Fate and recovery of ¹⁵N-labelled fertilizer urea applied to winter wheat in spring in the Canterbury region of New Zealand

Abstract

¹⁵N-labelled fertilizer urea was applied at increasing rates (0–200 kg N/ha), in spring, to winter wheat crops in the Canterbury region of New Zealand in three successive seasons (1993/94, 1994/95 and 1995/96). Recovery of fertilizer N by the crop (grain, chaff, straw and roots) ranged from 43–58% (mean 48%). The quantity of fertilizer N retained in the soil (0–40 cm), at harvest, ranged from 26–42%. Of the labelled N present in the soil, over 95% was present in organic form and 60–80% was retained in the surface 0–10 cm layer. Since soil organic matter represents a substantial sink for fertilizer N there is a need to characterize the nature of this organic pool of N more fully. The quantity of inorganic N present in the soil profile at harvest ranged from 20–46 kg N/ha and labelled fertilizer-derived N contributed less than 16% (mean 9·2%) to this inorganic pool. Loss of fertilizer N from the crop/soil system (i.e., labelled N not recovered in the crop or soil at harvest) varied from 12–26% (mean 18%). Losses were attributed mainly to denitrification since conditions were not conducive for ammonia volatilization or leaching of nitrate. In agreement with European research, it was concluded that almost all of the N at risk of leaching over the winter originates from mineralization of soil organic N and not from unused fertilizer-N applied in spring.