, Volume 76, Issue 1-3, pp 23-33

Nitrogen relationships in intensively managed temperate grasslands

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Summary

Most studies of N relationships in grassland have used cut swards. These have shown that for annual inputs of 200 to 400 kg N/ha from fertilizer or fixation, 55 to 80% of the N is recovered in harvested herbage. Generally, no more than 5 to 15% is lost through leaching and denitrification with most of the remaining N incorporated into soil organic matter. The relatively high efficiency of N use by cut swards reflects rapid uptake of N and the removal of a large part of the input in herbage. Inclusion of the grazing ruminant alters the efficiency of N use; only 5–20% of the input is recovered in meat or milk, and 75 to 90% of the N ingested is excreted, mainly as urea in urine. Application of N in urine ranges from 30–100 g/m2. Too much N is voided for effective recovery by the sward whilst soils usually contain insufficient C to allow appreciable immobilization. The surfeit is lost. Hydrolysis of urea is usually complete within 24 h of urine deposition. For urine-treated pasture in New Zealand (NZ) losses by NH3 volatilization of up to 66% of applied N are found during warm dry weather, with an average of 28% for a range of seasonal conditions. In the UK, the average rate of NH3 loss from an intensively grazed ryegrass sward was 0.75 kg N/ha/day during a 6-month season. NH 4 + remaining in the soil may be nitrified, nitrification being complete within 3 to 6 weeks. Although some NO 3 is recovered by plants, a substantial portion is leached and/or denitrified. On average such losses were 42%, with only 30% of the added N recovered by plants in urine-treated pasture in NZ. In the UK annual leaching of 150 to 190 kg N/ha has been observed for grazed swards receiving 420 kg N/ha/yr. Low retention of N by grazing ruminants results in a breakdown of N relationships in intensively managed grasslands. The substantial losses through NH3 volatilization, leaching and denitrification have serious agronomic, economic and environmental implications.