Total and Mineral Nitrogen in a Wheat-Based Rotation Trial under Dryland
 Mediterranean Conditions

Abstract

Cropping systems involving rotations have evolved in the Middle East region to cope with the everpresent constraint of limited soil moisture due to low and generally erratically distributed rainfall, and to the inherently low fertility mainly due to lack of nitrogen (N). Most dryland rotational systems involve conserving moisture in the fallow year for the subsequent cereal crop and input of N through Nfixation by legumes in the alternate year. Rotations have variable influences on crop yield, soil moisture, diseases, and especially nutrients. In this paper we report the influence of seven rotations, four N fertilizer application rates, and three stubble grazing management regimes in a 14-year cereal rotation trial in relation to total soil and mineral N (ammonium and nitrate). Total N was significantly higher in topsoil samples of the medic ( Medicago spp.) and vetch ( Vicia sativa ) rotation, with lowest values being in the fallow and melon (Citrullus vulgaris ) and continuous wheat rotation. Intermediate values were from the chickpea ( Cicer arietinum ) and lentil ( Lens culinaris ) rotations. The enrichment in total N in the medic rotation occurred down to 40-60 cm depth. By comparison with total N, mineral N values were about two orders of magnitude lower and showed no consistent trend with rotation. Interestingly, the mean effect of added fertilizer N was to consistently increase total N from 744 mg ha -1 in the control to 804 mg N kg -1 at the 90 kg -1 N application rate. Similarly, mineral N increased consistently with applied N showing a two-fold increase at the highest N rate. However, variable grazing of the stubble had no consistent effect on total or mineral N. Thus, in addition to the favourable effect on legume-based rotations on crop yields and water-use efficiency, this study clearly showed that forage legumes, and to a lesser extent food legumes, can lead to a build-up of soil N mainly as organic forms in semi-arid soils, which can subsequently contribute to cereal nutrition following mineralization. Such nutrient accumulation is critical for maintenance of soil quality and sustainability of land use, especially in rainfed Mediterranean cropping environments.