Seminar: Jeffrey Beem-Miller

The response of soil C to rising temperatures is uncertain, with the potential for gains or losses in the coming decades. Insight into these dynamics can be obtained from model-derived probability distributions of soil C ages and transit times. The aims of this study were two-fold: 1) assess the relationship between climatic and mineralogical factors and soil C age and transit time distributions, and 2) quantify the effect of soil mineral assemblages on the temperature response of soil C age and transit time distributions. I adopted a comparative modeling approach, with the models constrained by time series of radiocarbon measurements from heterotrophically respired CO2 and bulk soil. Observed soil C stocks and modeled transit times were used to estimate inputs, while pre-aging was quantified by fitting a lag time parameter in the models. I assessed parameter uncertainty using a Markov Chain Monte Carlo approach, and quantified relationships among climatic and mineralogical variables and model outputs with a Bayesian linear modeling framework.

The results demonstrate that mineral assemblages attenuate the temperature sensitivity of soil C ages and transit times in soils with mixed mineralogies. Mean annual soil temperature was not significant for explaining soil C ages. In contrast, mineral control of soil C ages was strong, and the significant relationship observed between parent material and transit time indicates that mineral assemblages play a role in controlling annual to decadally cycling soil C as well. This work highlights the importance of future studies to identify the specific mineralogical mechanisms controlling soil C cycling on short time scales. Radiocarbon time series of heterotrophically respired CO2 and bulk soils are uniquely suited for constraining distributions of C ages and transit times, and these distributions are the key for improving predictions of the soil C response to climate change.