INVITED: CHANGES IN SUBSURFACE MICROBIAL COMMUNITIES WITH SOIL AGE IN A MARINE TERRACE CHRONOSEQUENCE, SANTA CRUZ, CALIFORNIA

Ongoing research at a marine terrace chronosequence in Santa Cruz, CA includes quantification of biogeochemical and physical processes to calculate soil mineral weathering rates and to better understand processes of soil development. The chronosequence formed in marine sediments deposited on Miocene sandstones and mudstones, which are in part derived from Cretaceous granitoid rocks. The site has a Mediterranean climate and is dominated by grassland vegetation. This study investigated changes in subsurface microbial communities from the third and fifth terraces, dated at 137 and 226 ka respectively (Perg et al., 2001), and links the results to findings from the larger study. Preliminary results showed that microbial biomass carbon (MBC) was higher at the surface of terrace 3 than at the surface of terrace 5. The highest MBC found for terrace 5 was between 0.5 and 1 meter below the surface, and terrace 5 MBC decreased more gradually with depth than terrace 3 MBC. The terrace 5 subsurface MBC concentration peak and more gradual decline with depth may be due to increased development of a clay-rich, argillic horizon. The argillic horizon in the terrace 5 soil is thicker and has a significantly higher clay content (up to 48%) than the terrace 3 soil (up to 27%). In terrace 5, increased clay content results in increased soil water, which may increase the viability of soil microbes, particularly during the dry season when these samples were collected. Increased microbial population size at depth in older soils may result in the establishment of a positive feedback mechanism for pedogenesis within the argillic horizon. The formation of argillic horizons in a similar chronosequence was postulated to be due to the subsurface degradation of organic matter by microbes causing supersaturation of aluminum with respect to kaolinite, which contributed to the formation of the argillic horizon (White et al., 2005). This investigation of subsurface microbial communities suggests that soil microbial population size increases with the development of argillic horizons in older soils, which creates a positive feedback where the increased microbial population increases the rate of argillic horizon formation. Argillic horizon formation and development plays an important role in pedogenesis, pointing to soil microbial communities as important agents in soil development.