LONG-TERM TILLAGE, CROP-ROTATION, AND NITROGEN-FERTILIZER EFFECTS ON WHEAT YIELD UNDER RAIN-FED MEDITERRANEAN CONDITIONS

The combined long-term effects of tillage method and crop rotation on crop yield have not been studied in rainfed systems under Mediterranea n climates. A field study was conducted from 1988 to 1994 to determine the effects of tillage (TILL), crop rotation (ROT) and N fertilizer o n wheat (Triticum aestivum L.) yield in a rainfed Mediterranean region . Tillage treatments include no tillage (NT) and conventional tillage (CT). Crop rotations were wheat-sunflower (Helianthus annuus L.) (WS), wheat-chickpea (Cicer arietinum L.) (WCP), wheat-fababean (Vicia faba L.) (WFB), wheat-fallow (WF), and continuous wheat (CW), with N ferti lizer rates of 50, 100, and 150 kg N ha(-1), A split-split plot design with four replications was used. Differences in rainfall during the g rowing season had a marked effect on wheat yield. Amount of rainfall d uring the vegetative period for wheat (November-February) was highly c orrelated with yield because of the high water-retention capacity of V ertisols (Typic Haploxerert). In dry years, wheat yield was greater un der NT than under CT; the opposite was true in wet years. The TILL x R OT interaction was also significant in the drought years; the wheat yi eld under NT was greater for CW and the WFB and WF rotations than unde r CT. Wheat yields ranked by crop rotation were: WFB > WF much greater than WCP > WS much greater than CW. Wheat did not respond to N fertil izer when rainfall was below 450 mm during the growing season. Using t hese results strategies can be developed for establishing the N fertil izer rate applied to wheat as a function of rainfall, the preceding cr op, and residual N in soil in order to optimize wheat yield and reduce nitrate pollution to groundwater.