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

Abstract

Grass-legume associations may offer a way of improving the N efficiency of dairy farming, but may also have an adverse impact on the environment by increasing 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 unfertilized grass-clover (perennial ryegrass (Lolium perenne L.)/white clover (Trifolium repens L.)) and fertilized perennial ryegrass (300 kg N/ha) swards during 1997–2002. Furthermore, 1 year (2001) of N2 fixation in 1-, 2- and 8-year-old grass-clover pastures was determined. Nitrate leaching from grazed unfertilized grass-clover was always considerably lower than from grazed fertilized 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 6–8, leaching from grass-clover was only 9–13% of the comparable losses from ryegrass. Under the 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 with 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 the present study indicate that the higher leaching losses observed in fertilized grass compared with unfertilized 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.

Residual effect of clover-rich leys on soil nitrogen and successive grain crops

“…The effect of the age of the ley on the N leaching risk was insignificant in our experiments, which agrees with the findings of Hansen et al (2005) and Eriksen et al (2004). The mineral N level in soil at a depth of 60-90 cm was quite low, less than 5 kg ha -1 in Juva, indicating low N leaching risk.…”

“…The decrease in the yield and BNF (i.e. N content) of 2-to 8-yearold grass-clover ley led to decreased N leaching in studies of Eriksen et al (2004), but Hansen et al (2005) did not find any significant difference in N leaching after 1-or 10-year-old grass-white clover ley. This indicates that organic N mineralises more easily the younger the perennial plants are.…”

Challenges in modelling dissolved organic matter dynamics in agricultural soil using DAISY

“…In 1994 and 1995, the N 2 fixations were estimated as 232 and 408 kg N ha À1 year À1 , respectively (Søegaard et al, 2001), and in 1997 and 1998 as 263 and 124 kg N ha À1 year À1 , respectively (Hansen and Vinther, 2001). In 2001, the N 2 fixation was estimated as 101 kg N ha À1 year À1 (Eriksen et al, 2004). Continuous clover-grass in treatment A increased the SOM content in the simulations from approximately 140 to 143 t C ha À1 in the period of 1995-2003 which corresponds to an increase of 1.9%.…”

Effects of sward botanical composition on nitrogen and potassium leaching in cut grassland

“…Concerning the grasslands it has been found that nutrient leaching is high only from pastures, in cut grassland it is often estimated to be low (Eriksen, Vinther, 2002). Although nutrient leaching from grassland is lower than from arable land it is still important to find opportunities for its reduction.…”

“…Bouman et al (2010) showed that in midsummer when the percentage of white clover in the sward is high, N leaching also increases. Eriksen et al (2004) have reported that less leaching occurs from unfertilized grassland containing white clover than from fertilized grass-only grassland. Low and Armitage (1970) studies have demonstrated increased leaching in the swards with reduced legume content.…”