Influence of time of application on the fate of 15 N‐labelled urea applied to dairy pasture

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Urea ( 15 N‐enriched) was applied at 50 kg N/ha to dairy pasture in May, June, or August 1985. Initial pasture responses were 4.6, 6.2, and 6.2 kg dry matter (DM)/kg N respectively. Corresponding residual responses were 1.8, 2.6, and 4.9 kg DM/kg N respectively. Total plant recovery (shoots + roots) of 15 N by November was 31, 47, and 63% for May, June, and August applications respectively. During the slow‐growing period after the May application, about half the 15 N uptake was in the roots. Recovery of 15N in plant and soil (0 – 250 mm depth) by 5 – 6 months after application was 58 and 70% for May and June applications respectively. Measurements of inorganic soil 15 N with depth indicated that leaching had occurred. The high recovery (99%) following the August application was associated with rapid plant uptake and immobilisation by the soil microbial biomass. At the first harvest after application, the soil microbial 15 N (making up to 20% of total soil 15 N) was twice as large following the August application than after the May and June applications. This fraction declined significantly by the last harvest after the August application, coinciding with a relatively large residual pasture response to applied N.