Nitrous oxide emissions from soils and tree stems of rainforests and cacao agroforests in the Congo basin, Cameroon

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The humid tropical forest biome plays a dominant role in the global budget of the greenhouse gas nitrous oxide (N2O). However, the estimated source strength of N2O emissions from tropical rainforest soils using a bottom-up approach (extrapolation of measured/modeled soil-atmosphere flux data) has decreased in the past decades. This appears to be in contrast with top-down studies that used global N2O atmospheric observations and inversion methods to estimate N2O emissions, which continue to point at the tropics as an important N2O source. This apparent mismatch of the bottom-up and top-down estimates of N2O emissions may be explained in two ways. 1) Tropical trees on well drained soils may serve as a conduit for N2O produced in the soil, a potential emission pathway that has been largely ignored. Recent studies have shown that not only trees with aerenchyma (serving as channel for soil-atmosphere gas exchange) but also trees without this feature showed evidence of N2O emission. In a preliminary study we were able to show that the ability to emit N2O from tree stems is widespread in tropical trees in Cameroon. 2) Geographic bias of the existing field measurements may have reduced estimates of the N2O source strength of the humid tropical forests biome. Even though the majority of remaining tropical rainforests in the Amazon and Congo basins are located on relatively heavily weathered nitrogen-rich soils, most trace gas studies were conducted on less weathered soils which tend to have lower N availability and consequently emit less N2O. Especially measurements from the Congo basin in Africa are notably absent. In the preliminary study we showed that the forest sites where we plan to conduct our study have a high N availability which strongly hints that these sites will have high gaseous nitrogen losses. Using a combination of chamber methods, gas chromatography, 15N natural abundance signatures of N2O and 15N isotope tracing techniques, I propose to measure N2O fluxes from heavily weathered forest and agroforestry soils and from tree stems in the Congo Basin of Cameroon. The methods for soil N2O flux measurements and 15N tracing to N2O have been tested in tropical forest sites by my group. The method for measuring N2O emissions from tree stems we tested at two sites in Cameroon. This proposed study will provide the first ever robust measurements of soil N2O fluxes in a never-before measured important biome in Africa (rainforest in the Congo Basin), and will also provide the first evidence of whether or not tropical trees on well-drained soils serve as important conduits of N2O from the soil.