Soil organic matter (SOM) decomposition and organic phosphorus (P) cycling may help sustain plant productivity under elevated CO₂ (eCO₂) and low-P conditions. Arbuscular mycorrhizal (AM) fungi and their role in P-acquisition and SOM decomposition may become more relevant in these conditions. Yet, experimental evidence of AM fungi and P availability interactive effects on soil carbon (C) cycling under eCO₂ is scarce with the potential mechanisms of this control being poorly understood. We performed a pot experiment with soil and a grass from a low-P ecosystem where plant biomass and soil C cycling have been mostly unresponsive to eCO₂. We manipulated AM fungi, P, and CO₂ levels and assessed their impacts on soil C cycling and plant growth using continuous ¹³C plant labelling to isolate and measure short-term changes in total and SOM-derived fractions of respired CO₂, dissolved organic C (DOC) and microbial biomass (MBC), as relevant components of the soil C cycle. Increases in SOM decomposition and microbial C use were hypothesised to support plant growth under eCO₂ and low-P with AM fungi intensifying this effect. However, we did not detect simultaneous significant impacts of the three experimental factors. We observed instead increased root biomass and nutrient uptake with eCO₂ and AM presence and lower SOM-derived DOC and MBC with low-P, decreasing further with AM inoculation. Taken together, our findings in this model plant-soil system suggest that, AM fungi can support root biomass growth and nutrient uptake under eCO₂ and protect the SOM pool against decomposition even in low-P conditions. Contrary to reports from N-limited ecosystems, our results allow us to conclude that C and P biogeochemical cycles may not become coupled to sustain an eCO₂ fertilisation effect and that the role of AM fungi protecting the SOM pool is likely driven by competitive interactions with saprotrophic communities over nutrients.

土壤有机质 (SOM) 分解和有机磷 (P) 循环可能有助于在 CO ₂ (eCO ₂ ) 升高和低磷条件下维持植物生产力。丛枝菌根 (AM) 真菌及其在 P 获取和 SOM 分解中的作用可能在这些条件下变得更加相关。_{然而，在 eCO 2}下 AM 真菌和 P 有效性对土壤碳 (C) 循环的交互影响的实验证据很少，这种控制的潜在机制知之甚少。我们对来自低磷生态系统的土壤和草进行了盆栽实验，其中植物生物量和土壤碳循环对 eCO ₂几乎没有反应。我们操纵了 AM 真菌、P 和 CO ₂水平并评估其对土壤碳循环和植物生长的影响，使用连续的^{13 C 植物标记来分离和测量呼吸 CO} ₂、溶解有机碳 (DOC) 和微生物生物量 (MBC)的总和 SOM 衍生部分的短期变化，作为土壤碳循环的相关成分。假设 SOM 分解和微生物 C 使用的增加支持植物在 eCO ₂和低 P 下的生长，而 AM 真菌增强了这种效应。然而，我们没有检测到三个实验因素的同时显着影响。_{相反，我们观察到 eCO 2}增加了根生物量和养分吸收和 AM 的存在以及低 P 的 SOM 衍生的 DOC 和 MBC，随着 AM 接种进一步降低。总之，我们在这个模型植物-土壤系统中的研究结果表明，AM 真菌可以支持 eCO ₂下的根系生物量生长和养分吸收，并且即使在低磷条件下也可以保护 SOM 池免受分解。与来自 N 限制生态系统的报告相反，我们的结果使我们能够得出结论，C 和 P 生物地球化学循环可能不会耦合以维持 eCO ₂施肥效应，并且 AM 真菌保护 SOM 池的作用可能是由与营养物质上的腐生群落。