Microbial mining and accumulation of nutrient-rich mineral-associated organic matter limits carbon storage under elevated carbon dioxide in a Eucalyptus woodland

Atmospheric carbon dioxide (CO₂) concentration continues to rise, affecting terrestrial ecosystem processes and pools, including soil organic matter (SOM). Typically, elevated CO₂ (eCO₂) is associated with increased plant-derived C inputs into the soil, leading to the common expectation that SOM will accrue and slow the rise in atmospheric CO₂. We tested this at the EucFACE (Australia) by examining SOM functional pools and their nutrient stoichiometry. We found: (1) a significant decrease in C content of the mineral associated organic matter (MAOM-C) under eCO₂ (16%) compared to ambient CO₂ (aCO₂) over 12 years of CO₂ fertilization and (2) a significantly lower rate of new MAOM-C accumulation under eCO₂ (17%) compared to aCO₂ (38%), thereby challenging this common expectation. Enhanced substrate availability caused by eCO₂, along with low P availability, alters microbial activity, particularly due to the need of soil microbes to maintain a stoichiometric nutrient balance. We determined that microbes adopted two strategies to increase nutrient availability: (1) mining existing MAOM, leading to an overall decrease in MAOM, and (2) necromass recycling, resulting in limited accumulation of new MAOM. MAOM may have been more susceptible to microbial attack than POM, because of its higher nutrient content derived from the contribution of microbial necromass and a lower activation energy once released by minerals, providing a mechanistic explanation for the observed decrease in MAOM under eCO₂. These results question the role of MAOM as the persistent SOM pool and restrict the potential for greater SOM accumulation in soils under eCO₂.