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 Agenc y to use their judgment as to which soil variables would be most impac ted by five scenarios of climatic change in deserts of North America t hat could occur over the next 40 years. The following soil characteris tics were evaluated in terms of their potential for change: physical, chemical, and biological crusts; the vesicular layer; soil organic mat ter; organic C and N content; the C/N ratio; carbonate pool; inorganic N, P, and S; salinity levels; micro-element content; microbial commun ity composition; free-living microbial N fixation; denitrification; am monia volatilization; salinization rates; water infiltration; evaporat ion; lateral flow and leaching; wind and water erosion; and litter dec omposition. The Delphi approach was used to reach consensus on expecte d trends. Computer modeling was used to integrate and project interact ive changes. We expect physical and chemical crusting, vesicularity, a mmonium volatilization, soil erosion, and salt accumulation to increas e and microphytic crusts to decrease under all scenarios of climatic c hange. Both soil organic C and N will decline, especially under increa sed temperatures, whereas the C/N ratio will decline to its lowest ran ge of possible values. Both free and symbiotic N fixation should decli ne unless there is a shift to greater absolute precipitation during su mmers. Only slight changes in soil P, S, and trace element contents ar e expected under any of the five scenarios of climatic change. Product ion of litter will change relatively little, but its chemical quality will decrease and nutrient cycling will be accelerated when the vegeta tion shifts from perennials to annuals. Use of the Century Model showe d that soil organic matter is more sensitive to temperature changes th an precipitation. Indicators of desert ecosystem ''health'' that we id entified are relatively low albedo, patchiness of plant cover and inte rspace (trend depends on context), changes in drainage patterns and mi crorelief, biological crusting, and ratios of microbial biomass C to t otal organic C. Need for further research is outlined.