BIOGENIC CYCLING OF SILICON IN SOILS IN A MARINE TERRACE CHRONOSEQUENCE, SANTA CRUZ, CALIFORNIA

The uptake of Si by grasses from minerals in coastal soils found in a marine terrace chronosequence north of Santa Cruz, CA, was studied in order to estimate the extent of biological cycling and make corrections in the calculation of weathering rates of the local regolith. Pore water Si concentrations exhibit both progressive increases in the deep soils, by reason of plagioclase weathering, as well as increases within the rooting zone at shallow depths (< 1 m) as a result of evapotranspiration and biogenic cycling. Due to selective exclusion, pore waters exhibit increases with both Ge/Si and ³⁴Si/³⁰Si ratios during active growth of the grasslands from late winter to spring in response to the local Mediterranean climate. This Si pool taken up in the grasses is immobilized and contained in phytoliths until the end of growth followed by the onset of die-off and physical degradation of dried-out vegetation to litter. During the weeks following the start of a subsequent autumn-to-winter wet-weather period, the amorphous Si phytolith structures are exposed to weathering after further decomposition of wet litter by natural microbial processes culminating in the release of biogenic Si to the pore water profile, thereby lowering the Ge/Si and ³⁴Si/³⁰Si ratios in shallow soil water. While pore water Si levels track declining phytolith concentrations down to ~2 m in depth, laboratory analyses of alkaline extracts from soil treated with dilute base indicate that concentrations of total amorphous Si in the soil correlate more closely with those of total Al, pointing toward Si uptake in precursor clay phases.