Soil Phosphorus Dynamics are an Overlooked but Dominant Control on Mineral-Associated Organic Matter

Understanding the controls on soil organic matter (SOM) cycling is essential for predicting how future pedoclimatic conditions will affect soil carbon (C) sequestration and emissions. However, significant uncertainty surrounds the controls on SOM formation, persistence, and destabilization, especially for the more persistent pool, mineral-associated organic matter (MAOM). Here we argue that much of this uncertainty can be resolved by incorporating soil phosphorus (P) into conceptual and empirical models of SOM persistence because: (1) P forms the strongest bonds in the MAOM pool, resulting in greater relative enrichment and persistence; (2) this P enrichment regulates the formation of organo-organic associations within MAOM; (3) bioavailable P drives the production of microbial necromass and by-products, which shape contributions to MAOM; and (4) microbial P demand stimulates the destabilization of MAOM. Under this new framework, we propose specific consequences of global change, such as changes in pH and short-term flooding, on MAOM persistence. Given soil P's outsized role in MAOM formation, a better understanding of the abiotic and biotic controls on MAOM P enables us to form more accurate predictions of MAOM persistence and destabilization under global change.