The fate of residual nitrogen fertiliser applied to a ryegrass (Lolium perenne L.) seed crop

Abstract

Large amounts of the nitrogen (N) fertiliser applied to ryegrass seed crops remain within the soil at seed harvest and can potentially affect subsequent pasture production and environmental contamination. The fate of residual urea-¹⁵N-labelled fertiliser and the effect of previous fertiliser application on subsequent leaching losses and pasture production was assessed during a 9-month period after seed harvest using monolith lysimeters (diameter, 180 mm; length, 300 mm) in Canterbury, New Zealand. Results indicated that leaching losses and pasture uptake of residual ¹⁵N-labelled fertiliser were largely restricted by the immobilisation of ¹⁵N-labelled fertiliser into soil organic pools and the expanding root mass. Most of the ¹⁵N-labelled fertiliser remaining in the soil 9 months after the seed harvest was present within the humified organic matter (50%) and microbial biomass (40%) pools; the majority (62%) was anaerobically mineralisable. The ¹⁵N-labelled fertiliser that became available was largely recovered in rapidly expanding ryegrass roots, which increased 3–4-fold between seed harvest (December 1997) and pasture harvest (September 1998). Root mass, soil mineral N, and soil microbial biomass N were significantly (P < 0.05) greater in fertilised treatments than in controls at pasture harvest; clay-fixed N, anaerobically mineralisable N, and total N were not affected. The results indicated that, in the short term, N mineralisation rates were increased by previous fertiliser application but there was little evidence of a longer term effect on N mineralisation rates.