Predicting the interaction between the effects of salinity and climate change on crop plants

The human population is expected to double so there will be at least a doub led demand for food production. This will increase the demand for irrigatio n because irrigation gives a higher potential yield per unit area than non- irrigated agriculture, together with more yield stability. The demand for i rrigation will be especially focused in semi-arid regions supporting a larg e population, such as the Mediterranean basin and the Indo-Gangetic plain o f northern India and Pakistan. The 'hot/warm semi-arid' agro-climatic zone is the one projected most to expand in relative proportion as a consequence of climate change brought about by the increase in atmospheric carbon diox ide concentration. Irrigation in semi-arid climates is a major cause of sec ondary salinisation (that due to human activity) which already affects 1 ha in five of the irrigated lands. Increased demand for irrigation in semi-ar id climates, as a result of both population increase and climate change, wi ll tend to increase the extent of secondary salinisation. Any increase in t he extent of secondary salinisation could be offset by positive effects of elevated atmospheric CO2 on crop yield per unit area and per unit input of water. In protected environments, or where CO2 is the only experimental var iable, elevated CO2 usually enhances plant growth and water-use-efficiency in the short-term and can also do so in the longer term. However, far crop production in the field world-wide, elevated CO2 per se is not a factor tha t can be viewed separately from the climate change that it will bring about . Neither the anticipated 'CO2-fertilisation' nor the 'water-use-efficiency ' benefits to the plant of elevated CO2 is certain to outweigh the climatic effects of elevated CO2 on temperature, water availability and evaporative demand. Climate change is expected to cause a net increase in the proporti on of land classed as semi-arid. Raised temperatures may benefit some crops in some places but disadvantage others through increased evapotranspiratio n and thermal damage. Increased water-use-efficiency may not reduce leaf sa lt concentration in a saline environment. Buffering and feedback effects in both agricultural and ecological systems conspire to moderate or even to c onfound the anticipated gains in net assimilation and water-use-efficiency found in experimental systems. Elevated CO2 may not, therefore, provide the anticipated decrease in water-use, decrease in leaf salt concentration, an d increase in fixed carbon available for re-allocation: factors that might enhance crop performance under salinity stress. If these benefits are not r ealised then elevated atmospheric CO2 will exacerbate rather than moderate the problems of secondary salinity in agriculture. (C) 1999 Elsevier Scienc e B.V. All rights reserved.