Deep Soil Cores N-Mineralization

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Soil cores collected under mesquite (Prosopis glandulosa) trees at several sites and under Larrea at one site at the Jornada LTER were incubated to determine the N-mineralization potential.

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Field and electronic data sheets

*Undisturbed soil cores from the rooting zone of three trees in each ecosystem were removed using a split steel, continuous sampling tube, 1.56 m long with 6.5 cm i.d. This coring device, mounted on a truck, was modified from Kelley et al. (1947). The split-tube bit fits into an outer, rotating auger bit that served as a continuous casing to prevent cave-in. As the outer bit cut through the soil, the inner, nonrotating bit was pressed into the soil. Cores were collected at the edge of the mesquite canopy. The core retainer and the two halves of the split sampling tube were cleaned of all residual soil. Their interior surface was flame sterilized with 95% ethanol before being put together for sampling. Soil samples were removed from the surface 1 m of soil in 0.5 m increments, and thereafter in 1 m increments. Flame sterilized trowels and spatulas were used to replace each depth increment into a clean plastic bag. These were put into icecooled chests, and transported to the Univ. of California, Riverside, where they were subdivided for analysis. Drilling depth for each core was determined by either the absence of roots in two consecutive 1.56 m sampling tube lengths, or the presence of coarse, dry loose soil that could not be retained in the tube. The number of cores (three per ecosystem per sampling) collected was limited by the expense of obtaining the specialized drilling equipment used in this study. Sampling dates at New Mexico were in January 1986, the midpoint of the dormant season; late May 1986, during peak growth; and early October 1986 following the summer rains. The grassland site was not sampled on the Jan-Feb drillings. Each bag of soil representing a depth increment was mixed thoroughly before subsampling. Using trowels and spatulas flame sterilized with 95% ethanol, each soil sample was sub-divided for various analyses. Gravimetric water content of the soil samples was determined at the time of subsampling(weight of water/weight of dry soil). Soils for chemical analysis were then air-dried in a glass-house, ground to break-up clay and caliche aggregates, and passed through a 2 mm sieve. 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 mil (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.

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*Four mesquite ecosystems were stuided: playa, coppice dune, arroyo, and grassland. An ecosystem dominated by the non-legume, Larrea tridentata, and lacking mesquite was included as a reference. The arroyo, grassland, Larrea and a playa site were loacated on the NSF Jornada Long Term Ecological Research (LTER) site situated 40 km north of Las Cruces, NM, in the northern Chihuahuan desert. A coppice mesquite dune site was located on the adjacent USDA Jornada Experimental Range about 15 km from the above sites.


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