Nitrogen Fertilizer Reduction with Organic Residue Incorporation Mitigates Soil Organic Carbon Mineralisation by Regulating Microbial Community and CAZyme Genes

The co-application of organic matter and nitrogen (N) fertiliser has been recommended as an environmentally friendly practice. However, excessive N fertiliser use contributes to carbon (C) loss and several environmental problems. Our study aims to analyse the effects of reducing mineral N application under supplemental organic residue on organic C mineralisation and its microbial mechanisms. Therefore, a 120-day incubation experiment was conducted to assess the effects of incorporating 13C-labelled pruning branch residue combined with different N levels (225, BRN1; 180, BRN2; 160, BRN3; and 140 mg N·kg−1, BRN4) on CO2 emission, soil microbial community and functional genes in apple orchard soil. All treatments increased CO2 efflux and induced a positive priming effect (PE), expressed as promoted native soil organic C mineralisation. BRN1 produced the highest positive PE, followed by BRN2 and BRN4, and the lowest PE was observed under BRN3. We highlight the importance of plant- and bacteria-derived biomass (indicated by peptidoglycan) decomposition rather than the composition and structure of microbial communities in regulating organic C decomposition in response to pruning residue and N addition. In addition, reduced N treatments decreased abundance of C-degrading genes for plant-derived (hemicellulose, lignin) and bacteria-derived biomass (peptidoglycan) as compared with BRN1. Four near-complete bacterial genome bins were strongly correlated with PE; three of these bins were assigned to Actinomycetota and one to Acidobacteriota. Moreover, structural equation modelling indicated CAZyme-encoding genes had a stronger association with PE than microbial community. Collectively, coupling branch residue with N reduction has great potential for mitigating soil C loss. The optimal amount of N reduction should be further evaluated based on the fruit yield in the field.