The effects of microarthropods on nitrogen availability within the rhizosphere of Erioneuron pulchellum in a northern Chihuahuan desert ecosystem

Recent studies suggest that rhizosphere soil microarthropods may have a major role in determining soil nitrogen availability. Desert soil microarthropods are consumers of soil bacteria, fungi, and nematodes, thus they accelerate mineralization processes by causing turnover in immobilized nitrogen. Therefore, I hypothesized that changing densities of soil microarthropods would result in changes in nitrogen availability. In order to test this hypothesis, Erioneuron pulchellum rhizosphere soil samples were taken monthly from control plots, plots irrigated with 6 mm/wk., plots soaked with chlordane (to remove microarthropods), and plots treated with chlordane that were irrigated with 6 mm/wk. These samples were analyzed for available inorganic nitrogen (NO{dollar}sb3{dollar} and NH{dollar}sb4{dollar}), gravimetric soil moisture, plant shoot and root biomass, plant shoot and root total nitrogen, plant growth, microarthropod and nematode densities. Microarthropods and nematodes responded to water only after a long dry period in April 1987. Water seemed to deplete nitrogen from soils, enhancing turnover and rapid nitrogen mineralization in the first year. This resulted in nitrogen depletion in the second year. Nematodes increased density in response to elimination of microarthropods only during the unusual wet winter-spring 1986-87, when soil water potential was above {dollar}-0.4{dollar} MPa most of the time. Biocide treatment used to eliminate microarthropods, led to an increase in soil available nitrogen, but a decrease in plant root nitrogen. There was no significant difference in plant shoot biomass among treatments, and root biomass was higher only in the irrigated plots with microarthopods. There was no correlation between microarthropods and plant shoot or root total nitrogen among treatments. Over all treatments and dates root biomass was very low, averaging only 0.23 g per kg dry soil. These data suggest that in the live rhizosphere of Erioneuron pulchellum, if water is available, then soil biota and nitrogen dynamics might be regulated by organic matter. Also, there were no significant differences in nitrogen mineralization potentials of soils from the various treatments. These data indicate that soil microarthropods are not essential for nitrogen mineralization or other aspects of the nitrogen cycle in the rhizosphere of fluff grass.