[Key pathway of methane production and characteristics of stable carbon isotope of the Tuojia River waterbody.].

This study aimed at exploring the key pathway of methane production and clarifying the composition and distribution of carbon (C) isotopes in the Tuojia River waterbody in Hunan Pro-vince. We estimated CH concentrations and fluxes of four reaches (S, S, S and S) by a two-layer diffusion model and gas chromatography. The spatial and temporal distribution of CH flux and its relationship with environmental factors were examined. The key pathway of CH production was investigated by stable C isotope method to analyze the distribution characteristics of C isotope (δC) of water dissolved CH and seston/benthic organic matter. There was significant seasonal variability in water pH, with mean value of (7.27±0.03). The concentration of dissolved oxygen (DO) showed strong seasonal and spatial variations, with the range of 0.43-13.99 mg·L. The maximum value of DO occurred in S and differed significantly in summer and autumin. In addition, DO differed significantly in winter and other seasons in S, S and S. The concentration of dissolved organic carbon (DOC) showed a gradual increasing trend from source to estuary. The highest concentration of DOC (8.32 mg·L) was found in S, while the lowest was observed in S (0.34 mg·L). The electrical conductivity (EC) and oxidation-reduction potential (ORP) of water ranged from 17 to 436 μS·cm and from -52.30 to 674.10 mV, respectively, which were significantly different among the four reaches (P＜0.05). Water ammonium nitrogen (NH-N) and nitrate nitrogen (NO-N) concentrations were in the ranges of 0.30-1.35 (averaged 0.90±0.10) mg·L and 0.82-2.45 (averaged 1.62±0.16) mg·L, respectively. The dissolved concentration and diffusion flux of CH ranged from 0 to 5.28 μmol·L and from -0.34 to 619.72 μg C·m·h, respectively, with significant temporal and spatial variations. They showed a similar trend among reaches. Their values were highest in spring, followed by in winter and lowest in summer and autumn. Spatially, the CH concentration and flux followed the order of S>S>S>S. The correlation analysis showed that CH flux was positively correlated with NH-N and DOC. The pathway of CH production of all reaches was dominated by acetic acid fermentation, while there were obvious differences among the four reaches. The contribution of CH from acetic acid fermentation was greatest (87%) in S, followed by S(81%), S(78%) and S(76%). The mean value of the δC for dissolved CH, seston organic matter and benthic organic matter was -41.64‰±1.91‰, -14.07‰±1.06‰ and -26.20‰±1.02‰, respectively. There was a positive correlation between the δC of dissolved CH and benthic organic matter, whereas the δC value of dissolved CH was negatively correlated with CH flux.