finally NO3-N leaching from animal manure versus inorganic fertilizer was examined. In all studies we found in the literature, both the sequence and type of crops grown, and the input intensity of N was different in organic and conventional systems. Organic farming systems had on average a lower N input and more legumes in rotation. Average leaching of NO3-N from organic farming systems over a crop rotation period was somewhat lower than in conventional agriculture. If the different input intensities of N between organic and conventional systems were taken into account and corrected for, no differences in leaching losses between systems were found. However, a proper comparison of leaching between the two types of systems should take the yield into account. Attempting to do this in this review, we found only two studies which provided data for this. In both studies, specific conditions of the soil-a high organic matter content resulting in a high N mineralization at one site and a heavy clay texture resulting in very small leaching losses at the other site-did not enable us to come up with a clear-cut answer. Nevertheless, we could not find any evidence that nitrate leaching will be reduced by the introduction of organic farming practices, if the goal is to maintain the same crop yield levels as in conventional farming systems. Reduction of nitrate leaching is not a question of organic or conventional farming, but rather of introduction and use of appropriate counter- measures. This insight should guide our thinking when developing environmentally friendly and sustainable cropping systems.