Organic Farming Increases Nitrate Leaching from Soils under Cold-Temperate Conditions.
Gunnar Torstensson, Lars Bergström, Lennart Mattsson, and Holger Kirchmann. Swedish Univ of Agricultural Sciences, Dept of Soil Sciences, Box 7014, Uppsala, 750 07, Sweden
Nitrogen leaching was studied in three long-term field experiments in Sweden: Bjärröd (1989-1998), Mellby (1989-ongoing) and Lanna (1997-onging). Organic practices were compared to conventional farming with respect to yield, nutrient uptake, inorganic N in soil and N leaching. The three sites have different soil textures, clay loam, sand and heavy clay, respectively. The cropping systems studied were typical for each soil type comprising of mixed farming and crop production only. Although the experimental design of comparative organic and conventional cropping systems should only differ with respect to fertilizer and pesticide use, such experiments are often far more dissimilar than one may think of. This was also the case in the three studies described. The organic rotations included more legumes to cover the N demand than the conventional ones. The average N input was lower in the organic than conventional systems. However, during years after incorporation of green manure crops, N applications were higher in the organic rotation. Applications of animal manures/slurries were related to yield levels. As mean yields of the organic systems were throughout lower, less animal wastes were spread on organically managed soils. Whether a cropping system included cover (catch) crops or not was also affecting N leaching. At the sandy site (Mellby), both systems included cover crops, whereas at the other sites only the organic rotations included cover crops in order to increase N fixation. The application of fertilizer N was to a certain extent substituted by N2 fixation through legumes in the organic systems. However, the transfer of fixed N to the following crop was found to be the critical point. Mineralization of N from legume residues occurred to a large extent at times when there was no demand for crops – autumn/winter/early spring revealed by inorganic soil in N profiles. Thus, despite a lower N input to organic systems, the potential possible for leaching was higher. The lysimeter measurements showed that total N leaching from the organic systems was significantly higher throughout the years in two studies and occasionally higher in the third study than from the conventional ones. Leaching losses from the organic ones were highest after plowing of clover-rich grasslands and incorporation of legume residues. Also low-yielding crops such as organic potatoes resulted in higher leaching losses than from conventional systems. Yields of the organic systems amounted to only 50-75% of those in the conventional ones. If leaching of N was expressed per unit of product, organic production systems increased leaching more than 100%. We concluded that N leaching was highly increased through organic farming practices. Lack of synchrony between N release from organic N sources and crop demand was the main reason. Organic farming seems not be an environmental benefit for ground- and surface waters under cool-temperate conditions.