The fate of autumn-, late winter- and spring-applied nitrogen fertilizer in a perennial ryegrass (Lolium perenne L.) seed crop on a silt loam soil inCanterbury, New Zealand

The use of fertilizer nitrogen (N) during seasons other than spring to incr ease seed yield of perennial ryegrass (Lolium perenne L.) has raised concer ns about possible environmental contamination from fertilizer N. The fate o f N-15-labelled fertilizer applied at several rates during autumn, late win der and/or spring was measured using silt loam (Eutric Cambisol (FAO)) mono lith lysimeters (500mm diameter, 600 mm deep) during two, I-year lysimeter studies in Canterbury, New Zealand. Total ryegrass uptake of late winter an d spring applied N-15-labelled fertilizer at harvest was greater (40 and 43 %, respectively) than N fertilizer applied in autumn (18%) in 1996 and 1997 , reflecting plant demand and soil immobilization in the different seasons. Consequently, in conjunction with spring N fertilizer, late winter N ferti lizer application had significantly (P<0.05) greater affects on seed yields than autumn applied N fertilizer. Losses of late winter applied N fertilizer (9%) were significantly lower th an from N fertilizer applied during autumn (23%) or spring (19%) because of conditions which encouraged leaching and denitrification during autumn and ammonia volatilization during spring. Risk:benefit analysis showed that, under the climatic and soil conditions p revailing during these experiments, autumn N fertilizer application was ass ociated with high risk of losses and small benefits in crop N recovery. Lat e winter and spring N fertilizer had substantial bene fits with much lower risk. (C) 2001 Elsevier Science B.V. All rights reserved.