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.

我们对地中海生态系统将如何应对预期的降水变化的理解存在重大差距。特别是，关于深层土壤碳动力学对气候变化的响应的数据有限。在这项研究中，我们想沿着加利福尼亚草原的降水梯度检查表层土壤和底土之间的碳和氮动力学。我们专注于加利福尼亚三个草原地点的有机物组成，从干热状态（~ 300 毫米降水;MAT： 14.6 \（\boldsymbol{^\circ{\text{C}} }\）） 到潮湿、凉爽的状态（~ 2160 毫米降水量/年;MAT： 11.7 \（\boldsymbol{^\circ{\text{C}} }\））.我们确定了土壤碳和氮总元素浓度、稳定同位素组成 （δ¹³C， δ¹⁵N） 和土壤有机质组成 （SOM） 的变化通过漫反射红外傅里叶变换光谱 （DRIFTS） 测量到 1 m 土壤深度。我们根据 β （\（{\varvec{\beta}}\）） 测量了土壤有机质 （SOM） 中的碳持久性，该参数基于碳同位素组成随深度的斜率和周转的代理。此外，我们检查了 δ¹⁵N 和 C：N 值之间的关系，以推断 SOM 的微生物加工程度。正如预期的那样，我们在最潮湿的场地表面测量了最大的碳储量，但底土中的碳储量分别集中在最潮湿和最干燥的地点 Angelo 和 Sedgwick。根据 DRIFTS，最湿位 Angelo 的深层土壤 （x3E 30 cm） 的 C：N 值最低，最高δ¹⁵N，简单植物来源有机质的比例最大。 这些结果表明，在我们的研究地点中，有机物在深处的稳定机制不同。我们推断，在我们最潮湿的地点，与深处的活性矿物的结合赋予了最大的稳定性。相比之下，我们最干燥的地点 Sedgwick 的有机物受到的微生物加工最多。本研究结果表明，降水模式对深层土壤碳储存、组成和稳定性具有重要意义。