Fate of fertiliser N applied to winter wheat growing on a Vertisol in a Mediterranean environment

A field study using N-15 was conducted on a Vertisol in semi-arid Morocco to assess the fate and efficiency of fertiliser N split applied to winter wheat (Triticum aestivum L.). Splitting of fertiliser N is highly crucial in semiarid regions, considering the increased moisture stress towards the end of the growing season. A N fertilisation rate of 100 kg N ha(-1) was split according to two schemes: i) 25% at planting, 50% at tillering and 25% at stem elongation; or ii) 50% at tillering and 50% at stem elongation. The application of 100 kg N ha(-1) increased the vegetative dry matter production with more than 2000 kg dry matter ha(-1) in comparison with the control treatment. Nitrogen fertilisation had no significant effect on the grain yield production. Moreover, the 1000 grain weight decreased from 32 to 26 g due to N fertilisation. Total N uptake was about 50 kg N ha(-1) higher for the fertilised plants in comparison with the unfertilised plants, but it was not affected by the splitting pattern of the fertiliser N. Recoveries of N-15-labelled fertiliser by the plant (above-ground plant parts plus roots from the upper 20 cm layer) were low (31% and 24% for the 3-split and 2-split application, respectively). More N in the plant was derived from fertiliser when applied early in the growing season than when applied late in the season. About 13% of the N in the plants was derived from the 50 kg N ha(-1) at tillering, while only 5% was derived from the N application (50 kg N ha(-1)) at stem elongation. At harvest, a high residual of fertiliser-derived N was found in the 0-90 cm profile (62% and 72%, for the 3-split and 2-split application, respectively). Less than 10% of the applied N could not be accounted for, the amount being highest for the application at tillering. This N not accounted for was mainly ascribed to denitrification after an important rainfall event. The application of fertiliser N led to an increase of about 20 kg N ha(-1) in soil N uptake by the crop (positive ANI). The results suggested a dominant influence of moisture availability on the fertiliser N uptake by wheat.