Uptake and residual value of ¹⁵N-labelled fertilizer applied to first and second year grass seed crops in New Zealand

Abstract

This study was established to quantify the uptake of ¹⁵N-labelled nitrogen (urea) applied in the first and second years of perennial ryegrass (Lolium perenne L.), tall fescue (Festuca arundinacea Schreb.) and browntop (Agrostis capillaris L.) seed crops, and the availability of the residual fertilizer N to a subsequent wheat (Triticum aestivum L.) crop under field conditions in Canterbury, New Zealand. Total recovery of ¹⁵N-labelled nitrogen fertilizer was approximately 100% when fertilizer was applied to the grass seed crops in spring. At harvest in year 1, grass straw and seed contained 34–47% and 6–15% of the applied N respectively; 27–35% remained in the soil (0–150 mm depth). Recovery of ¹⁵N in straw and soil was higher in fescue and ryegrass than in browntop, but recovery in roots was lower. At harvest in year 2, most of the ¹⁵N was present in the soil (30–37%) with only small amounts in the seed (0·7–1·0%), straw (3·6–4·9%) and roots (5·2–12·7%). In year 3, 2·5–3·5% of the residual ¹⁵N was recovered in the wheat and 18–26% in soil. Losses of ¹⁵N were minimal until ploughing after the second harvest, when there was an apparent loss of 11–35% of the fertilizer N applied. Losses were not directly associated with the fertilizer but indirectly following release of fertilizer N previously immobilized in plant roots and soil microorganisms. Small losses also occurred directly from autumn-applied N, probably through leaching. Despite these losses, overall there was an accumulation of fertilizer N in the soil organic pool, suggesting that ryegrass fescue and browntop seed crops have a role in contributing to the N fertility of the soil.