SOME APPROACHES TO DISTINGUISHING BETWEEN LITHOGENIC AND BIOGENIC PROCESSES IN SOILS

While recent interest in critical zone processes emphases the importance of interactions between the lithosphere and biosphere, many of these connections remain ill-defined due to the complexity of the processes involved. This paper investigates the application of elemental and isotopic partitioning in order to distinguish between the sources and pathways of mineral nutrients in soils of a marine terrace soil chronosequence near Santa Cruz, California. Attributes of the field site, in terms of distinguishing between lithogenic and biogenic-dominated processes, include shallow-rooted annual grassland vegetation, a Mediterranean climate with well-defined wet and dry seasons, a simple soil protolith consisting predominately reworked granitic sand and the formation of a shallow argillic horizon which serves as a hydrology filter between shallow and deep soil horizons.

In soils above 1.5 m, exchange and pore water Ca and K are relatively high and proportionally decrease, compared to Mg and Na, down to the top of abiotic horizon as a result of mineral weathering. These trends correspond to an exponential decline in root density and SOM and fit the general distributions expected for biological cycling in which the mineral nutrients, which are used more extensively and limit plant growth, i.e. K and Ca, have shallower distributions. Although K is the dominate cation in vegetation, it maintains the lowest proportions in pore water and on the exchange complex indicating that it is very efficiently cycled in the living vegetation and leaf litter. Pore water and exchange Sr/Ca and ⁸⁷Sr/⁸⁶Sr indicate mixed sources from rainfall and mineral weathering with Ca being selectively mobilized during the growth cycle which also results an approximate δ⁴⁴Ca difference of about -2. In contrast, the vegetation produces a slight increase in δ²⁶Mg of +0.25 relative to precipitation. Ge/Si and δ³⁰Si exhibit variations corresponding to seasonal phytolith production and dissolution.