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

Abstract

The human population is expected to double so there will be at least a doubled demand for food production. This will increase the demand for irrigation because irrigation gives a higher potential yield per unit area than non-irrigated agriculture, together with more yield stability. The demand for irrigation will be especially focused in semi-arid regions supporting a large population, such as the Mediterranean basin and the Indo–Gangetic plain of 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 dioxide concentration. Irrigation in semi-arid climates is a major cause of secondary salinisation (that due to human activity) which already affects 1 ha in five of the irrigated lands. Increased demand for irrigation in semi-arid climates, as a result of both population increase and climate change, will tend to increase the extent of secondary salinisation. Any increase in the extent of secondary salinisation could be offset by positive effects of elevated atmospheric CO 2 on crop yield per unit area and per unit input of water. In protected environments, or where CO 2 is the only experimental variable, elevated CO 2 usually enhances plant growth and water-use-efficiency in the short-term and can also do so in the longer term. However, for crop production in the field world-wide, elevated CO 2 per se is not a factor that can be viewed separately from the climate change that it will bring about. Neither the anticipated `CO 2-fertilisation' nor the `water-use-efficiency' benefits to the plant of elevated CO 2 is certain to outweigh the climatic effects of elevated CO 2 on temperature, water availability and evaporative demand. Climate change is expected to cause a net increase in the proportion of land classed as semi-arid. Raised temperatures may benefit some crops in some places but disadvantage others through increased evapotranspiration and thermal damage. Increased water-use-efficiency may not reduce leaf salt concentration in a saline environment. Buffering and feedback effects in both agricultural and ecological systems conspire to moderate or even to confound the anticipated gains in net assimilation and water-use-efficiency found in experimental systems. Elevated CO 2 may not, therefore, provide the anticipated decrease in water-use, decrease in leaf salt concentration, and increase in fixed carbon available for re-allocation: factors that might enhance crop performance under salinity stress. If these benefits are not realised then elevated atmospheric CO 2 will exacerbate rather than moderate the problems of secondary salinity in agriculture.