Recovery of wheat residue Nitrogen15 and residual effects of N15 fertilisation in a wheat - wheat cropping system under Mediterranean conditions

For cropping systems, quantification of the fate of fertiliser N and residual N is essential, to develop management practices that optimise N use efficiency. A field study was conducted in Saharan Morocco to assess the fate of fertiliser N in a wheat (Triticum durum var. Karim) - wheat cropping sequence. Therefore 85 kg ha-1 N as labelled ammonium sulfate (9.764% atomic excess) was applied in a three-split application. Fertiliser N recovery by wheat in the first year was 33.1%. At harvest, 64.8% of fertiliser N was found in the 0 - 80 cm profile as residual fertiliser-derived N; 2.1% of the applied N could not be accounted for year 1996/1997. The N unaccounted for was mainly ascribed to denitrification. The recovery of the residual labelled fertiliser N by the subsequent wheat crop was 6.4% for the treatment without residue incorporation, and 7.4% in the case of residue incorporation. The un recovered N after the second cropping season was 15.6 and 11.8% without and with residue incorporation, respectively. Loss of labelled N by the soil-plant system was not due to leaching but to denitrification. In the treatment (N+*R) with labelled residue incorporation, the %N recovery by plants was 16.2%, indicating the mineralisation of the residue applied