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The woody legume, mesquite (Prosopis glandulosa) has expanded from its historical habitats (playas and arroyos) to recently occupied grassland and dune habitats during the desertification of perennial grasslands in the Chihuahuan Desert. We studied historical and recently occupied sites, having hypothesized that the trophic structure and population density of soil microarthropods and nematodes associated with the surface root system of mesquite would differ in sites representing historical and recent habitats, and that the N mineralization potential would be lower in the recent habitats. Our results showed that net N mineralization potential did not differ significantly among the sites, even though soil nutrient concentrations and texture varied widely. Concentrations of organic C, N, and P were lowest in the recent dune habitat and highest at the playa. Very low concentrations of P in the dune and grassland soils implicated P as a limiting factor in these systems. The bacterial-feeding and omnivore-predator functional groups made up the largest fraction of the nematode community at most of the sites. The high density of plant-feeding nematodes at the playa indicated that herbivory is potentially most important at this site. Total microarthropod densities did not vary significantly among habitats, with Collembola densities highest in the mesquite dunes. Grazers were the dominant microarthropod functional group. While both C and N pool sizes were higher in the historical habitats, a higher substrate lability in the recent habitats appeared to support biota populations and N mineralization rates equivalent to those in the playa and arroyo. Differences in soil properties and biota among historical and recent mesquite habitats may be important for understanding the changes that have occurred in Chihuahuan Desert ecosystems during desertification.

The nematode community in litter and soil was examined for a year in the Chihuahuan desert, before and after supplemental rainfall application. Proportions of nematode-active or anhydrobiotic forms and population densities were determined for 3 treatments: control (natural rainfall), a single, large (25-mm) monthly irrigation pulse, and 4 smaller (6-mm) irrigations spaced at weekly intervals. In litter the greatest nematode abundance was in the 6 mm week-1 treatment (48 nematodes 20 g-1 litter). Bacteriovores and fungivores accounted for approximately 95% of the numbers and biomass in all treatments. In soil, water amendments had no significant effect (P < 0.05) on annual mean densities of total nematodes, fungivores, bacterivores, or omnivore predators. Phytophage densities were greater on both irrigation treatments, with highest densities (9268 m-2) in the 6 mm week-1 soils, which was 5.9% of the total soil nematode density. Total densities of individual trophic groups were not significantly different before or after rainfall. Soil nematode densities fluctuated independently with trophic group, month, and season. Bacterial feeders and omnivore predators were the largest contributor to total soil nematode density and biomass. Prior to irrigation, there were no differences in the percentage of anhydrobiotes on the three treatments. Anhydrobiotes decreased after irrigation in all treatments, and were significantly lower in soils of the larger, monthly irrigation. Nematodes were inactive (anhydrobiotic) and decoupled from decomposition processes when soil water matric potentials reached -0.4 MPa.

Free-living nematodes may be attacked and eaten by soil mites which are normally considered fungivores or saprophages. Three species in the genus Tyrophagus, common inhabitants of grassland soils and also common pests in stored products, museums and laboratories, are predators of nematodes. All active stages of the mites will voraciously consume nematodes. When offered nematodes and a choice of other food (baker's yeast and algae), 11% of the Tyrophagus putrescentiae, 23% of the T. zachvatkini, and 56% of the T. similis tested fed on nematodes. Tyrophagus zechvatkini and similis were reared on a diet consisting entirely of nematodes, and developed at rates similar to a fungal diet and produced viable offspring. Agar cultures of Aphelenchus avenae which were inoculated with five mating pairs of Tyrophagus zachvatkini had populations that were one-third less than mite-free controls. Observations indicate that nematodes may be attacked by tyrophagid mites when in a dry, anhydrobiotic state.