Abstract
A rise in atmospheric CO 2 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 CO 2 ( e CO 2 ) affects these pools is unclear. In this study, we examined how 12 years of e CO 2 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 e CO 2 caused significant decreases in MAOM‐total C (16%), MAOM‐organic N (15%), and MAOM‐organic P (16%) compared to ambient CO 2 ( a CO 2 ), 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 e CO 2 ‐induced changes in plant inputs but instead suggest that the increased belowground C inputs under e CO 2 , 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 e CO 2 in soils with low P availability.