Recovery of 15N-labelled urea applied to wheat (Triticum aestivum L.) in the Ethiopian highlands as affected by P fertilization

Soil degradation and depletion of soil nutrients are among the major factors threatening sustainable cereal production in the Ethiopian highlands. Fertilizer use as a means of mitigating nutrient depletion has become widely accepted. The fate of fertilizer nitrogen (N) applied to a semidwarf bread wheat system was determined in microplots receiving 41 kg N ha(-1) in the form of urea labelled with 5.617 % atom excess (15) N, without and with the application of phosphorus (P) at 20 kg P ha(-1) . The study was conducted in three different agro-ecological zones at 2200, 2400 and 2800 m above sea level (masl) (Kulumsa, Asasa and Bekoji, respectively) in the south-eastern Ethiopian highlands. Yield and yield response to N diminished with elevation, while the response to P increased with elevation, with a significant increase in wheat grain yield observed at Bekoji (2800 masl). N harvest index and N use efficiency improved in response to P application at Asasa (2400 masl). Crop removal accounted for 25-64 and 36-64 % of the applied N without and with P, respectively. The corresponding fractions of (15) N recovered in the soil ranged from 2.9 to 3.5 % and from 3.2 to 3.7 %, respectively. Recoveries of applied (15) N were 67 and 52 % at Kulumsa and Asasa, but only 25 % at the highest elevation site, Bekoji. Fertilizer P significantly enhanced N recovery at Asasa (60 %) and Bekoji (36 %), but not at Kulumsa, where the soil had a high level of residual P from previous applications. The fraction of (15) N recovered in the soil-plant system was rather low at Bekoji. However, the rate of (15) N recovery was significantly improved (by 37 %) when fertilizer P was added to the soil. Across all locations, the unaccounted (15) N may have been lost primarily through denitrification and volatilization.