DESERTIFICATION: CHANGES IN SOIL BIOGEOCHEMISTRY IN THE CHIHUAHUAN DESERT

Changes in vegetation cover-a loss of grasslands in favor of shrublands dominated by creosotebush and mesquite-characterize the desertification process in the Chihuahuan Desert of New Mexico and West Texas. The Jornada Basin Long-Term Ecological Research (LTER) Project has synthesized changes in the landscape mass-balance of plant-essential soil nutrients that accompany this change in vegetation. Receipts of nitrogen are derived from atmospheric deposition (2.5 kgN/ha/yr) and symbiotic nitrogen-fixation by mesquite (21 kgN/ha/yr), whereas losses are dominated by wind erosion (14 kgN/ha/yr), surface runoff (< 5 kgN/ha/yr), and denitrification (4 - 6 kgN/ha/yr). With the transition to shrub-dominated landscapes, nutrient losses predominate over inputs, so that the soils show a progressive depletion of soil nutrients. Moreover, the distribution of soil nutrients in shrublands is markedly patchy, with high concentrations of nutrients under shrubs and relatively barren soils in the shrub interspaces. The internal cycle of nutrients in each vegetation type (25-40 kgN/ha/yr) exceeds the new inputs of nutrients to the ecosystem, so that decomposition and nutrient mineralization beneath shrubs fosters their persistence on the landscape. Lower plant production is the expected and traditional outcome of arid-land degradation, but changes in the spatial distribution of soil resources may be a more effective index of desertification.