Efficiency of nitrogen in wheat under Mediterranean conditions: effect of tillage, crop rotation and N fertilization

Within the framework of a long-term experiment started in 1986, a 3-year field study was carried out under rainfed Mediterranean conditions to determine the effect of tillage, crop rotation and N fertilization on the efficiency of nitrogen in wheat. Tillage treatments included no tillage (NT) and conventional tillage (CT). Crop rotations were wheat–sunflower (Helianthus annuus L.) (WS), wheat–chickpea (Cicer arietinum L.) (WC), wheat–faba bean (Vicia faba L.) (WB), wheat–fallow (WF) and continuous wheat (WW). Nitrogen fertilizer rates were 0, 50, 100 and 150 kg N ha−1. A split–split plot design with four replications was used. The high rainfall recorded over the three study years (60% above average) had a negative influence on wheat growth and yield. Soil NO3−-N and mineralized N content did not vary with tillage treatment. Residual NO3−-N was greater in WW and in the WB and WF rotations, whereas lower levels were recorded with the WS rotation. Mineralized N levels fell sharply for WW and were higher for WS and WF. Wheat yield was greater with CT than with NT. Clear differences in yield were recorded for all 2-year rotations with respect to WW. Wheat yield showed no additional response to N fertilizer rates above 100 kg ha−1. N use efficiency (NUE) and N uptake efficiency (NUpE) were greater with CT than with NT. These indices, together with N utilization efficiency (NUtE), N harvest index (NHI), N physiological efficiency (NPE), N agronomic efficiency (NAE) and N apparent recovery fraction (NRF) generally pointed to greater NUE for the WS rotation, and poor efficiency for wheat monoculture. Moreover, all indices except NPE fell as N fertilizer rates increased, indicating poor N utilization.