Nitrogen uptake by sunflower as affected by tillage and soil residual nitrogen in a wheat–sunflower rotation under rainfed Mediterranean conditions

Nitrogen fertilization of sunflower crops in semiarid climates is a controversial practice, since N availability is governed by rainfall patterns. A 4-year field study was carried out to determine the effects of tillage system and soil residual N on sunflower grain yield, N accumulation, N utilization efficiency (NUtE) and N harvest index (NHI) in a wheat–sunflower rotation under rainfed Mediterranean conditions. Tillage treatments included no-tillage (NT) and conventional-tillage (CT). Nitrogen-fertilizer rates were 0, 50, 100 and 150 kg N ha−1 applied only to wheat. A split-plot design with four replications was used. The heavy rainfall recorded in 3 of the 4 effective study years strongly influenced results. Tillage system failed to exert a significant influence on any of the indexes studied except grain N content, which was higher under CT than under NT. Soil residual N affected sunflower N accumulation and grain N content, both of which increased significantly with rising doses of N applied to the preceding wheat crop. In contrast, residual N did not influence grain yield. Accordingly, both NUtE and NHI declined with increasing N rates applied to wheat. The unusual incidence of excessive rainfall over the study period suggests that further long-term studies are required to determine effects under the dry conditions which are more typical of this region.