**4. Conclusions**

This study investigated the variation of soil respiration and its influencing factors under di fferent irrigation levels with and without aeration in a greenhouse tomato system. Aeration had a significant effect on tomato root length, as well as dry biomass of leaf, stem, and fruit, while no significant di fferences on other parameters were observed. As irrigation amount applied into soils increased, soil respiration increased in conjunction with its influencing factors, and the e ffect was significant on the dry biomass of leaf, fruit, and root. Soil respiration was significantly correlated with soil water-filled pore space, the abundance of soil bacteria and fungi. These results indicate that irrigation combined with aeration would increase soil physical and biotic variables, which stimulate more CO2 release. The application of reduced irrigation and aeration has potential for alleviating CO2 emissions.

**Author Contributions:** Conceptualization, H.C. (Hui Chen) and H.C. (Huanjie Cai); Data curation, H.C. (Hui Chen) and Z.S.; Formal analysis, Z.S.; Funding acquisition, H.C. (Huanjie Cai); Investigation, H.C. (Hui Chen), Z.S. and Y.Z.; Methodology, H.C. (Hui Chen), H.C. (Huanjie Cai) and Y.Z.; Software, H.C. (Hui Chen); Validation, H.C. (Huanjie Cai); Writing—original draft, H.C. (Hui Chen); Writing—review and editing, Z.S., H.C. (Huanjie Cai) and Y.Z.

**Funding:** This work was supported by the National Key Research and Development Program of China (2016YFC0400201) and the National Natural Science Foundation of China (51309192).

**Conflicts of Interest:** The authors declare no conflict of interest.
