Trace gas fluxes from soils and tree stems of rainforests and cacao agroforests in the Congo Basin, Cameroon
von Najeeb Al-Amin Iddris
Datum der mündl. Prüfung:2020-05-26
Betreuer:Prof. Dr. Edzo Veldkamp
Gutachter:Prof. Dr. Edzo Veldkamp
Gutachter:Prof. Dr. Alexander Knohl
EnglischTropical rainforests play a crucial role in biogeochemical cycles and global climate dynamics. Yet, research efforts to quantify the main sources and sinks of trace greenhouse gases lags behind that of other biomes. The African continent is among the least researched regions worldwide, and the effects of land-use change on trace greenhouse gases are identified as an important research gap in the greenhouse gas budget of Africa. Recent studies in wetland and temperate forests have provided evidence for tree stem nitrous oxide (N2O) and methane (CH4) emissions, but the magnitudes of tree contributions to total (soil + stem) N2O and CH4 emissions from tropical rainforests on heavily weathered soils remain unknown. Given these knowledge gaps, this thesis consists of two studies aimed at quantifying the changes in stem and soil N2O and CH4 fluxes, and soil carbon dioxide (CO2) fluxes with forest conversion to cacao agroforestry. The study was conducted at three sites (villages) in central and southern Cameroon, all located on heavily weathered soils. To assess the impact of land-use change on stem and soil greenhouse gas fluxes, we studied two land-use systems at each site: the reference forest and the converted cacao agroforestry system. At each site, we selected four replicate plots (2500 m2 each) for each land use. Soil and stem greenhouse gas fluxes were measured monthly using vented static chambers (4 chambers per plot) and stem chambers (6 trees per plot), respectively, from April 2017 to April 2018. On each measurement period, we also measured known soil and climatic controlling factors. The aim of the first study was to quantify the changes in stem and soil N2O fluxes with forest conversion to cacao agroforestry. Additionally, we conducted a 15N tracing experiment at one of the sites as a follow-on study to elucidate the source of stem N2O emissions. Our findings revealed that trees on well-drained, heavily weathered soils served as an important N2O emission pathway, with the potential to overlook up to 38% of fluxes in the forests, and up to 15% of fluxes in cacao agroforests, if tree stems are not considered in the ecosystem N2O budget. 15N-isotope tracing from soil mineral N to stem-emitted 15N2O suggest that emitted N2O from stems originated predominantly from N2O produced in the soil. Additionally, forest conversion to cacao agroforestry systems had no effect on stem and soil N2O emissions, because of similarities in soil moisture and soil texture, absence of fertilizer application, and comparable presence of leguminous trees in both land uses, which can compensate for N export from harvest or other losses. For our second study, we investigated the changes in stem and soil CH4 fluxes and soil CO2 fluxes with forest conversion to cacao agroforestry. Conversion of forest to cacao agroforestry had no effect on stem and soil CH4 and CO2 fluxes. The lack of differences may be due to the comparable soil texture and soil moisture content between the two land uses, which influences gas diffusivity into and out of the soil. All the studied trees emitted measurable CH4 at some point during the study period. In both land uses, tree stems were net sources of CH4, while the soils were net CH4 sinks. Our upscaling suggests that tree stem emissions offset 3–18% of the annual soil CH4 sink in both land uses. This study provides the first year-round and spatially replicated quantifications of stem and soil trace gas fluxes for the Congo Basin, with key implications for improved estimates of trace gas budgets for Africa. Our results show for the first time that, N2O and CH4 emissions from tree stems on well-drained soils are apparently widespread and detectable in many tropical trees in Africa. As discussed in the synthesis chapter, even low stem trace gas emissions at the ecosystem level can upscale to significant fluxes globally. These findings emphasize the need for additional studies on tree stem fluxes in order to constrain their magnitudes and mechanisms, and to refine global greenhouse gas budgets.
Keywords: Africa; Cacao agroforest; Congo Basin; Ferralsol; Land-use change; nitrous oxide; methane; carbon dioxide; Oxisol; Trace greenhouse gases; Stem N2O emissions; Stem CH4 emissions; tropical rainforest; Soil N2O emissions; Soil CH4 fluxes; Soil CO2 emissions