Nitrate leaching and N2-fixation in grasslands of different composition, age and management

Summary

Grass-legume associations may offer a way of improving the N efficiency of dairy farming, but may also have adverse impact on the environment by increased leaching losses. Nitrate leaching from four cropping sequences with different grassland frequency and management (long-term grazed, long-term cut, cereals followed by 1 and 2-year grazed leys) were investigated on a loamy sand in central Jutland for both unfertilised grass-clover (perennial ryegrass [Lolium perenne L.]/white clover [Trifolium repens L.]) and fertilised perennial ryegrass (300 kg N/ha) during 1997-2002. Furthermore, one year (2001) of N2 fixation in 1, 2 and 8-year-old grass-clover pastures was determined. Nitrate leaching from grazed unfertilised grass-clover was always considerably lower than from grazed fertilised ryegrass. The effect of grassland age on nitrate leaching was insignificant in grass-clover but clear in grazed ryegrass, where levels increased dramatically with sward age. In production years 6 to 8, leaching from grass-clover were only 9-13% of the comparable losses from ryegrass. Under cutting regime grass-clover showed a significant reduction in both yield and N-removal with increasing sward age whereas for ryegrass these figures did not show any decreasing trend. N2 fixation was lower in 8-year-old swards compared to fully established 2-year-old swards as a consequence of lower dry matter production, lower clover content and a lower proportion of clover-N derived from the atmosphere. The results from this study indicate that the higher leaching losses observed in fertilised grass than in unfertilised grass-clover systems were caused by 1) a reduction in N2-fixation in grass-clover over time, and 2) a reduction in dry matter production in grass-clover over time lowering the grazing intensity and the recycling of grassland N via animal excreta.