Regional differences in soil stable isotopes and vibrational features at depth in three California grasslands

There are major gaps in our understanding of how Mediterranean ecosystems will respond to anticipated changes in precipitation. In particular, limited data exists on the response of deep soil carbon dynamics to changes in climate. In this study we wanted to examine carbon and nitrogen dynamics between topsoils and subsoils along a precipitation gradient of California grasslands. We focused on organic matter composition across three California grassland sites, from a dry and hot regime (~ 300 mm precipitation; MAT: 14.6 \(\boldsymbol{^\circ{\text{C}} }\)) to a wet, cool regime (~ 2160 mm precipitation/year; MAT: 11.7 \(\boldsymbol{^\circ{\text{C}} }\)). We determined changes in total elemental concentrations of soil carbon and nitrogen, stable isotope composition (δ¹³C, δ¹⁵N), and composition of soil organic matter (SOM) as measured through Diffuse Reflectance Infrared Fourier Transformed Spectroscopy (DRIFTS) to 1 m soil depth. We measured carbon persistence in soil organic matter (SOM) based on beta (\({\varvec{\beta}}\)), a parameter based on the slope of carbon isotope composition across depth and proxy for turnover. Further, we examined the relationship between δ¹⁵N and C:N values to infer SOM’s degree of microbial processing. As expected, we measured the greatest carbon stock at the surface of our wettest site, but carbon stocks in subsoils converged at Angelo and Sedgwick, the wettest and driest sites, respectively. Soils at depth (> 30 cm) at the wettest site, Angelo, had the lowest C:N and highest δ¹⁵N values with the greatest proportion of simple plant-derived organic matter according to DRIFTS. These results suggest differing stabilization mechanisms of organic matter at depth across our study sites. We infer that the greatest stability was conferred by associations with reactive minerals at depth in our wettest site. In contrast, organic matter at our driest site, Sedgwick, was subject to the most microbial processing. Results from this study demonstrate that precipitation patterns have important implications for deep soil carbon storage, composition, and stability.