Five mesquite dunes were irrigated by pumping water from storage tank located 200 m from the site. Water was applied to the dunes with an overhead sprinkler (later replaced by a drip irrigation system to minimize wind drift) located at the apex of each dune. Irrigation began in March with approximately 2.5 cm applied 2-3 times/week for 6 weeks to initially wet the dry soil. Thereafter high water availability was maintained by weekly irrigation. Five nonirrigated dunes served as control. Soil samples were collected (hand augered) under mesquite (Prosopis glandulosa) trees at sand dune site on the Jornada LTER site to three depths, 0-50 and 50-100, and 100-150cm. Soil samples were placed in plastic bags, tied, and stored in portable coolers. Soil subsamples for nutrient analysis were placed in polyethylene bags and stored at 2 to 7 C to maintain soil moisture content at field levels until the start of the incubations. After sieving (2-mm mesh), duplicate 25-g soil samples in 40-ml plastic vials were covered with 0.5 ml (0.0125-mm thick) polyethylene film to permit aeration and reduce moisture loss (Bremner and Douglas, 1971; Westermann and Crothers, 1980). The samples were incubated at 29 C for 1 week (one full set) and 4 weeks (duplicate set). Moisture content was adjusted to 0.1 g/g as the soils were placed in the vials and was maintained weekly using a syringe to add water through a small hole in the polyethylene film (Fisher et al., 1987). Soils were extracted with 2N KCl (Keeney and Nelson, 1982) for exchangeable NH4+ and NO3- and analyzed colorimetrically on a Technicon autoanalyzer.