Inland waters have been recognized as a significant source of carbon dioxide (CO₂) to the atmosphere at the global scale. Fluxes of CO₂ between aquatic systems and the atmosphere are calculated from the gas transfer velocity and the water–air gradient of the partial pressure of CO₂ (pCO₂). Currently, direct measurements of water pCO₂ remain scarce in freshwaters, and most published pCO₂ data are calculated from temperature, pH and total alkalinity (TA). Here, we compare calculated (pH and TA) and measured (equilibrator and headspace) water pCO₂ in a large array of temperate and tropical freshwaters. The 761 data points cover a wide range of values for TA (0 to 14 200 μmol L⁻¹), pH (3.94 to 9.17), measured pCO₂ (36 to 23 000 ppmv), and dissolved organic carbon (DOC) (29 to 3970 μmol L⁻¹). Calculated pCO₂ were >10% higher than measured pCO₂ in 60% of the samples (with a median overestimation of calculated pCO₂ compared to measured pCO₂ of 2560 ppmv) and were >100% higher in the 25% most organic-rich and acidic samples (with a median overestimation of 9080 ppmv). We suggest these large overestimations of calculated pCO₂ with respect to measured pCO₂ are due to the combination of two cumulative effects: (1) a more significant contribution of organic acids anions to TA in waters with low carbonate alkalinity and high DOC concentrations; (2) a lower buffering capacity of the carbonate system at low pH, which increases the sensitivity of calculated pCO₂ to TA in acidic and organic-rich waters. No empirical relationship could be derived from our data set in order to correct calculated pCO₂ for this bias. Owing to the widespread distribution of acidic, organic-rich freshwaters, we conclude that regional and global estimates of CO₂ outgassing from freshwaters based on pH and TA data only are most likely overestimated, although the magnitude of the overestimation needs further quantitative analysis. Direct measurements of pCO₂ are recommended in inland waters in general, and in particular in acidic, poorly buffered freshwaters.

Carbon dioxide (CO₂) evasion from inland waters is an important component of the global carbon cycle. However, it remains unknown how global change affects CO₂ emissions over longer time scales. Here, we present seasonal and annual fluxes of CO₂ emissions from streams, rivers, lakes, and reservoirs throughout China and quantify their changes over the past three decades. We found that the CO₂ emissions declined from 138 ± 31 Tg C yr⁻¹ in the 1980s to 98 ± 19 Tg C yr⁻¹ in the 2010s. Our results suggest that this unexpected decrease was driven by a combination of environmental alterations, including massive conversion of free-flowing rivers to reservoirs and widespread implementation of reforestation programs. Meanwhile, we found increasing CO₂ emissions from the Tibetan Plateau inland waters, likely attributable to increased terrestrial deliveries of organic carbon and expanded surface area due to climate change. We suggest that the CO₂ emissions from Chinese inland waters have greatly offset the terrestrial carbon sink and are therefore a key component of China’s carbon budget.

A significant amount of carbon is transported to the ocean as dissolved organic carbon (DOC) in rivers. During transport, it can be transformed through microbial consumption and photochemical oxidation. In dark incubation experiments with water from the Tana River, Kenya, we examined the consumption of DOC through microbial decomposition and the associated change in its carbon stable isotope composition (δ¹³C). In 15 of the 18 incubations, DOC concentrations decreased significantly by 10 to 60 %, with most of the decomposition taking place within the first 24–48 h. After 8 days, the remaining DOC was up to 3 ‰ more depleted in ¹³C compared with the initial pool, and the change in δ¹³C correlated strongly with the fraction of DOC remaining. We hypothesize that the shift in δ¹³C is consistent with greater microbial lability of DOC originating from herbaceous C₄ vegetation than DOC derived from woody C₃ vegetation in the semi-arid lower Tana. The results complement earlier findings that the stable isotope concentration of riverine DOC does not necessarily reflect the proportion of C₃ and C₄-derived DOC in the catchment: besides spatial distribution patterns of different vegetation types, processing within the river can further influence the δ¹³C of riverine OC.