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

Summary

Introduction
Cut grassland systems usually have high N efficiency and consequently low nitrate leaching, whereas the introduction of grazing animals increases the loss potential dramatically (Jarvis, 2000). It has been demonstrated that nitrate leaching from unfertilised grass-clover swards is lower than from mineral-N fertilised swards grazed by cattle. The possible explanation for this is that the N2 fixation by pasture legumes is regulated by a natural feedback mechanism driven by soil inorganic N levels. The feedback mechanism acts as a limit to N inputs from legumes and consequently regulates the potential for N losses (Ledgard, 2001). In this paper we report 5 years (1997-2002) of nitrate leaching from four cropping sequences with different grassland frequency and management for both unfertilised grass-clover and fertilised monoculture grass. Furthermore, one year of N2 fixation in 1, 2 and 8-year-old grass-clover pastures is reported.
Materials and methods
Four cropping sequences (long-term grazed, long-term cut, cereals followed by 1 and 2 year grazed leys) were established for both unfertilised grass-clover (perennial ryegrass [Lolium perenne L.]/white clover [Trifolium repens L.]) and fertilised perennial ryegrass (300 kg N ha-1). Each plot was equipped with ceramic suction cups and nitrate leaching was estimated during 1997-2002 as described by Eriksen (2001). In 2001, N2 fixation was estimated in 1, 2 and 8-year-old grass-clover pastures using an enriched 15N-dilution method as described by Vinther and Jensen (2000). In the cropping sequence with cut grass, annual herbage production was determined during 1994-2001.
Results and discussion
Nitrate leaching from grazed unfertilised grass-clover was always considerably lower than from grazed fertilised ryegrass. In production years 6-8, losses from grass-clover were only 9-13% of the comparable losses from ryegrass, confirming the findings of others in systems with grazing cattle. This indicates a lowering of N inputs in grazed grass-clover swards with increasing age, since many data have suggested that nitrate leaching from grass systems are similar, at similar total N inputs, regardless of the source of N (Ledgard, 2001).
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. During the lifetime of the old grass-clover swards, N2-fixation dropped from 260 kg N ha-1 in the first three years (Eriksen, 2001) to 115 kg N in production year 8, in contrast to the fertilised grass sward that received 300 kg N ha-1 in all years. Thus, over the years a considerable difference between sward types has built up which may, at least partly, explain the differences in nitrate leaching. Following year 8, leaching from ryegrass exceeded that of grass-clover by on average 108 kg N ha-1.
The deposition of dung and urine by cows during grazing is unknown, as one herd (for practical and economic reasons) grazed all plots. However, some relationship is expected between grass production and manure deposition simply because cows graze more in highly productive plots which inevitably leads to higher deposition. As dry matter production was maintained in older cut ryegrass swards and decreased by almost 50% in cut grass-clover swards, deposition of excreta must be expected to be proportionally higher in ryegrass swards (assuming some similarity between cut and grazed grass production). This was confirmed by higher and considerably more variable nitrate concentrations in drainage extracted by the ceramic suction cups in old grazed ryegrass swards compared with grass-clover, probably due to urination by the grazing cattle. These observations suggest that the high nitrate leaching loss in ryegrass was not directly due to the fertiliser input, but rather an indirect effect of fertilisation on grass production leading to increased grazing and thus increased recycling of N in dung and urine to the sward.