Uptake Efficiency and Partitioning of Soil and Fertilizer N Sources By Canola, Wheat, and Pea.

The indigenous soil N supply is often not factored into many N use efficiency equations. In order to determine the relative contribution of residual soil N, a greenhouse study was conducted in which wheat, inoculated field pea, and canola plant received labeled (¹⁵NH₄)₂SO₄ fertilizer at rates of 6, 60, 180 or 420 mg N kg soil^-1. The proportion of plant N derived from fertilizer (%Ndff), soil (%Ndfs), and atmosphere (%Ndfa) was measured using the isotope dilution technique. Multiple nitrogen recovery indices were calculated, including the recovery of ¹⁵N fertilizer by crops, N uptake efficiency (plant N/N supply), available N use efficiency (plant N/[plant N +residual N]), and the apparent fertilizer N recovery ([fertilized plant N- control plant N]/fertilizer rate).  Canola and wheat derived similar proportions of plant N from fertilizer, ranging from 4 to 65%. N fertilization had a significantly negative impact on %Ndfa by field pea, which declined from 53-3%. Nevertheless, a majority of plant N was derived from fertilizer only when fertilizer N contributed to more than 62% of the N supply. For all crops, uptake efficiency, apparent fertilizer N recovery, and available N use efficiency exceeded ¹⁵N crop recovery. Furthermore, less than 2% of the N supplied remained after the harvest of all crops, with the exception of pea fertilized at the highest rate. Our results highlight the importance of indigenous soil N in supplying N under limiting conditions or high losses, and supports the inclusion of indigenous N supply in N use indices.