Chapter 2 Nitrogen in Rainfed and Irrigated Cropping Systems in the Mediterranean Region

The Middle East region, embracing West Asia and North Africa (WANA), is characterized by a Mediterranean climate which dictates the agriculture of the region. Being one of the centers of origin of settled farming, where cereals and pulses originated along with sheep and goats, crop production is largely rainfed and is dependent on the limited rainfall (200-600 mm) during the cool moist season in late fall to late spring. The agricultural sector has changed in recent decades with intensification and pressure on land use. As elsewhere, increased chemical fertilizer use, especially nitrogen (N), has had an impact on the transition from a traditional system to a more market driven one with increased inputs. This review examines the varied aspects of N in the soils and cropping systems as reflected by research at The International Center for Agricultural Research in the Dry Areas (ICARDA) in Syria in collaboration with other countries of the West Asia-North Africa region, especially in Morocco and other countries north and south of the Mediterranean. The synthesis, therefore, reflects a broad overview of conditions that impinge an N nutrition of crops and the evolution of N research achievements since the advent of commercial fertilization over three decades ago. With few exceptions, the soils of the Mediterranean region are low in organic matter and consequently in the reserves of total N, thus posing a limit of growing crops without fertilizer N or biological N fixation (BNF) through legumes. Soil calibration studies established the value of the soil nitrate test as a predictor of crop response with field trials to establish application rates for the main crops. Applicability is influenced by depth of sampling and the extent of mineralization. Dryland crop responses to N varied widely throughout the region from 30 to 150 kg N ha- 1, being dependent on soil N status and seasonal rainfall as the major determinant of yields. Splitting the N application was only advantageous in higher rainfall areas. Residual N from BNF by food and forage legumes influenced soil N supply for cereals and relative responses to N fertilizer. The contribution of rhizobia fixation to all the major legumes was quantified using 15N along with management factors that influenced BNF by legumes. Where legumes were newly introduced to a region, rhizobial inoculation was considered necessary. With cereal responses to fertilizer N, differences between varieties were highlighted. Where urea or ammonium-N fertilizers were used, volatilization was the main loss mechanisms rather than leaching or denitrification. Considerable work was done on N use within crop rotation systems and components of the N cycle defined along with inputs from urine and feces from grazing animals. Forage legumes were shown to enhance total soil N and both labile and biomass N, with the least influence from fallow. These N forms were shown to fluctuate during the year as moisture and temperature conditions changed. Fertilizer N use had a positive effect on grain quality with increased protein, as well as soil organic matter (SOM) and thus soil quality. The significant change of the gradual introduction of supplemental irrigation in traditional rainfed cropping areas and its implications for use of models to describe the complex nature of N in dryland cropping systems was described. With the likelihood of a continuation of intensification of the dryland cropping systems in the Mediterranean region, N fertilizer use will inevitably increase and along with it the need for greater use efficiency in the interest of production economics and the environment. While limited use has been made of modeling of N, this approach is likely to be of more significance in integrating the varied facets of N under Mediterranean cropping conditions. © 2009 Elsevier Inc. All rights reserved