DENITRIFICATION IN THE TOP-SOIL OF MANAGED GRASSLANDS IN THE NETHERLANDS IN RELATION TO SOIL TYPE AND FERTILIZER LEVEL

During the growing season of 1989 and 1990 denitrification measurements were carried out in grazed perennial rye-grass swards on sand, loam and peat soils, each receiving two rates of nitrogen fertilizer. Denitrification was measured monthly, using a coring system with acetylene inhibition. Denitrification rates were highly variable throughout the season but tended to increase with increasing soil water content. High denitrification rates (0.1 kg N ha(-1) day(-1)) were observed in spring and autumn, and were associated with rainfall or grazing in combination with a high soil water content. Estimated seasonal denitrification losses ranged between 1 and 21% of the fertilizer input, and showed no significant effect of fertilizer application rate. In 1990, denitrification losses were significantly higher on the loam soil. However, this difference was rather a result of different timing of sampling after rainfall or grazing, than of soil type differences. In the growing season of 1991, field experiments were carried out in the same sites, to measure denitrification losses during the first 8 or 10 days after fertilizer application. Fertilizer application significantly increased denitrification rates. Losses following one fertilizer application ranged from 6 to 9% of the fertilizer input, but only on a wet soil.