Spent mushroom substrate as a substitute for chemical fertilizer changes N-cycling genes and reduces N2O emission in different textured soils

Spent mushroom substrate (SMS) is a by-product of mushroom production, which can be used as an organic fertilizer. However, studies on the effect of SMS as a substitute for chemical fertilizer on soil N₂O emissions and relative mechanisms are scarce. A laboratory incubation experiment was conducted to examine the effects of SMSs derived from Pleurotus eryngii (SMS-PE) and Agaricus bisporus (SMS-AB) production on N₂O emissions from a sandy loam and a clay loam soil. Six treatments for each soil were established: no fertilization (CK); chemical N fertilizer (U); two types of SMSs alone (SMS-PE and SMS-AB); and their half substitution with chemical N fertilizer (SMS-PEU and SMS-ABU). The recalcitrance and aliphaticity indices of SMS-PE determined by ¹³C nuclear magnetic resonance spectra were lower than those of SMS-AB. N₂O emissions from the SMS-PEU and SMS-ABU decreased by 21.5–58.5% compared to the urea alone treatment. The emission of N₂O was controlled by soil NH₄⁺-N and AOB, indicating that nitrification is primarily responsible for N₂O production. The increased NH₄⁺-N concentration in the SMS-PEU treatment increased N₂O emissions by 32.4% compared to the SMS-ABU treatment of the sandy loam soil. The N₂O emissions from the clay loam soil treated with SMS-PEU depended on NO₃⁻-N concentration and absolute abundance of nirS, nirK, and the nir/nos ratio, suggesting the presence of the simultaneous nitrification and denitrification. Thus, the application of SMS-PE with a higher proportion of available organic C increased denitrification gene abundance and likely enhanced denitrification-associated N₂O emissions (by 72.6%) in the clay loam soil compared to SMS-ABU. Overall, our results reveal that substituting chemical N with SMSs, particularly SMS-AB with lower NH₄⁺-N and available C concentrations, is a promising strategy to mitigate N₂O emissions while reducing chemical N consumption.