conservation (reduced) tillage (CT), by leaving the straw of the preceding crop on the surface as a mulch, chiselling (25±30 cm depth) after wheat, and discharrowing (5±7 cm depth) after sun¯ower, just before sowing the next wheat crop. More details of the complete agronomic operations in each treatment can be found in Moreno et al. (1997). Awheat (cultivar`yavaros')±sun¯ower (cultivar `¯orasol') crop rotation was established in each treatment, under rainfed conditions. Wheat was yearly fertilized with 400 kg ha of a complex fertilizer 15N±15P2O5±15K2O applied before sowing. Sunflower was not fertilized. Seeding
of sun¯ower was late February (5 March in 1997) and
harvest on late July, except for 1995 (10 July).
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