Atmospheric methane is a potent greenhouse gas that plays a major role in controlling the Earth's climate. The causes of the renewed increase of methane concentration since 2007 are uncertain given the multiple sources and complex biogeochemistry. Here, we present a meta-data analysis of methane fluxes from all major natural, impacted and human-made aquatic ecosystems. Our revised bottom-up global aquatic methane emissions combine diffusive, ebullitive and plant-mediated and/or fluxes from several sediment-water-air interfaces. We emphasize the high variability of methane fluxes within and between aquatic ecosystems and a positively skewed distribution of empirical data, making global estimates sensitive to statistical assumptions and sampling design. We find aquatic ecosystems contribute (median) 41% or (mean) 53% of total global methane emissions from anthropogenic and natural sources. We show that methane emissions increase from natural to impacted aquatic ecosystems, and from coastal to freshwater ecosystems. We argue that aquatic emissions will likely increase due to urbanization, eutrophication and positive climate-feedbacks, and suggest changes in land-use management as potential mitigation strategies to reduce aquatic methane emissions. Main text:Methane (CH4) is the second most important greenhouse gas after carbon dioxide (CO2), accounting for 16 to 25% of atmospheric warming to date 1,2 . Atmospheric methane nearly tripled since pre-industrial times with a steady rise between 1984 and 2000 (8.4 ± 0.6 ppb yr -1 ) 3 , little or no growth between 2000 and 2006 (0.5 ± 0.5 ppb yr -1 ) 3 , and a renewed growth to present day (2007 to 2020: 7.3 ± 0.6 ppb yr -1 ) 3-6 . Whether the renewed increase is caused by emissions from anthropogenic or natural sources, or by a decline in the oxidative capacity of the atmosphere, or a combination of all three factors remains unresolved [7][8][9] . Depending on the approach used, total Rivers (ice-corrected) 5.8 (1.8-21.0) 30.5 ± 17.1 This study Lakes (ice-cover, ice-melt corrected) < 0.001 km 2 21.2 (9.1-53.5) 54.5 ± 48.5 This study 0.001 -0.01 km 2 13.2 (5.6-33.1) 31.1 ± 23.7 This study 0.01 -0.1 km 2 4.4 (1.4-16.7) 22.4 ± 18.4 This study 0.1 -1 km 2 3.0 (1.1-8.0) 9.9 ± 7.0 This study > 1 km 2 14.0 (6.0-31.0) 33.0 ± 45.0 This study All lakes 55.8 (23.3-142.3) 150.9 ± 73.0 This study Reservoirs (ice-cover, ice-melt corrected) < 1 km 2 0.4 (0.1-1.3) 2.4 ± 4.7 This study > 1 km 2 14.7 (8.7-27.1) 22.0 ± 6.4 This study All reservoirs 15.1 (8.8-28.4) 24.3 ± 8.0 This study Freshwater wetlands 150.1 (138.3-164.6) 148.6 ± 15.2 Saunois et al. 11 (A) Freshwater aquaculture ponds 4.4 (0.4-7.9) 14.0 ± 18.8 This study Rice cultivation 29.9 (24.9-32.1) 29.8 ± 6.7 Saunois et al. 11 (B) Total inland waters 261.0 (197.5-396.2) 398.1 ± 79.4 This study Estuaries 0.23 (0.02-0.91) 0.90 ± 0.29 This study Coastal wetlands Saltmarshes 0.18 (0.02-0.89) 2.00 ± 1.51 This study Mangroves 0.21 (0.06-0.77) 1.46 ± 0.91 This study Seagrasses 0.13 (0.07-0.21) 0.18 ± 0.19 This study Tidal flats 0.17 (0.04...

Significance Stream/river carbon dioxide (CO 2 ) emission has significant spatial and seasonal variations critical for understanding its macroecosystem controls and plumbing of the terrestrial carbon budget. We relied on direct fluvial CO 2 partial pressure measurements and seasonally varying gas transfer velocity and river network surface area estimates to resolve reach-level seasonal variations of the flux at the global scale. The percentage of terrestrial primary production (GPP) shunted into rivers that ultimately contributes to CO 2 evasion increases with discharge across regions, due to a stronger response in fluvial CO 2 evasion to discharge than GPP. This highlights the importance of hydrology, in particular water throughput, in terrestrial–fluvial carbon transfers and the need to account for this effect in plumbing the terrestrial carbon budget.

High suspended sediment (SPS) concentration exists in many rivers of the world. In the present study, the effects of SPS concentration on denitrification were investigated in airtight chambers with sediment samples collected from the Yellow River which is the largest turbid river in the world. Results from the nitrogen stable ((15)N) isotopic tracer experiments showed that denitrification could occur on SPS in oxic waters and the denitrification rate increased with SPS concentration; this was probably caused by the presence of low-oxygen microsites in SPS. For the water systems with both bed-sediment and SPS, the denitrification kinetics fit well to Logistic model, and the denitrification rate constant increased linearly with SPS concentration (p < 0.01). The denitrification caused by the presence of SPS accounted for 22%, 38%, 53%, and 67% of the total denitrification in systems with 2.5, 8, 15, and 20 g L(-1) SPS, respectively. The activity of denitrifying bacteria in SPS was approximately twice that in bed-sediment, and the denitrifying bacteria population showed an increasing trend with SPS concentration in both SPS and bed-sediment, leading to the increase of denitrification rate with SPS concentration. Furthermore, the denitrification in bed-sediment was accelerated by increased diffusion of nitrate from overlying water to bed-sediment under agitation conditions, which accompanied with the presence of SPS. When with 8 g L(-1) SPS, approximately 66% of the increased denitrification compared to that without SPS was attributed to denitrification on SPS and 34% to agitation conditions. This is the first report of the occurrence of denitrification on SPS in oxic waters. The results suggest that SPS plays an important role in denitrification in turbid rivers; its effect on nitrogen cycle should be considered in future study.

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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.

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Many previous studies have used δ(15)N and δ(18)O of nitrate (δ(15)NNO3 and δ(18)ONO3) to determine the nitrate sources in rivers but were subject to substantial uncertainties and limitations, especially associated with evaluating the atmospheric contribution. The Δ(17)O of nitrate (Δ(17)ONO3) has been suggested as an unambiguous tracer of atmospheric NO3(-) and may serve as an additional nitrate source constraint. In the present study, triple nitrate isotopes (δ(15)NNO3, Δ(17)ONO3, and δ(18)ONO3) were used for the first time to assess the sources and sinks of nitrate in the Yellow River (YR) basin, which is the second longest river in China. Results showed that the Δ(17)ONO3 of the water from the YR ranged from 0‰ to 1.6‰ during two normal-water seasons. This suggested that unprocessed atmospheric nitrate accounted for 0-7% of the total nitrate in the YR. The corrected δ(15)NNO3 and δ(18)ONO3 values with atmospheric imprints being removed indicated that the main terrestrial sources of nitrate were sewage/manure effluents in the upstream of the YR and manure/sewage effluents and ammonium/urea-containing fertilizer in the middle and lower reaches which made comparable contributions to the nitrate. In addition, there was a significant positive relationship between δ(15)NNO3 and δ(18)ONO3 values of river water (p < 0.01) which may signal the presence of denitrification. This study indicates that the triple nitrate isotope method is useful for assessing the nitrate sources in rivers, especially for the measurements of atmospheric nitrate contribution.

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