## 1 Introduction  
When faced with the discharge of human pollutants and animal waste and the decomposition of biological residues, water can retain a large number of small-molecule organic matter (OM) [1, 2]. The substrate of wetland, such as rivers, lakes, and marshes, comprises sediment, gravel, and many other base materials [3]. A large number of microbial communities live in the wetland substrate. The small-molecule OM in the overlying water can be settled, accumulated, degraded, and transformed by the microbial activities, thus the concentration of small-molecule OM is reduced and water eutrophication is alleviated in the wetlands [4; 5]. Meanwhile, the above-mentioned metabolic process can seriously affect the carbon sequestration, nitrogen removal, and other functions of wetlands [6], while the attention on the accumulation and degradation of small-molecule OM in wetlands is far from enough. At present, most relevant studies focus on the effects of exogenous carbon addition on the greenhouse gas and nitrogen emissions [7; 8], but the internal dynamics and affecting mechanisms are still not clear. Many scholars have studied the dynamic impacts of exogenous carbon addition on the nitrogen cycle and carbon cycle in soils and wetland sediments. Organic fertilizers, as one compound of exogenous nutrition, can directly input labile carbon into soils, stimulating the degradation of soil organic carbon (OC) by microorganisms [9]. Additionally, the addition of organic fertilizers significantly enhances the abundance of genes related to nitrification and denitrification [10]. Furthermore, the inputs of chemical N fertilizers (ammonium- or nitrate-based) can not only stimulate N\({}_{2}\)O, but also promote CH\({}_{4}\) and CO\({}_{2}\) emissions [11]. Exogenous carbon addition enriches the relative abundances of proteobacteria and planctomycetes that carry C and N cycling genes, while inhibiting the growth of oligotrophic groups such as Verrucomicrobia [12]. Moreover, the results have indicated that exogenous carbon addition can significantly decrease soil nitrate nitrogen (NO\({}_{3}\)\({}^{-}\)-N) and ammonium nitrogen (NH\({}_{4}\)\({}^{+}\)-N) contents, and reduce the nitrogen mineralization by inhibiting nitrification [13; 14]. The addition of monosaccharides or synthetic root secretions can rapidly raise the nitrogen-fixing bacteria abundance and nitrogen fixation rate in sediments, thus improving the organic nitrogen accumulation [15]. [PERSON] et al. (2019) indicated that exogenous biochar addition can elevate soil fertility, crop yield, CO\({}_{2}\) absorption, and organic carbon accumulation [16]. [PERSON] et al. (2018) suggested little increase in organic carbon storage when exogenous carbon is continuously added to carbon-saturated soils for years [17]. The addition of plant residues can increase the prim effect and promote the native original organic carbon decomposition [18]. [PERSON] et al. (2016) reported that the effects on exogenous glucose addition are mainly manifested in the first 3 days, with glucose mostly being degraded and the mineralization of native organic carbon being induced [19]. Moreover, the impacts of glucose addition on bacteria were more significant than fungi, and bacterial abundance was remarkably elevated with glucose addition [16], while the impacts on wetland OC dynamics were still not deep enough. This research identified the main dynamics of nitrogen, carbon, and microbial activities after the addition of various amounts of exogenous carbon. The CO\({}_{2}\) assimilation by autotrophic microbes and biology-derived OM input are also important aspects of sediment carbon fixation and sequestration, while the affecting performance is less studied. Mostly, autotrophic microbes fix CO\({}_{2}\) through the Calvin cycle [20]. Ribulose-1,5-bisphosphate carboxylase oxygenase (RubisCO) is the key enzyme for controlling the Calvin cycle rate, which is performed in four forms (I, II, III, and IV), with different structures, functions, and O\({}_{2}\) sensitivities [21].