Abstract
Globally, tropical forests are assumed to be an important source of atmospheric nitrous oxide (N2O) and sink for methane (CH4). Yet, although the Congo Basin comprises the second largest tropical forest and is considered the most pristine large basin left on Earth, in situ N2O and CH4 flux measurements are scarce. Here, we provide multi-year data derived from on-ground soil flux (n = 1558) and riverine dissolved gas concentration (n = 332) measurements spanning montane, swamp, and lowland forests. Each forest type core monitoring site was sampled at least for one hydrological year between 2016 - 2020 at a frequency of 7-14 days. We estimate a terrestrial CH4 uptake (in kg CH4-C ha−1 yr−1) for montane (−4.28) and lowland forests (−3.52) and a massive CH4 release from swamp forests (non-inundated 2.68; inundated 341). All investigated forest types were a N2O source (except for inundated swamp forest) with 0.93, 1.56, 3.5, and −0.19 kg N2O-N ha−1 yr−1 for montane, lowland, non-inundated swamp, and inundated swamp forests, respectively.
Highlights
Tropical forests are assumed to be an important source of atmospheric nitrous oxide (N2O) and sink for methane (CH4)
While strong uptake rates for CH4 can mostly be attributed to diffusion, facilitated by the coarse texture of highly weathered tropical lowland forest soils[3], high N2O emissions are mostly attributed to excess nitrogen relative to phosphorus in these soils[4]
Even though soil moisture and temperature are known drivers of N2O and CH4 fluxes, the combined variability explained by water-filled pore space (WFPS) and soil temperature was very low
Summary
Tropical forests are assumed to be an important source of atmospheric nitrous oxide (N2O) and sink for methane (CH4). We quantified seasonal soil–atmosphere fluxes of N2O and CH4 from the three major tropical forest types (montane, lowland, swamp) within the Congo Basin at a weekly to fortnightly resolution using the static chamber method[24]. Initial short-term campaigns with daily measurements were conducted in 2016 and 2017 at three lowland locations (MaringaLopori-Wamba Landscape, Yangambi Biosphere Reserve, Yoko Forest Reserve) and one montane location (Kahuzi-Biéga National Park) to constrain spatiotemporal variation (Fig. S1). As aquatic ecosystems are increasingly considered as potential emission hotspots within the terrestrial landscape, we report dissolved N2O and CH4 concentrations from headwater streams draining the same catchments in which soil flux core sites were located. Fluxes previously measured from the Congo River main stem[26] likely underestimate total aquatic losses by a significant margin, since relatively high fluxes have been observed to take place higher in the watershed[27]
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