Organic Carbon and Lignin Protection by Metal Oxides Versus Silicate Clay: Comparative Study Based on Wetland and Upland Soils

Abstract Mineral binding of organic carbon (OC) is one of the key mechanisms promoting soil organic carbon (SOC) persistence, which is vital for the long‐term sequestration of atmospheric carbon dioxide into soils. However, the relative importance of two main types of soil minerals ‐ metal oxides and silicate clay—in SOC protection remains unclear, hampering our ability to predict and protect this important pool of persistent SOC. Here, we analyzed OC bound to metal oxides (OC metal ) versus silicate clay (OC clay ) in soils from contrasting environments (i.e., wetlands and uplands) using sequential dissolution by dithionite and hydrofluoric acid. We found that OC metal % was higher than OC clay % in both wetlands and uplands, indicating the dominance of metal oxides in SOC protection. The sum of OC metal % and OC clay % constituted a higher fraction of SOC in wetlands than uplands, suggesting an underappreciated role of mineral (in particular, metal oxide) protection in wetland SOC preservation. Furthermore, using lignin phenol analysis in tandem, ∼23% of lignin phenols were additionally released after the removal of silicate clay, providing a potential means to assess “hidden” lignin in mineral matrices. Given the wide use of lignin phenols as tracers for terrestrial plant‐derived OC, our results suggest that the contribution of terrestrial or plant‐derived carbon to sedimentary carbon stocks or mineral‐protected SOC deserves reassessment. These findings highlight the differential role of various soil minerals in the protection of SOC and its components in contrasting terrestrial environments, which warrants better recognition for understanding the distribution and dynamics of mineral‐protected SOC.