Phosphorus availability and arbuscular mycorrhizal fungi limit soil C cycling and influence plant responses to elevated CO2 conditions

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.