Comparative Analysis of the Nitrogen Effect of Common Agricultural Practices and Rotation Systems in a Rainfed Mediterranean Environment

The nitrogen (N) effect of legumes is one of the main reasons for their inclusion in rotation systems and their success in rainfed agriculture of Mediterranean areas. The comparative analysis of this effect in relation to alternative systems or practices is essential for a comprehensive appreciation in their merit. This field experiment was comprised of four three-year rotation cycles. Wheat (Triticum turgidum durum) was seeded for two consecutive years after common vetch (Vicia sativa L.), treated in three different ways, and after fallow and compared with three wheat monocultures: the conventional one, the continuous straw incorporation, and the sewage sludge incorporation once every three years. Wheat grain and straw yields and N uptake were compared among treatments. Results showed that rotation systems that included vetch were the most promising for improving sustainability. Maximum N uptake and the greatest yield surpluses were obtained when wheat followed vetch incorporated during flowering. When vetch in the rotation was cut for hay or left to fill grains subsequent wheat showed also enhanced yields. Fallow affected the rotation system's fertility due to the incorporation of volunteer plants into the soil. Sewage sludge sustained production without the need for inorganic fertilization during three years. Straw incorporation always gave the smallest yields and N harvests, presumably due to soil N immobilization.