The world's largest tropical peatland complex is found in the central Congo Basin. However, there is a lack of in situ measurements to understand the peatland's distribution and the amount of carbon stored in it. So far, peat in this region has been sampled only in largely rain-fed interfluvial basins in the north of the Republic of the Congo. Here we present the first extensive field surveys of peat in the Democratic Republic of the Congo, which covers two-thirds of the estimated peatland area, including from previously undocumented river-influenced settings. We use field data from both countries to compute the first spatial models of peat thickness (mean 1.7 ± 0.9 m; maximum 5.6 m) and peat carbon density (mean 1,712 ± 634 MgC ha −1 ; maximum 3,970 MgC ha −1 ) for the central Congo Basin. We show that the peatland complex covers 167,600 km 2 , 36% of the world's tropical peatland area, and that 29.0 PgC is stored below ground in peat across the region (95% confidence interval, 26.3-32.2 PgC). Our measurement-based constraints give high confidence of globally significant peat carbon stocks in the central Congo Basin, totalling approximately 28% of the world's tropical peat carbon. Only 8% of this peat carbon lies within nationally protected areas, suggesting its vulnerability to future land-use change.

The forested swamps of the central Congo Basin store approximately 30 billion metric tonnes of carbon in peat1,2. Little is known about the vulnerability of these carbon stocks. Here we investigate this vulnerability using peat cores from a large interfluvial basin in the Republic of the Congo and palaeoenvironmental methods. We find that peat accumulation began at least at 17,500 calibrated years before present (cal. yr bp; taken as ad 1950). Our data show that the peat that accumulated between around 7,500 to around 2,000 cal. yr bp is much more decomposed compared with older and younger peat. Hydrogen isotopes of plant waxes indicate a drying trend, starting at approximately 5,000 cal. yr bp and culminating at approximately 2,000 cal. yr bp, coeval with a decline in dominant swamp forest taxa. The data imply that the drying climate probably resulted in a regional drop in the water table, which triggered peat decomposition, including the loss of peat carbon accumulated prior to the onset of the drier conditions. After approximately 2,000 cal. yr bp, our data show that the drying trend ceased, hydrologic conditions stabilized and peat accumulation resumed. This reversible accumulation–loss–accumulation pattern is consistent with other peat cores across the region, indicating that the carbon stocks of the central Congo peatlands may lie close to a climatically driven drought threshold. Further research should quantify the combination of peatland threshold behaviour and droughts driven by anthropogenic carbon emissions that may trigger this positive carbon cycle feedback in the Earth system.

<p>The Cuvette Centrale wetland area in the central Congo Basin is home to the most extensive peatland complex in the tropics, which covers an estimated 167,600 km<sup>2</sup> and stores approximately 29.0 petagrams of carbon belowground (Crezee et al., 2022). However, relatively little is known about these peat swamp forests, which span both the Republic of the Congo (ROC) and the Democratic Republic of the Congo (DRC). In ROC, peatlands are mostly found in interfluvial basins, where they form wide but shallow peat domes (Davenport et al., 2020), which are largely rain-fed, have ombrotrophic (low-nutrient) status and are covered by distinct hardwood- or palm-dominated forest vegetation types (Dargie et al., 2017). In contrast, peatlands in DRC can be found in different hydro-geomorphological settings, including relatively narrow river valleys along the Congo River’s eastern tributaries.</p> <p>Here, we present insights from <em>in situ</em> measurements of water table depths, peat and water chemistry, and vegetation characteristics along three of these tributaries, the Ruki, Busira and Ikelemba Rivers. Peat deposits (≥ 30 cm of ≥ 65% organic matter) were found in seasonally inundated swamp forests close to rivers, which are characterised by large (>1 m) wet season inundations. This shows that peat formation in the central Congo Basin is not only confined to permanently waterlogged swamps with relatively stable water tables year-round, predominating in ROC’s interfluvial basins, but also occurs in swamp forests that experience greater water table seasonality. Seasonal inundations are due to riverbank overflow during the wet season, and potentially upland runoff from higher <em>terra firme</em> ground. Since the Congo River’s eastern tributaries are highly acidic (blackwater) rivers, the seasonally inundated, river-fed swamps are still of low nutrient status, allowing peat to form. Nonetheless, they are typically characterised by shallower peat deposits than permanently waterlogged swamp forests. Comparison of vegetation plot data identifies a distinct forest type in the seasonally inundated, river-fed peat swamps: a hardwood swamp forest dominated by <em>Oubanguia africana</em> and <em>Guibourtia demeusei</em>. This vegetation type was previously described by Evrard (1968), but was not known to overlie peat soils.</p> <p>Our findings identify distinct rain-fed and river-fed peatlands in the central Congo Basin. These two peatland types may face different threats to their integrity. While rain-fed peatlands will be susceptible to future climate change if rainfall levels decrease, those that are also maintained by riverbank overflow, may be susceptible to changes in upstream rainfall patterns. This, as well as infrastructure projects that affect river dynamics, will likely have larger impacts on peatland functioning than previously thought.</p>

The global recognition of modern agricultural practices’ impact on the environment has fuelled policy responses to ameliorate environmental degradation in agricultural landscapes. In the US and the EU, agri-environmental subsidies (AES) promote widespread adoption of sustainable practices by compensating farmers who voluntarily implement them on working farmland. Previous studies, however, have suggested limitations of their spatial targeting, with funds not allocated towards areas of the greatest environmental need. We analysed AES in the US and EU—specifically through the Environmental Quality Incentives Program (EQIP) and selected measures of the European Agricultural Fund for Rural Development (EAFRD)—to identify if AES are going where they are most needed to achieve environmental goals, using a set of environmental need indicators, socio-economic variables moderating allocation patterns, and contextual variables describing agricultural systems. Using linear mixed models and linear models we explored the associations among AES allocation and these predictors at different scales. We found that higher AES spending was associated with areas of low soil organic carbon and high greenhouse gas emissions both in the US and EU, and nitrogen surplus in the EU. More so than successes, however, clear mismatches of funding and environmental need emerged—AES allocation did not successfully target areas of highest water stress, biodiversity loss, soil erosion, and nutrient runoff. Socio-economic and agricultural context variables may explain some of these mismatches; we show that AES were allocated to areas with higher proportions of female producers in the EU but not in the US, where funds were directed towards areas with less tenant farmers. Moreover, we suggest that the potential for AES to remediate environmental issues may be curtailed by limited participation in intensive agricultural landscapes. These findings can help inform refinements to EQIP and EAFRD allocation mechanisms and identify opportunities for improving future targeting of AES spending.

The Cuvette Centrale is the largest tropical peatland complex in the world, covering approximately 145,000 km2 across the Republic of Congo and the Democratic Republic of Congo. It stores ca. 30.6 Pg C, the equivalent of three years of global carbon dioxide emissions and is now the first trans-national Ramsar site. Despite its size and importance as a global carbon store, relatively little is known about key aspects of its ecology and history, including its formation, the scale of greenhouse gas flows, its biodiversity and its history of human activity. Here, we synthesise available knowledge on the Cuvette Centrale, identifying key areas for further research. Finally, we review the potential of mathematical models to assess future trajectories for the peatlands in terms of the potential impacts of resource extraction or climate change.