Nitrogen cycling (deposition, leaching and N trace gas) in lowland tropical forests of the Congo Basin

Congo Basin forests are exposed to high nitrogen inputs by atmospheric deposition which so far could not be matched by measured nitrogen losses and plant uptake (Bauters et al., 2019, Makelele et al., 2022, Barthel et al., 2022). High nitrogen deposition in the Congo Basin mainly originates from biomass burning as practiced during shifting cultivation (Bauters et al., 2018). Overall, this N budget imbalance suggests that other gaseous N-losses, such as N₂ fluxes, may play a major role in the N-cycle of Afrotropical forests (Barthel et al., 2022) although not yet quantified as such in the Congo Basin. Thus, the main research question we focus upon here: Can the high fire-derived nitrogen deposition be balanced by N₂ emissions in the Congo Basin?  To answer this key question, we first used permanently installed throughfall and lysimeter networks, sampled weekly, in Gilbertiodendron dewevrei forests to analyse nitrogen deposition and leaching (ammonium, nitrate, nitrite, total dissolved nitrogen (TDN), dissolved inorganic nitrogen (DIN) and dissolved organic nitrogen (DON)) over a full year. Secondly, we measured weekly soil greenhouse gas fluxes (CO₂, CH₄ and N₂O) in the same forest over a full year based on manual static chamber measurements. Thirdly, we used the He/O₂ gas-flow-soil-core method to measure for the first time N₂ and N₂O fluxes and calculated the denitrification product ratio (N₂O/(N₂O+N₂)). Our results confirmed, in line with previous studies, high atmospheric N deposition in the Gilbertiodendron dewevrei forest with substantially low in-situ soil nitrous oxide fluxes. Furthermore, Gilbertiodendron dewevrei forest, with ectomycorrhizal symbiosis, showed highest reduction rates of N₂O to N₂ (complete denitrification). Therefore, the N budget imbalance in the Congo basin, and especially the Gilbertiodendron dewevrei forests of the Congo Basin, can be explained by high N₂ emissions from denitrification processes.