EFFECTS OF CLIMATIC-CHANGE ON THE EDAPHIC FEATURES OF ARID AND SEMIARID LANDS OF WESTERN NORTH-AMERICA

A group of specialists was asked by the Environmental Protection Agency to use their judgment as to which soil variables would be most impacted by five scenarios of climatic change in deserts of North America that could occur over the next 40 years. The following soil characteristics were evaluated in terms of their potential for change: physical, chemical, and biological crusts; the vesicular layer; soil organic matter; organic C and N content; the C/N ratio; carbonate pool; inorganic N, P, and S; salinity levels; micro-element content; microbial community composition; free-living microbial N fixation; denitrification; ammonia volatilization; salinization rates; water infiltration; evaporation; lateral flow and leaching; wind and water erosion; and litter decomposition. The Delphi approach was used to reach consensus on expected trends. Computer modeling was used to integrate and project interactive changes. We expect physical and chemical crusting, vesicularity, ammonium volatilization, soil erosion, and salt accumulation to increase and microphytic crusts to decrease under all scenarios of climatic change. Both soil organic C and N will decline, especially under increased temperatures, whereas the C/N ratio will decline to its lowest range of possible values. Both free and symbiotic N fixation should decline unless there is a shift to greater absolute precipitation during summers. Only slight changes in soil P, S, and trace element contents are expected under any of the five scenarios of climatic change. Production of litter will change relatively little, but its chemical quality will decrease and nutrient cycling will be accelerated when the vegetation shifts from perennials to annuals. Use of the Century Model showed that soil organic matter is more sensitive to temperature changes than precipitation. Indicators of desert ecosystem ''health'' that we identified are relatively low albedo, patchiness of plant cover and interspace (trend depends on context), changes in drainage patterns and microrelief, biological crusting, and ratios of microbial biomass C to total organic C. Need for further research is outlined.