Journal Article

The riverine source of CH.sub.4 and N.sub.2O from the Republic of Congo, western Congo Basin

Fiske, Gregory J, Poulsen, John, Salter, Matthew E, Mann, Paul J, Dinga, Bienvenu, Upstill-Goddard, Robert C, Barnes, Jonathan, Holmes, Robert M

2017

Overview

We discuss concentrations of dissolved CH.sub.4, N.sub.2 O, O.sub.2, NO.sub.3 .sup.- and NH.sub.4 .sup.+, and emission fluxes of CH.sub.4 and N.sub.2 O 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; CH.sub.4 and N.sub.2 O only). Dissolved inorganic nitrogen (DIN: NH.sub.4 .sup.+ + NO.sub.3 .sup.- ; wet season) was dominated by NO.sub.3 .sup.- (63 ± 19 % of DIN). Total DIN concentrations (1.5-45.3 µmol L.sup.-1) were consistent with the near absence of agricultural, domestic and industrial sources for all three land types. Dissolved O.sub.2 (wet season) was mostly undersaturated in swamp forest (36 ± 29 %) and tropical forest (77 ± 36 %) rivers but predominantly supersaturated in savannah rivers (100 ± 17 %). The dissolved concentrations of CH.sub.4 and N.sub.2 O were within the range of values reported earlier for sub-Saharan African rivers. Dissolved CH.sub.4 was found to be supersaturated (11.2-9553 nmol L.sup.-1 ; 440-354 444 %), whereas N.sub.2 O ranged from strong undersaturation to supersaturation (3.2-20.6 nmol L.sup.-1 ; 47-205 %). Evidently, rivers of the ROC are persistent local sources of CH.sub.4 and can be minor sources or sinks for N.sub.2 O. During the dry season the mean and range of CH.sub.4 and N.sub.2 O concentrations were quite similar for the three land types. Wet and dry season mean concentrations and ranges were not significant for N.sub.2 O for any land type or for CH.sub.4 in savannah rivers. The latter observation is consistent with seasonal buffering of river discharge by an underlying sandstone aquifer. Significantly higher wet season CH.sub.4 concentrations in swamp and forest rivers suggest that CH.sub.4 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 %) N.sub.2 O saturation but wet season values were overall significantly lower than in either tropical forest or savannah rivers, which were always supersaturated (103-266 %) and for which the overall means and ranges of N.sub.2 O were not significantly different. In swamp and forest rivers O.sub.2 saturation co-varied inversely with CH.sub.4 saturation (log %) and positively with % N.sub.2 O. A significant positive correlation between N.sub.2 O and O.sub.2 saturation in swamp rivers was coincident with strong N.sub.2 O and O.sub.2 undersaturation, indicating N.sub.2 O consumption during denitrification in the sediments. In savannah rivers persistent N.sub.2 O supersaturation and a negative correlation between N.sub.2 O and O.sub.2 suggest N.sub.2 O production mainly by nitrification. This is consistent with a stronger correlation between N.sub.2 O and NH.sub.4 .sup.+ than between N.sub.2 O and NO.sub.3 .sup.- . Our ranges of values for CH.sub.4 and N.sub.2 O emission fluxes (33-48 705 µmol CH.sub.4 m.sup.-2 d.sup.-1 ; 1-67 µmol N.sub.2 O m.sup.-2 d.sup.-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 CH.sub.4 and N.sub.2 O gas transfer velocities. Even so, because we did not account for any contribution from ebullition, which is quite likely for CH.sub.4 (at least 20 %), we consider our emission fluxes for CH.sub.4 to be conservative.

• Publisher: Copernicus GmbH
• Subject: Chemical properties / Methane / Nitrous oxide / Rivers