## 4 Discussion
### Effects of Fungal and Bacterial Functional Groups on Paddy Field In Situ CO2 Fluxes  
Among the predicted fungal functional groups, the endomycorrhizal-plant pathogen-undefined saprotroph functional group was largely represented by Ceratobasidiaceae, with dominant relative abundances in NKA. Ceratobasidiaceae are able to rice blight, where in stems and leaves become yellow and wilt, leading to weakened photosynthetic ability [67]. The plant pathogenic fungal functional group was largely represented by the genus _Gaeumanmonyces_, which also exhibited higher abundances in NKA. Some fungi of this genus are capable of infesting wheat and resulted in wheat allozyme diseases. The plant pathogenic and endomycorrhizal-plant pathogen-undefined saprotroph abundances were positively correlated with CO2 in situ emission. The infected plants transmit infection information to soil through root exudates, causing the bacterial quorum sensing [68] leading to compensatory increase in soil respiration. In addition, Ca2+ affect activity of the antifungal proteins released by bacteria [69], which in turn influences the resistance of crops to allozyme disease. Therefore, the interaction between pathogenic microbes and antagonistic microbes in KA and NKA should be further studied.  
The abundance of dung saprotroph-endophyte-litter saprotroph-undefined saprotroph group were negatively correlated with in situ CO\({}_{2}\) emission. This functional group was mostly represented by _Podospora_ that are dominant in KA. [PERSON] reported that _Podospora anserine_, a late colonizer of herbivous dung, has high NADPH oxidative activity [70]. The oxidase promotes the formation of H\({}_{2}\)O\({}_{2}\) and acts specifically on the more recalcitrant fraction of lignocellulosic, which may promote the degradation of organic matter. [PERSON] reported that NADPH oxidase can also promote the sexual reproduction and ascospore germination of the filamentous fungus _Podospora anserine_[71]. The products of lignocellulosic degradation by fungi are likely to be used by bacteria, which improved utilization efficiency of organic matter and increased thermally stable SOC [72]. These results suggest that the relationship between soil microorganisms including _Podospora_ and the thermal stability of SOC in KA is worth further study.  
CO\({}_{2}\) in soils not only changed general microbial taxa, but also can be assimilated by autotrophic bacteria and turned into microbial biomass [73]. In the present study, autotrophic bacteria (such as aerobic nitrite oxidizers and nitrifiers) using CO\({}_{2}\) as the only carbon source were dominant in KA (as shown in Figure 6a). This was similar to the results of our previous investigation, which showed that high abundance of carbon fixing bacteria was in KA [25]. Among the predicted bacterial functional groups, the abundances of aerobic nitrite oxidizers and nitrifiers were negatively correlated with in situ CO\({}_{2}\) fluxes. These functional groups derive from the aerobic nitrite oxidation and nitrification activities associated with OTU831 (_Nitrospira_), which were dominant in KA. Ammonia-oxidizing bacteria (AOB) are autotrophic microorganisms using CO\({}_{2}\) as a carbon source in autotrophy [74]. Nitrification substrates are NH\({}_{3}\) molecules rather than NH\({}_{4}\)\({}^{+}\) ions, and a neutral alkaline karst environment might make the chemical equilibrium of NH\({}_{3}\) and NH\({}_{4}\)\({}^{+}\) tend towards NH\({}_{3}\) molecule production. Consequently, the abundances of AOB and their ability to fix CO\({}_{2}\) might be improved by increasing the abundances of NH\({}_{4}\)\({}^{+}\)-N [75; 76]. Therefore, the lower in situ CO\({}_{2}\) emission in KA might also contribute to autotrophic nitrification by _Nitrospira_[77].