Limited information is available for how soil compaction affects carbon dioxide (CO2) efflux under different tillage systems. To improve understanding of the effects of compaction and tillage on soil CO2 efflux, an incubation study consisting of no tillage (NT), moldboard plow (MP), and ridge tillage (RT) was conducted to explore the relationships between CO2 efflux and bulk density, as well as pore size distribution under different levels of bulk densities, and the thresholds of bulk density and the volume of pore size above which CO2 efflux were affected in northeast China. Results showed that there was a significantly negative correlation (r = -0.990, p<0.05; r = -0.986, p<0.05; and r = -0.992, p<0.01, respectively, for NT, MP, and RT) between CO2 efflux and bulk density, whereas the correlation was significantly positive (r ranges from 0.75 to 0.85, p<0.05 for each tillage practice under bulk densities of 1.0-1.6 g/cm3) for the volume of small macropores (30-100 μm). The critical value of bulk density for impeding CO2 efflux was more produced in 1.6 g/cm3 and the volume of small macropores affected CO2 efflux variation greatly. Ridge tillage is a better tillage practice for impeding soil CO2 efflux than no tillage, as evidenced by the lesser volume of small macropores.
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