Quantifying the effect of petrogenic carbon on SOC turnover for two Rocky Mountain soils: When are petrogenic carbon corrections required?

Petrogenic organic carbon (OC_petro), derived from sedimentary rocks, is an often overlooked and poorly quantified source of soil organic carbon (SOC), which may influence measured or modeled SOC composition, age, and stability. In this study, we exploited differences in thermochemical stability between OC_petro and biogenic SOC (OC_bio) using stepped elemental analysis to quantify the fractional contribution of OC_petro to total SOC (f_petro), and we conducted a sensitivity analysis to estimate the effects of OC_petro on modeled SOC transit times and system ages. Specifically, we compared the effects of accounting for OC_petro inputs in SOC turnover modeling (using SoilR) for two montane meadow soils that are underlain by Cretaceous Mancos Shale. At these sites, we estimate that OC_petro comprises 7%–9% of the total SOC stock (f_petro = 0.07–0.09). However, accounting for OC_petro as a mixture of inert and passive C or as completely inert C had negligible effects on SOC transit times and system ages, suggesting that there is a threshold of OC_petro content under which there is minimal effect on calculated SOC turnover. Based on our sensitivity analysis, we estimate this threshold to be f_petro = 0.125, further supporting that the accurate calculation of OC_petro remains an important factor in estimating SOC turnover.