Chemical Oxygen Demand as a Measure of Fluvial Organic Matter Oxidation State

The oxidative ratio (OR) of the terrestrial biosphere is directly related to the size of the terrestrial biosphere carbon sink. In turn, OR of naturally occurring organic matter can be directly related to the oxidation state of the carbon in naturally occurring organic matter (C_ox). Chemical oxygen demand (COD) is a widely measured water quality parameter that has been used as a short-term substitute for the biochemical oxygen demand (BOD). Here, we propose that if the concentration of reduced species is known, then COD measurement can be used to assess the oxidation state (C_ox) of fluvial organic C. Using a Bayesian hierarchical modeling approach, this study analyzed 21 years of water quality monitoring across England to calculate C_ox of fluvial organic matter. The study showed that (a) COD could not be considered separately from the reduced species (e.g., NH₄) commonly occurring in freshwater water samples, but it was still possible to calculate the Cox of dissolved organic carbon (DOC) and particulate organic carbon (POC). (b) The median C_ox of DOC was 0.23 with a 95th percentile range of −0.1 to 0.4. (c) The median C_ox of POC was 0.20 with a 95th percentile range of 0.03–0.37. (d) The estimated C_ox in fluvial systems confirms that BOD is decoupled from the production of CO₂. Including new C_ox estimates in the global estimate of OR gives a new median value of 1.059 with a 95th percentile range of 1.047–1.071, giving the annual flux of CO₂ to land (f land) of 1.45 ± 0.1 Gt C/ year.