The ecological relevance of fast-cycling mineral-associated organic matter – a dynamic pool of 'persistent’ soil carbon and nitrogen

Longstanding theories and models classify mineral-associated organic matter (MAOM) as the large (~60%) but slow-cycling and persistent portion of the soil organic matter (SOM) pool. Strong physico-chemical interactions and diffusion limitations restrict the turnover of MAOM, allowing carbon and nitrogen bound therein to persist in soil for as long as centuries to millennia. However, MAOM is a chemically and functionally diverse pool with a substantial portion cycling at relatively fast (i.e., minutes to years) timescales. Despite a growing body of evidence for the heterogenous and multi-pool nature of MAOM, we lack consensus on how to conceptualize and directly quantify fast-cycling MAOM and its ecological significance. We demonstrate the dynamic qualities of fast-cycling MAOM vary based on 1) the chemistry of the mineral particles and organic matter, 2) the complex set of interactions between OM and the mineral matrix, and 3) the presence and strength of destabilizing forces that lead to decomposition or loss of MAOM (i.e., plant-microbe interactions, land use change, agricultural intensification, and climate change). Finally, we discuss potential implications and research opportunities for how we measure, manage, and model the dynamic subfraction of this otherwise persistent pool of SOM.