Some concepts of soil organic carbon characteristics and mineral interaction from a review of literature

In the past decades, the molecular structure of soil organic carbon (SOC) has been regarded as a pivotal criterion for predicting organic carbon (OC) turnover in soils. However, newly emerging evidence indicates that molecular structure does not necessarily predetermine the persistence of OC in soils and that environmental factors (e.g., soil structure, availability of resources and diversity of microorganisms) exert an additional influence upon SOC turnover. Among these potential factors, adsorption to soil minerals and occlusion within soil aggregates have been universally demonstrated to shield SOC from decomposition. In this review, we identified the uncertainties involved in examining the turnover of specific SOC fractions (lignin, humic substances (HS), coal and black carbon (BC)) in soils. Moreover, we concluded that the role of minerals in SOC adsorption and stability depends on the mineralogy, chemical properties of SOC and soil conditions. Characterization of SOC chemical composition in different soil size fractions (e.g., sand, silt and clay) shows that different-sized minerals potentially protect different types of SOC. Aromatic C may be adsorbed to minerals in the coarse silt/sand fractions and preserved there, while fine-sized (fine silt and clay) minerals generally associate with microbial-derived SOC. Finally, by tabulating the data from the 13C turnover time and 14C ages of bulk SOC and specific SOC fractions (carbohydrate, lignin, aliphatic C, HS, and BC), we obtained further validation that molecular structure does not exclusively determine the turnover rate of OC in soils. Furthermore, the 13C turnover time and 14C age of SOC consistently increased with increasing soil depth, which may be partially attributed to the larger protective potential of SOC by minerals and the unfavorable conditions for biodegradation in the subsoils. Because the limitations of 13C and 14C-dating techniques have largely been neglected, they are emphatically discussed in this review. It is suggested that more geomorphic and spectroscopic evidence is paramount to further explore the mechanisms underlying the persistence of OC in soils.