The inhibitor additivity ratio (IAR)was used to evaluate whether antibiotics exerted non-target effects (Beare et al., 1990). It was estimated by the equation: ðABÞþðACÞ AD , where A, B, C, and D represent N2O (or CO2) fluxes in antibiotic-free control, cycloheximide amended soil, streptomycin-amended soil, and both antibioticsamended soil, respectively. An IAR of ~1.0 indicated that inhibitory effects of streptomycin and cycloheximide were not confounded