Soil microorganisms of a native shrub and exotic grasses along a nitrogen deposition gradient in southern California

Both anthropogenic nitrogen deposition and exotic annual grass invasion are occurring in the coastal sage scrub vegetation (CSS) of southern California. A study was carried out to determine the effects of these changes on soil microbial communities. Soils were sampled under exotic grasses and the native shrub Artemisia californica along an urban-to-rural N deposition gradient, and in N-fertilized plots in a rural site with low deposition. Arbuscular mycorrhizal (AM) colonization of A. californica was highest in low-N soils, but annual grasses (mainly Bromus madritensis) were colonized primarily by a fine endophyte that showed no pattern of response to soil N level. In addition, annual grasses generally had higher colonization by nonmycorrhizal, primarily septate, fungi in high N soils, while nonmycorrhizal colonization of A. californica was low in all soils. Spore density declined in the rhizosphere of A. californica with elevated N, but not of B. madritensis. Fatty acid methyl esters (FAME) were extracted from soil and their profiles were used to describe the soil microbial communities. Principal components analysis of FAME profiles showed a significant but weak relationship with levels of soil N. At the low N-deposition site that received N fertilizer, host plant was more important than soil N in determining FAME profiles. The most abundant fatty acids were the biomarkers for AM fungi. The different microbial communities of the two species, especially the predominance of fine endophyte and nonmycorrhizal fungi in roots of B. madritensis, warrant further research on functional responses to understand how these microorganisms may be involved in the invasion of native shrubland by exotic grasses.