Document: CON-3-58-12

Soil microorganisms and fatty acid methyl ester profiles along a nitrogen deposition gradient.

SIGUENZA, C.*, E.B.ALLEN, M.F.ALLEN and D.E.CROWLEY

University of California, Riverside, CA 92521 USA ¹

Abstract:
Anthropogenic nitrogen deposition is a problem in Southern California, as is the invasion of Mediterranean annual grasses into the local coastal sage scrub (CSS) vegetation. This study was carried out to determine the effect of these changes on soil microbial communities. Soils were sampled in several sites under exotic grasses and under Artemisia californica a native shrub, along a N deposition gradient. Microbial abundance was assessed by measuring active fungal hyphae and bacteria on soil samples stained with europium chelate. Fatty acid methyl esters (FAME) were extracted from soils and their profiles were used to describe soil microbial community differences along the gradient. Arbuscular mycorrhizal fungi spores were extracted and counted and roots from Artemisia californica and exotic grasses stained by standard methods to evaluate colonization. Microbial abundance showed no differences in the first year of sampling but in the second year the sites with high N levels were more bacterial dominated while the sites with lower N were fungal dominated. Principal components analysis of FAME profiles showed defined clusters representing sites along the gradient in both years. The differences in FAME profiles between soils under A. californica and under exotic grasses are not clear in all the sites, probably because exotic grasses have invaded the understory of most shrubs all along the gradient. However, N deposition is affecting the composition of the soil microbial communities. Mycorrhizal spore number and colonization was affected by the N gradient. Sites with high levels of N had fewer spores or less colonization than sites with lower N levels. In several sites, the spore numbers were significantly higher under A. californica than under exotic grasses. In CSS the high soil N levels and the vegetation shift were negatively affecting the microbial community.

Keywords: Soil microbes, mycorrhizae, Coastal Sage Scrub, exotic grasses

This abstract is being presented at: 9:45 AM in session:
Oral Session #23: Soil Ecology.