Nitrogen mass balances in conventional, integrated and ecological cropping systems and the relationship between balance calculations and nitrogen runoff in an 8-year field experiment in Norway

For a cropping system to be sustainable, should not only the soil nitrogen (N) content be preserved but also the N runoff be minimised. Finding a simple but robust way to estimate N runoff would thus be a great advantage when evaluating cropping systems. In this study all major N flows in six different cropping systems, each covering 0.18 ha of a separately pipe drained field lysimeter, located in southeast Norway, were either measured or estimated over a period of X-years. The effect of the cropping system on the soil N content was evaluated using mass balances of total N, and the usefulness of such N balances to predict N runoff(total N losses via drainage and surface water) was investigated. The experiment included systems with conventional arable cropping (CON-A), integrated arable-cropping (INT-A), ecological arable cropping (ECO-A), conventional forage cropping (CON-F), integrated forage cropping (I:VT-F), and ecological forage cropping (ECO-F). All the arable cropping systems resulted in a net reduction in the calculated soil N pool, and the reduction increased with decreasing N input. The only system, which did not alter the soil N content, war; CON-E The largest net reduction was estimated fur EGO-A and ECO-F, which averaged 45 and 43 kg N ha(-1) per year, respectively. The N runoff from the systems was in the range of 18-35 kg N ha(-1) per year, with highest losses from the two conventional and lowest from the two ecological systems and INT-E The forage systems had lower N runoff than the arable systems. The INT-F system appeared to be the most favourable system in terms of both soil N balance and N runoff. When the annual (May-April) N balance, calculated as N-fertiliser+N-slurry-N-harvest. the annual precipitation and the precipitation from the previous year were used as predictors in a linear regression model, 87 and 65% of the variation in N runoff could be explained from the arable and forage cropping systems, respectively. The average N balance calculated for all years, on its own predicted 86% of the variation in N runoff from the arable systems. Mass N balances were thus found to be a useful tool for predicting N runoff, especially in systems with mainly arable crops. (C) 2000 Elsevier Science B.V. All rights reserved.