Arbuscular mycorrhizal (AM) fungi form symbiotic associations with about two-thirds of plants in a wide range of ecosystems. AM fungi often increase plant growth through enhancing plant nutrient acquisition. They also act as a sink for plant carbohydrates, thereby enhancing carbon inputs belowground. Accumulating evidence shows that AM fungi directly mediate plant residue decomposition and that elevated CO₂ and N inputs modify the functioning and activity of AM fungi. However, few experiments have examined whether alterations in AM fungi induced by elevated CO₂ and N inputs influence residue decomposition and plant N acquisition. We conducted a microcosm experiment to quantify the effect of elevated CO₂ and N inputs on mycorrhizal mediation of residue decomposition and plant N uptake in a highly N-limited soil. A nylon mesh bag with a pore size of 20µm was designed to isolate effects of AM fungi from growing roots, using a mycorrhizal mixture and wild oats (Avena fatua L.) as model species. ¹³C and ¹⁵N dual labeled organic residues were used to examine the C mineralization and subsequent ¹⁵N transfer by AM external hyphae.

Results/Conclusions

Results showed that both elevated CO₂ and N additions significantly increased root and shoot biomass, and the percentage of root colonization by AM fungi. Elevated CO₂ significantly enhanced mycorrhizal stimulation of residue decomposition. Elevated CO₂ and N additions interactively stimulated mycorrhizally-mediated plant ¹⁵N uptake, increasing shoot ¹⁵N by 87% and 164% under no-N addition and N-addition treatments, respectively. These findings suggest that elevated CO₂ and low to moderate N inputs may facilitate residue decomposition and nutrient cycling through stimulating mycorrhizal activities, particularly in highly N-limited ecosystems.