The riverine source of tropospheric CH4 and N2O from the
Republic of Congo, Western Congo Basin
Authors: Upstill-Goddard Robert C. , Salter Matthew E. , Mann Paul J. , Barnes Jonathan , Poulsen John , Dinga Bienvenu , Fiske Gregory J. , Holmes Robert M. 
Published: November 2016

Abstract. Abstract. We report concentrations of dissolved CH4, N2O, O2, NO3− and NH4+, and corresponding CH4 and N2O emissions for river sites in savanna, swamp forest and tropical forest, along the Congo main stem and in several of its tributary systems of the Western Congo Basin, Republic of Congo, during November 2010 (41 samples; ''wet season'') and August 2011 (25 samples; ''dry season''; CH4 and N2O only). Dissolved inorganic nitrogen (DIN: wet season; NH4&amp;plus; &amp;plus;  NO3−) was dominated by NO3− (63 ± 19 % of DIN), total DIN concentrations (1.5–45.3 nmol L−1) being consistent with small agricultural, domestic and industrial sources. 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 %). Dissolved CH4 and N2O were within previously reported ranges for sub-Saharan African rivers. While CH4 was always super-saturated (11.2–9553 nmol L−1; 440–354 400 %), N2O ranged from strong under-saturation to strong super-saturation (3.2–20.6 nmol L−1; 47–205 %). Evidently, rivers of the ROC are persistent local sources of tropospheric CH4 but can be small sources or sinks for N2O. Dry season concentration means and ranges of CH4 and N2O were indistinguishable for all three land types and seasonal differences in means and ranges were not significant for N2O for any land type or for CH4 in savannah rivers; the latter is consistent with seasonal buffering of river discharge by an underlying sandy-sandstone aquifer. By contrast, swamp and forest river CH4 was significantly higher in the wet season, possibly reflecting CH4 derived from floating macrophytes during flooding and/or enhanced methanogenesis in adjacent flooded soils. Swamp rivers exhibited both low (47 %) and high (205 %) N2O saturations 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 co-varied negatively with log % CH4 and positively with % N2O. The strong positive N2O–O2 correlation in swamp rivers was coincident with strong N2O and O2 under-saturation, indicating N2O consumption by sediment denitrification. In savannah rivers persistent N2O super-saturation and a negative N2O–O2 correlation may indicate N2O production mainly by nitrification, consistent with a stronger correlation between N2O and NH4&amp;plus; than between N2O and NO3−. Our range in CH4 and N2O emissions fluxes (33–48 705  μmol CH4 m−2 d−1; 1–67 μmol N2O m−2 d−1), is wider than previously estimated for sub-Saharan African rivers but it includes 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 for CH4 is likely to be at least 20 %, our emissions estimates for CH4 are probably conservative.