Congo Basin Carbon Cycle Responses to Global Change.

The Congo Basin and its contiguous forests harbor globally significant carbon stocks, estimated at 65 gigatons of C (GtC) above and belowground. Despite rising temperatures and intensifying droughts, they have remained a carbon sink, albeit weak: 0.26-0.50 GtC yr.-1 carbon uptake since 1980. However, these forests' carbon stocks and fluxes, including gross primary productivity, respiration, net primary productivity, and riverine carbon transport, remain poorly quantified. This limits understanding of the region's role in the global carbon cycle, its vulnerability to environmental change, and its potential as a long-term carbon sink. We review and quantify Congo Basin and contiguous forest carbon stocks and fluxes and synthesize the current knowledge on how key global change drivers shape the region's carbon cycle. We find limited responses to long-term precipitation variability, but declining stocks and fluxes in response to long-term and increasing temperature and drought frequency. Land cover and land use changes, largely driven by small-scale agriculture, logging, and agro-industrial expansion, reduce carbon stocks, ecosystem structure and functioning, and animal-mediated ecosystem services. In contrast, large-scale savanna biomass burning delivers phosphorus and nitrogen to Congo Basin forests via cross-equatorial winds, providing additional nutrients and supporting carbon sequestration. In situ studies suggest that CO2 fertilization has increased intrinsic water-use efficiency, although its effects may be modulated by climate change, and its impacts on biomass accumulation remain uncertain. Legacy effects from historical land use and climate change likely shaped present-day vegetation structure, yet their relative influence is unclear. High-resolution carbon monitoring, improved remote sensing, and strengthened in situ measurement networks are needed to quantify the impacts of these key drivers and their interactions on the Central African carbon cycle. This is needed to inform conservation strategies and advance understanding of the region's future as a carbon sink under global change pressures.