ECOLOGY MEETS GEOCHEMISTRY: A NATIONAL ASSESSMENT OF THE BIOGEOGRAPHY OF MICROBIAL COMMUNITIES IN SOILS

The field of biogeochemistry, which examines the cycling of macronutrients (e.g., carbon, nitrogen) and trace nutrients (e.g., Fe, Cr), is an outgrowth of ecology and geochemistry and has generated tremendous growth in both of these fields. Similarly, the physical and chemical characterization of soils has improved our understanding of ecosystem processes and our understanding of the biotic (both microbial and plant) influence on soils and geochemistry. The link between geochemistry and ecology is often made through ties to the effects of both these disciplines on microbial communities, which derive energy and nutrients through both organic and inorganic sources and are structured by climate, landform, and soil chemical and physical properties

The extent to which ecological principles can be applied to geochemical investigations is being explored using data collected through the U.S. Geological Survey and Canadian Geological Survey sponsored Geochemical Landscapes Program, a geochemical and microbial survey of soils across the North American Continent. There are three primary goals of this work: 1) we asked what geochemical factors structure the composition of the microbial community; 2) we asked what ecological principles can be used to predict the structure of the microbial community across many geochemical environments; 3) we asked how the ecological knowledge can be used to assess the microbial community influence on carbon cycling in soil. A broad look at the data indicate that organic carbon is a primary driver in the magnitude of microbial indicators, including phospholipids fatty acid concentrations and β-glucosidase activity. Higher amounts of organic carbon tend to be associated with cooler, moist climates. When the data are sorted by ecoregions and land-use more detailed patterns fall out. Beta-glucosidase activity and PLFA biomarkers for fungi are elevated in the Mediterranean scub and Central Valley of Californa, which has hot dry summers and wet, mild winters. We also found that agriculture influences size and structure of microbial communities. The balance of fungi and bacteria can be strongly influenced by the degree of disruption of soil structure through plowing and grazing. Thus, biogeography is significant controlling factor on soil microbial ecology.