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 in Canterbury, New Zealand

Abstract

The use of fertilizer nitrogen (N) during seasons other than spring to increase seed yield of perennial ryegrass ( Lolium perenne L.) has raised concerns about possible environmental contamination from fertilizer N. The fate of 15N-labelled fertilizer applied at several rates during autumn, late winter and/or spring was measured using silt loam (Eutric Cambisol (FAO)) monolith lysimeters (500 mm diameter, 600 mm deep) during two, 1-year lysimeter studies in Canterbury, New Zealand. Total ryegrass uptake of late winter and spring applied 15N-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 fertilizer 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 than 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 prevailing during these experiments, autumn N fertilizer application was associated with high risk of losses and small benefits in crop N recovery. Late winter and spring N fertilizer had substantial benefits with much lower risk.