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The riverine source CH4 and N2O from the Republic of Congo, Western Congo Basin

Lookup NU author(s): Professor Robert Upstill-Goddard, Dr Jonathan Barnes, Hengda Ding

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This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).


Abstract

We discuss concentrations of dissolved CH4, N2O, O2, NO3- and NH4+, and emission fluxes of CH4 and N2O for river sites in the Western Congo Basin, Republic of Congo (ROC). Savannah, swamp forest and tropical forest samples were collected from the Congo main stem and seven of its tributaries during November 2010 (41 samples; "wet season") and August 2011 (25 samples; "dry season"; CH4 and N2O only). Dissolved inorganic nitrogen (DIN: NH4+ + NO3-; wet season) was dominated by NO3- (63 ± 19% of DIN). Total DIN concentrations (1.5-45.3 mol L-1) were consistent with the near absence of agricultural, domestic and industrial sources for all three land types. Dissolved O2 (wet season) was mostly under-saturated in swamp forest (36 ± 29%) and tropical forest (77 ± 36%) rivers but predominantly super-saturated in savannah rivers (100 ± 17%). The dissolved concentrations of CH4 and N2O were within the range of values reported earlier for sub-Saharan African rivers. Dissolved CH4 was found to be super-saturated (11.2 - 9553 nmol L-1; 440-354444%), whereas N2O ranged from strong under-saturation to super-saturation (3.2-20.6 nmol L-1; 47-205%). Evidently, rivers of the ROC are persistent local sources of CH4 and can be minor sources or sinks for N2O. During the dry season the mean and range of CH4 and N2O concentrations were quite similar for the three land types. Wet and dry season mean concentrations and ranges were not significant for N2O for any land type or for CH4 in savannah rivers. The latter observation is consistent with seasonal buffering of river discharge by an underlying sandstone aquifer. Significantly higher wet season CH4 concentrations in swamp and forest rivers suggest that CH4 can be derived from floating macrophytes during flooding and/or enhanced methanogenesis in adjacent flooded soils. Swamp rivers also exhibited both low (47%) and high (205%) N2O saturation but wet season values were overall significantly lower than in either tropical forest or savannah rivers, which were always super-saturated (103-266%) and for which the overall means and ranges of N2O were not significantly different. In swamp and forest rivers O2 saturation co-varied inversely with CH4 saturation (log %) and positively with % N2O. A significant positive correlation between N2O and O2 saturation in swamp rivers was coincident with strong N2O and O2 under-saturation, indicating N2O consumption during denitrification in the sediments. In savannah rivers persistent N2O super-saturation and a negative correlation between N2O and O2 suggest N2O production mainly by nitrification. This is consistent with a stronger correlation between N2O and NH4+ than between N2O and NO3-. Our ranges of values for CH4 and N2O emission fluxes (33-48705 mol CH4 m-2 d-1; 1-67 mol N2O m-2 d-1) are within the ranges previously estimated for sub-Saharan African rivers but they include uncertainties deriving from our use of "basin-wide" values for CH4 and N2O gas transfer velocities. Even so, because we did not account for any contribution from ebullition, which is quite likely for CH4 (at least 20%), we consider our emission fluxes for CH4 to be conservative.


Publication metadata

Author(s): Upstill-Goddard RC, Salter ME, Mann PJ, Barnes J, Poulsen JR, Dinga B, Fiske GJ, Holmes RM

Publication type: Article

Publication status: Published

Journal: Biogeosciences

Year: 2017

Volume: 14

Pages: 2267-2281

Print publication date: 05/05/2017

Online publication date: 05/05/2017

Acceptance date: 06/04/2017

ISSN (print): 1726-4170

ISSN (electronic): 1726-4189

Publisher: Copernicus GmbH

URL: https://doi.org/10.5194/bg-14-2267-2017

DOI: 10.5194/bg-14-2267-2017


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