INFLUENCE OF PARENT LITHOLOGY ON SOIL MICROBIAL COMMUNITIES AS DEFINED BY PHOSPHOLIPID FATTY ACIDS

Phospholipid fatty acids (PLFAs) are a major structural component of microbial cellular membranes and are used to quantify biomass and characterize structure of soil microbial communities. Applications of PLFA analysis have included the relation of microbial communities to plant community structure, impact of contaminants (e.g., MTBE and Hg) and agricultural practices. We are exploring the influence that geology has on soil microbial biomass and community structure as a key to forming a baseline for use in ecological studies of differing landscapes.

The influence of soil parent lithology on PLFA biomass and community structure was explored in soils from the California Coastal Ranges and Sierra Nevada Foothills. Upland soils with a bedrock parent (serpentinite, mineralized serpentinite, marine siltstone) were collected in the Cache Creek watershed in the California Coastal Ranges, with alluvial soils collected along Cache Creek. Samples were also collected from the Sierra Nevada Foothills to address the influence of the geomorphic setting and climate on serpentine soils. Soils from these parent materials have distinct PLFA biomass and community structure that are likely related to Ca/Mg ratios and toxicity effects of chromium from serpentinite and mercury from mineralized serpentinite). For instance, PLFA biomarkers (e.g., 18:3ω6c, 18:1ω9c) for fungi are a greater percentage of the total PLFA biomass in siltstone soils than in serpentine soils from the Coastal Ranges. Serpentine soils tend to have a greater PLFA biomass than do siltstone soils, possibly due to reduced competition for nutrients from the sparser vegetation associated with serpentine soil. Organic carbon and total PLFA concentrations are greater in serpentine soils from the Sierra Foothills than in those from the Coastal Ranges; however, the concentration of the PLFA biomarker for fungi is greater in the Coastal Range serpentine soils. Mean annual precipitation is greater in the Coastal Range sites (860 mm) than in the Sierra Foothills sites (680 mm), which would enhance fungal growth. An important geochemical difference between these soils is that chromium in the Coastal Range soils is more recalcitrant to acid digestion and thus likely less bioavailable. Fungal growth in Sierra Foothills serpentine soils thus may be inhibited by chromium.