SOIL MICROBIAL COMMUNITY CHARACTERISTICS ALONG AN ELEVATION GRADIENT IN THE LAGUNA MOUNTAINS OF SOUTHERN CALIFORNIA

Authors: Collins, Harold; Cavigelli, Michel

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/20/2003
Publication Date: 7/17/2003
Citation: COLLINS, H.P., CAVIGELLI, M.A. SOIL MICROBIAL COMMUNITY CHARACTERISTICS ALONG AN ELEVATION GRADIENT IN THE LAGUNA MOUNTAINS OF SOUTHERN CALIFORNIA. SOIL BIOLOGY AND BIOCHEMISTRY. 2003. V. 35. P. 1027-1037.

Interpretive Summary: Spatial variability of soil resources in desert ecosystems is largely controlled by the spatial organization of perennial plants, which create relatively stable fertile islands, areas under plant canopies that have greater soil organic C and N, C and N mineralization potential, soil moisture, and reduced daytime temperatures due to litter accumulation and root influences. These patterns of nutrient heterogeneity in deserts are mainly controlled by plant cover rather than by the species forming that cover. As rainfall in deserts increases, plant community composition changes, plant density increases, and differences in soil resources between vs. under plants are likely to decrease. Because of these patterns of long- term direct and indirect plant effects in deserts, deserts provide an ideal laboratory for studying plant effects on soil microbial community structure and function in situ. The large differences in soil resource levels and environmental conditions between vs. under plants likely select for different soil microbial communities. We sampled soil between and under plants at four sites along an elevation gradient in the Laguna Mountains in the western Sonoran Desert to test the effects of site, plants and plant type on soil microbial community structure and function. Soil properties were clearly influenced by both site and plants along the transect. From Ocotillo to Laguna Summit, elevation, climate, and percent plant cover increased while sand content decreased. These factors contributed to increasing patterns of soil C and N fractions, mineralization rates and changes in microbial community structure and function with elevation.

Technical Abstract: We sampled soil between and under plants at four sites in the Laguna Mountains in the western Sonoran Desert to test the effects of site, plants and plant type on soil microbial community structure and function. The four sites differ in elevation,soil type and plant community composition and spatial distribution. Fertile islands, defined as areas beneath plants with greater soil resources than bare areas,are present at all sites but the difference between soils under vs. between plants was greatest at the lower elevations. At lower elevations,fertile islands had a strong influence on soil microbial community structure and function. Based on FAME biomarker and PC analyses, we found that soil microbial communities between plants at the lowest elevation had proportionally more Gram negative bacteria than all other soils. At the two higher elevation sites there were fewer and more subtle differences in microbial community structure in soils ssampled between vs.under plants,consistent with the fertile island phenomenon being less distinct with increasing rainfall. Differences in soil microbial communities under plants among the four sites were also small and subtle, suggesting that the plant influence is more important than plant type in controlling microbial community structure. Microbial community function, measured using Biolog plates, showed the strongest differences between samples taken under vs. between plants at the two lower elevation sites. Microbial community function also seemed to be influenced more by plant presence than by plant type.