The need to update and refine concepts relating to mineral-associated organic matter saturation in soil

Mineral-associated organic matter (MAOM) is an important carbon reservoir in relation to soil organic carbon sequestration over long timescales, and its saturation status is pertinent to the judicious formulation of agricultural management practices and global climate mitigation strategies. However, the existence of MAOM saturation is controversial due to the ambiguity of the MAOM concept and the variability of the underlying model. Based on this, we update and extend the concept of MAOM saturation into theoretical and apparent components, and propose the hypothesis that clay minerals, microbial communities, and input organic matter regulate the apparent saturation of MAOM. The theoretical saturation of MAOM represents the maximum sequestration potential of MAOM in natural soil ecosystems, which may require update of many current models of global carbon sequestration. In contrast, apparent saturation of MAOM capacity can be derived by comparing natural ecosystems with ecosystems subject to anthropogenic disturbances, and by monitoring single ecosystems on annual time scales over many years. Future research therefore needs to consider some new indicators and models to study MAOM saturation.