Low N2O and variable CH4 fluxes from tropical forest soils of the Congo Basin

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.

Inland Waters Increasingly Produce and Emit Nitrous Oxide

“…our simulated N 2 O concentrations (0.36−1.1 μg N 2 O-N/L) in the Congo River basin also agree with Congo measurements (0.2−1.1 μg N 2 O-N/L),116 with reported undersaturation areas of N 2 O in Congo 117 reproduced in our simulated N 2 O spatial pattern (Figure4). This fair agreement between model predictions and independent observations for N 2 O, TN, DO, and hydrological flows in global inland waters gives confidence to the overall methodologically consistent approach.3.2.…”

Global distribution and climate sensitivity of the tropical montane forest nitrogen cycle

“…This value is consistent with that found for soil enzyme activities ( V max 1.5-2) measured across tropical elevational gradients 3 , 42 and results from litter decomposition 30 and soil nitrogen mineralization studies 43 across tropical forests. The global convergence of temperature sensitivities for soil N content and δ 15 N suggests a strong kinetic underpinning of the inverse relationship between soil N accumulation and fractionating loss pathways (i.e., gaseous versus hydrological), consistent with decreasing microbial denitrification with increasing tropical elevation 17 , 18 . However, such space-for-time analyses cannot determine microbial temperature sensitivity at a given elevation/ambient temperature 14 , nor account for potential acclimation to past or future temperatures 42 .…”

“…The elevational distributions of these fluxes (e.g., biological N fixation, gaseous and hydrologic losses) are poorly quantified for tropical montane forests globally. For example, while soil N 2 O emissions have been shown to decline with increasing elevation in some montane forests 17 , 18 , consistent with lower N availability, observations of large hydrologic losses of N from other montane forests 19 , 20 suggest high N availability at the ecosystem level. Furthermore, the size and geographic distribution of tropical montane forest soil N pools are poorly constrained.…”

Conservative N cycling despite high atmospheric deposition in early successional African tropical lowland forests

“…The N 2 O emissions in the 60 years-old secondary forest was double the amount emitted in the lowland OG forests (Fig. 4, see also Gallarotti et al 2021;Barthel et al 2022) and above the global tropical forest N 2 O emissions mean (Werner et al 2007), but it was similar to the emissions in a secondary forest in Cameroun (Verchot et al 2020) and OG forests in Ghana (Castaldi et al 2013) and Congo (Serca et al 1994). The N 2 O emission peak in the 60 years-old secondary forest additionally indicates that the N cycle is recovering to its status in a mature forest, following shifting cultivation (Winbourne et al 2018).…”