Coupled carbon-nitrogen cycling controls the transformation of dissolved inorganic carbon into dissolved organic carbon in karst aquatic systems

The cycling of carbon (C) and nitrogen (N) in karst aquatic systems has been shown to be closely related, with coupled control of dissolved organic carbon (DOC) concentrations through the metabolic pathways of subaquatic communities. However, the coupled CN cycling involving in the transformation of dissolved inorganic carbon (DIC) into DOC has not been well-explored. In this study, we chose the Lijiang River, a typical karst aquatic system in Southwest China as our study area and documented its diurnal and seasonal variations in terms of several hydrochemical and isotopic parameters to identify how to couple cycling for C and N. The results of the Bayesian stable isotope-mixing model showed that approximately 50% and 72% of the total DOC formed in summer and winter, respectively, represented autochthonous organic carbon in the Lijiang River. Diurnal monitoring results revealed that DIC and NO3− transformations were primarily controlled by metabolic processes (photosynthesis and respiration) of subaquatic communities, accompanying DOC formation, in the Lijiang River. The consumption of DIC and NO3− by aquatic photosynthesis was in the ratio of 9:1 (mol/mol) to produce autochthonous DOC, accompanying the enriched δ13CDIC, δ15N-NO3− and δ18O-NO3−, with a daily variation of 7.9‰, 10.6‰ and 11.2‰, respectively. On the diurnal scale, 6.2% of the total DIC and 7.1% of the total NO3− were consumed by metabolic processes of subaquatic communities and these values were consistent with their corresponding values on the interannual scale. However, the proportions of DIC and NO3− utilized in the dry season were higher than those in the wet season. Approximately 1.18 × 107 kg C/yr of DIC and 1.64 × 106 kg N/yr of NO3− were converted into organic matter by the aquatic photosynthesis, with 80% and 79% of the total DIC and NO3− consumption respectively occurring in the wet season. Furthermore, the coupled CN cycling involving DIC and NO3− can promote the production of autochthonous DOC, constituting a relatively long-term natural C and N sinks in karst aquatic systems. •Coupled CN cycling in a karst aquatic system is controlled by metabolic processes.•Most DOC was autochthonous OC formed in the coupled CN cycling.•The ratio of consumption of DIC and NO3− by aquatic photosynthesis was 9:1.