Soil Microbial Fingerprints, Carbon, and Nitrogen in a Mojave Desert Creosote-Bush Ecosystem

469 Soil Sci. Soc. Am. J. 71:469–475 doi:10.2136/sssaj2005.0283 Received 26 Aug. 2005. *Corresponding author ([email protected]). © Soil Science Society of America 677 S. Segoe Rd. Madison WI 53711 USA The patchy distribution of soil nutrients in shrub–desert ecosystems has been well characterized (Schlesinger et al., 1990), but less is known about how soil microbial community composition, microbial respiration, and soil N-cycling processes contribute to the “islands of fertility” that form under shrub canopies. These islands are locations where plant litter accumulates, where wind-borne material may be trapped under shrubs (Garcia-Moya and McKell, 1970; Schlesinger et al., 1996; Holzapfel and Mahall, 1999), and where the soil may be protected from raindrop impact, erosion, and runoff. Kieft et al. (1998) described distinct islands of C, N, and microbial biomass under L. tridentata in New Mexico and showed that temporal variation of these soil resources was greatest under L. tridentata shrubs, compared with soils under grasses in a nearby grassland biome or to soils in bare spaces in either the shrubland or grassland. Spatial heterogeneity of microclimatic conditions under shrubs vs. open spaces between shrubs has been documented in the cool, semiarid shrublands of eastern Utah (Forseth et al., 2001), where shrubs lowered soil temperatures and reduced soil water content, particularly in the upper 20 cm of the soil, compared with open microhabitats, although soil N, P, and organic matter showed little shrub island effect. Shrub-associated islands of fertility in the Mojave Desert of California, where some individual L. tridentata have existed for several thousand years (Vasek, 1980), are well developed in terms of soil nutrient variation (Schlesinger et al., 1996). This distinct patchiness makes the Mojave Desert a suitable location to explore the variation in microbial communities, soil respiration, and soil N isotopic composition that may accompany changes in N-cycling processes as a function of proximity to shrubs. Stable soil N isotopic signatures (δ15N) are a convenient tracer of net soil N-cycling effects because they represent the balance of biological N transformations (Amundson and Baisden, 2000). Previous work using soil δ15N has suggested less conservative cycling of N in hot, dry locations, where losses due to isotopefractionating processes result in higher soil δ15N signatures than at cooler, more moist sites (Amundson and Baisden, 2000; Austin and Vitousek, 1998; Riley and Vitousek, 1995; Schulze et al., 1991). In arid zones, processes that favor loss of 14N (and thus increase soil δ15N) may occur during brief episodes of denitrifi cation following rainfall events; by NH3 volatilization, which is favored in dry, alkaline soils with low cation exchange capacity; or as loss of NOx and N2O during nitrifi cation and denitrifi cation (Mummey et al., FO R EST, R A N G E & W LD LA N D SO LS Soil Microbial Fingerprints, Carbon, and Nitrogen in a Mojave Desert Creosote-Bush Ecosystem