Nutrient-Rich Mineral-Associated Organic Matter Limits Carbon Storage Under Elevated Carbon Dioxide in a Low...

A rise in atmospheric CO₂ concentration can have positive or negative effects on soil organic carbon (SOC) pools, with likely impacts on soil nutrient availability, which can in turn, drive ecosystem-level impacts. Much of the soil nutrients are locked in the more stable mineral-associated organic matter (MAOM) pool compared to the more labile particulate organic matter (POM) pool, but how elevated CO₂ (eCO₂) affects these pools is unclear. In this study, we examined how 12 years of eCO₂ affected the POM and MAOM C, nitrogen (N), and phosphorus (P) pools at two different depths (0-10 and 10-20 cm) in a low P, native Eucalyptus woodland. Across soil depths, we found that 12 years of eCO₂ caused significant decreases in MAOM-total C (16%), MAOM-organic N (15%), and MAOM-organic P (16%) compared to ambient CO₂ (aCO₂), but no effects on C, organic N (N_org), and organic P (P_org) in POM. The MAOM had consistently lower C: P_org and C: N_org ratios (228 and 12.4, respectively) than POM (655 and 24.1, respectively). Our results cannot be explained by eCO₂-induced changes in plant inputs but instead suggest that the increased belowground C inputs under eCO₂, in combination with low soil P availability, triggered a rhizosphere priming effect on the MAOM pool. Since MAOM was richer in P than POM, microbes may have preferentially mined MAOM for P (and N) to meet their P demand, thereby enhancing the decomposition of MAOM more than POM. While uncertainties remain about the fate of P after microbial death in this ecosystem, this study highlights that the nutrient-rich MAOM pool is vulnerable to rhizosphere priming, thereby restricting the potential for greater SOC accumulation under eCO₂ in soils with low P availability.