## 4 Discussion
### Relationships between Carbon Emissions and Hydrothermal Factors  
The soil water content and soil temperature are important factors that affect soil CO\({}_{2}\) emissions [27]. [PERSON] et al. [29] found that the CO\({}_{2}\) emission fluxes tended to decrease as the soil water content increased in tobacco fields, and indicated that the CO\({}_{2}\) diffusion resistance was large when the soil water content was high, which was not conducive to CO\({}_{2}\) emissions. Furthermore, [PERSON] et al. [75] showed that the relationship between the CO\({}_{2}\) emission flux and soil water content conformed to an exponential decay equation in the maize growing season, and the R\({}^{2}\) value for the fitted equation increased as the biochar application amount increased. Similar to previous studies, we found that the relationships between the CO\({}_{2}\)-C emission fluxes and soil water contents under different treatments all conformed to an exponential decay equation, and the goodness of fit of the equation increased as the amount of added biochar increased. However, increases in the soil temperature can promote microbial metabolism and the decomposition of organic carbon in soils [76; 77], which will increase soil CO\({}_{2}\) emissions. Thus, [PERSON] et al. [26] showedthat the CO\({}_{2}\) emission flux increased exponentially as the soil temperature increased in a wheat-maize rotation system. Furthermore, [PERSON] et al. [78] showed that the goodness of fit of the equation for the relationship between the CO\({}_{2}\) emission flux and soil temperature decreased after applying biochar. Similar to most previous studies, we found that the relationships between the CO\({}_{2}\)-C emission fluxes and soil temperatures under different biochar application amounts all conformed to an exponential growth equation, and the R\({}^{2}\) values for the fitted equation decreased as the amount of added biochar increased.  
The soil water content and soil temperature are also important factors that affect soil CH\({}_{4}\) emissions (or uptake) [79; 80]. The relationship between the soil water content and CH\({}_{4}\) emission varies under different soil water conditions, where an increase in the soil water content from a low level can enhance the microbial activity and increase CH\({}_{4}\) uptake [81], whereas a decrease in the soil water content from a high level will make the environment less anaerobic and reduce CH\({}_{4}\) emissions [82]. However, there is no significant correlation between the CH\({}_{4}\) uptake flux and soil water content when the soil water content is in a moderate range with little fluctuation [27; 83]. In the present study, the soil water content was moderate range under the different treatments, so we found no significant correlation between the CH\({}_{4}\)-C uptake flux and soil water content under each treatment. In addition, methanogenic bacteria and methane-oxidizing bacteria differ in terms of their preferred temperature ranges and sensitivity to temperature [84; 85], and thus the effects of variations in the soil temperature on CH\({}_{4}\) emissions are relatively complex. In particular, [PERSON] et al. [86] found that the relationship between the CH\({}_{4}\) emission flux and soil temperature in a desert steppe soil conformed to a U-shaped quadratic equation. Furthermore, [PERSON] et al. [83] showed that the goodness of fit of the equation for the relationship between the CH\({}_{4}\) uptake flux and soil temperature increased after applying biochar. Similar to previous studies, we found that the relationships between the CH\({}_{4}\)-C uptake fluxes and soil temperatures under different treatments all conformed to an inverse U-shaped quadratic equation, and the R\({}^{2}\) value for the fitted equation increased after applying biochar.