## 4 Discussion
### Effect of Fungal and Background Community Composition On Paddy Field In Situ CO\({}_{2}\)  
Fungal-bacterial co-occurrence network analysis indicated a greater degree of putative mutualistic symbiotic relationships between fungi and bacteria (58.36%) compared with putative competitive relationships (41.64%) of KA and NKA. The presence of a typical \"core\" plant-microb microbiota has been documented in several environmental studies [20; 53]. These microbiomes may interact through direct or indirect mechanisms of carbon sequestration and can promote plant development [53; 54]. Significantly higher relative abundance of Acidobacteria Gp6 in KA of this study (average relative abundance of 7.05%) was negatively correlated with in situ CO2 fluxes. Acidobacterium Gp6 may be the top microbial species in paddy soil in karst areas, as it is present in aggregate networks of different particle size grades [29]. Previous studies have demonstrated that the average relative abundance of Acidobacteria Gp6 was positively correlated with SOC and pH [55] in addition to amylase activity [56]. These results suggest the presence of coordinated activities of bacteria in KA with respect to SOC and pH. In addition, elevated CO2 treatments have been shown to increase the relative abundance of Acidobacteria Gp6 [57], presumably because some Acidobacteria exhibit photosynthetic capacity. Some acidobacterial genomes contain _pscA_ genes that encode the Fenna-Matthews-Olson (FMO) protein that binds to the bacterial chlorophyll molecule (BCh1) in photoresponse I, enabling chlorophyll-based photosynthetic capacity [58]. In this study, some strains of Acidobacteria Gp6 who absorb CO2 for photosynthesis might have led to reduced CO2 emissions in KA soils. Nevertheless, the carbon sequestration capacity of Acidobacteria Gp6 taxa in KA soils deserves our attention.  
_Nitrospira_ (OTU9) was also a top taxon in the internal bacterial correlation network and was dominant in NKA. _Nitrospira_ abundance were negatively correlated with pH and TOC [59], and are highly resistant to high ammonia, high pH, salt, and SOC environments. In particular, the _Nitrospira_ was most likely represented by the single-step nitrifying bacteria comammox _Nitrospira_ that drives full nitrification [60]. Comammox _Nitrospira_ encode a single gene cluster containing both _amo_ and _hao_ genes, but do not use Ca2+ (e.g., calcium nitrate and calcium nitrate) in nitrification reactions [61]. Instead, comammox _Nitrospira_ require oxygen molecules to activate ammonia during nitrification [62], which are key adaptations for red soil environments with low carbon, nitrogen, and Ca2+ concentrations. Thus, OTU9 (comammox _Nitrospira_) is probably an important indicator in NKA rather than KA.  
The internal fungal correlation network indicated that OTU69 (_Entericellopsis_) and OTU139 (Sordariales) as the core groups within the soil fungal networks in KA and NKA, respectively. _Entericellopsis_ is an alkalophilic endophytic fungus with the ability to resist pathogenic peptide synthesis [63] and it enhances the plant host's ability to cope with environmental stresses, while also contributing to healthy host-plant growth and reproduction [64]. The neutral-alkaline pH in KA resulted in increased relative abundances of _Entericellopsis_ that may then intermittently raise the quantity and quality of algae during the flooding periods within rice fields. Further, most water column algae in KA can use CO2 and free HCO3- to conduct photosynthesis and produce organic carbon. The synergistic interaction between algae and their endophytic fungi in photosynthesis of KA therefore requires further investigation [65]. _Stellarospora_ belongs to the Sordariales group, whose abundances were positively correlated with CO2 emission. Some studies have shown that Sordariales are highly adapted to tropical soil environments that experience warming due to elevated atmospheric CO2[66]. Thus, _Stellarospora_ might be regarded as a biomarker indicator to distinguish NKA from KA.