Residual effect of nitrogen fertilisation in a wheat-sunflower cropping sequence on a Vertisol under semi-arid Mediterranean conditions

The residual effect of the common N fertilisation (a three-split application of a total of 100 kg N ha(-1)) for winter wheat on the following sunflower crop was studied in a Vertisol under semi-arid Mediterranean growing conditions. For this study an experimental plot having a N-15 labelled microplot was installed. Of the fertiliser N applied to the previous winter wheat crop, 69.2 and 0.8% was left at winter wheat harvest in the soil profile (up to 90 cm) and the wheat stubble, respectively. The recovery of the residual labelled fertiliser N by the subsequent sunflower crop (Helianthus annuus, L.) was 3.6% of the fertiliser added to the previous winter wheat crop. The possible reasons for this low plant recovery were immobilisation of the fertiliser N and the limited N uptake by the drought affected sunflower crop, At harvest of the sunflower 49.7% of the added labelled fertiliser was still left in the soil profile up to 90 cm depth. The total recovery of fertiliser N over the two growing seasons was 83%. About 15% of the residual fertiliser N at wheat harvest was not recovered at sunflower harvest. Unrecovered N-15 was presumably lost by denitrification during periods of temporary waterlogging of the surface soil during irrigation and due to leaching beyond 90 cm depth. It is concluded that, in spite of the carry-over effect of fertiliser N from one growing season to another under the given conditions, the immediate residual effect of fertiliser N on the following crop was low. This means that for optimal financial profit fertiliser N must be applied at rates on an annual basis, according to the crop needs. (C) 1998 Elsevier Science B.V. All rights reserved.