Arbuscular mycorrhizal fungi mitigate earthworm-induced N2O emissions from upland soil in a rice-rotated wheat farming system

Earthworms are important for soil processes in arable cropping systems, and affect the development of arbuscular mycorrhizal fungi (AMF) which may combinely play a crucial role in soil nitrogen (N) cycling. However, the information is limited about their interaction on N2O emissions. A two-factorial microcosm field experiment was established to identify the interactive effects of earthworms and AMF on N2O emissions from a rice-rotated wheat field and explore its underlying mechanisms about nitrification and denitrification processes. The results showed that earthworm addition (+E) increased cumulative N2O emissions during the whole growth stage by 236.7 % compared with no earthworms (−E). Cumulative N2O emissions during the jointing and booting stages contributed to 58.7 % of that during the whole growth stage. Higher N2O emissions under +E were attributed to changes in soil organic carbon (SOC) and NH4+ concentration. SOC was positively correlated to gene abundances associated with N2O reduction (e.g., nosZ), and earthworm activity enhanced SOC mineralization and reduced SOC concentration, thus increasing N2O emissions. A negative correlation between NH4+ and N2O emissions, and lower NH4+ under +E indicated that earthworms enhanced nitrification intensity using NH4+ as substrates. Interestingly, AMF normal (+A) significantly decreased cumulative N2O emissions by 37.3 % compared with mycorrhizal suppression (−A) and the interaction with earthworms can further decreased N2O emissions by 37.0 % compared with -E-A (P < 0.05). This can be explained by higher NH4+, lower NO3− concentrations, and lower abundance of nitrification-associated genes (e.g., AOA and AOB) at the booting stage under +A, suggesting that AMF reduced N2O emissions mainly through lowering nitrification intensity. The results highlight that earthworms increase N2O emissions while AMF can mitigate earthworm-induced N2O emissions, and enhance our understanding on the soil N cycle in terms of N2O production in the presence of AMF and earthworms.