Studies of ecosystem processes on the Jornada Experimental Range in southern New Mexico suggest that longterm grazing of semiarid grasslands leads to an increase in the spatial and temporal heterogeneity of water, nitrogen, and other soil resources. Heterogeneity of soil resources promotes invasion by desert shrubs, which leads to a further localization of soil resources under shrub canopies. In the barren area between shrubs, soil fertility is lost by erosion and gaseous emissions. This positive feedback leads to the desertification of formerly productive land in southern New Mexico and in other regions, such as the Sahel. Future desertification is likely to be exacerbated by global climate warming and to cause significant changes in global biogeochemical cycles.

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Soil was collected in 30‐cm depth increments to 90 cm from beneath Prosopis glandulosa Torr. var. glandulosa (L. Benson) M. C. Jtn. (mesquite) and from the nonvegetated area between mesquite trees in a phreatophytic stand in the Sonoran Desert of southern California. Total N, NO‐3‐N, NH+4‐N, organic C, NaHCO3‐extractable PO3‐4‐P, and saturation extract K+ were significantly higher beneath mesquite, while Na+ and Cl‐ were significantly higher between mesquite trees. Differences in pH, the osmotic potential of saturation extracts, saturation percent, and SO2‐4‐S were nonsignificant. Large amounts of N (1.68 g kg−1 of soil) have accumulated in the surface 30 cm beneath mesquite. This N most likely had been symbiotically fixed by mesquite. Over 20% of the N in this ecosystem occurred as NO‐3. This unusual NO‐3 accumulation was possible since leaching and denitrification were probably limited by aridity. The Na+ adsorption ratio of saturation extracts (SAR), 0–30 cm, was significantly lower at the center of tree canopies (7.9) than in soil between trees (17.3) and also was lower than the groundwater (12.7). Foliar analysis indicated mesquite was excluding Na+. Consequently, the decomposition of mesquite litter produced a soil with a lower SAR than the nonvegetated soil. Soil N was determined at six additional Sonoran Desert sites. Nitrogen accumulation beneath mesquite was related to soil texture and water regime. Total soil N was highest on low relief phreatophytic sites with high clay content (1.34 g of N kg−1 of soil, 0–15 cm) and lowest in aeolian sand dunes (0.18 g of N kg−1 of soil, 0–15 cm). Woody legumes such as mesquite, through the accumulation of symbiotically fixed N and the accretion of other nutrients in the surface beneath their canopies, may be important in maintaining the long‐term productivity of some desert ecosystems.

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