Soil organic matter storage and turnover along granite and basalt climate transects in California uplands

To understand how soil organic matter (OM) is affected by parent material in conjunction with climate, I compared carbon storage and turnover along climate gradients in granite versus basalt-derived soils of the Sierra Nevada and Cascade range of northern California. The transects spanned mean annual temperatures (MAT) of 5 to 18°C and corresponding mean annual precipitation (MAP) values of 140 to 30 cm. Regardless of parent material, clay accumulation and carbon storage peaked at intermediate values of MAT and MAP. However the basalt soils were found to have more clay and more uniform mineralogy; kaolins dominated all clay phases except those of the two coolest granite soils.; To evaluate the role of mineralogy on OM turnover I used density to separate mineral species and characterized the resultant fractions based on x-ray diffraction, carbon, nitrogen, 13C, 15N, and radiocarbon. Density successfully separated OM at different stages of decomposition, but fast and slow-cycling carbon pools co-occur within these fractions. To analyze carbon turnover I used density and chemical treatments to separate three soil fractions, which were modeled as two carbon pools---a 'fast' decadal pool and a 'slow' centennial pool. The proportion of fast carbon, summed over all soil fractions (CtFAST), and the overall flux-weighted carbon turnover time (T) were linearly and positively correlated with MAT and parent material had no significant effect on the mean response of either of these variables. However, along the basalt gradient CtFAST exhibited heightened sensitivity to MAT and clay + silt reinforced the effect of MAT on carbon cycling.; In summary, climate governs gross carbon storage and turnover patterns along either gradient, but the long-term effect of a climate gradient superimposed on distinct parent materials is reflected in texture and mineralogical differences that modulate finer scale response of physical soil fractions and modeled carbon pools to MAT. This research highlights a case in which the sensitivity of carbon stability to climate as well as the sign of the effect of texture on carbon stability depends on parent material and indicates that parent material deserves more consideration in order to understand the mechanisms that regulate carbon cycling at sub-regional scales.