Tropical forests play a central role in global carbon (C) cycles due to the high exchange rate of carbon between plants, soil, and the atmosphere. Nutrient availability in tropical forest systems controls these exchanges via their impact on tree growth, carbon productivity, and stocks. Research shows that local edaphic factors such as soil parent material and topography codetermine nutrient availability. However, the process knowledge of how tropical forests respond to changes in nutrients, the chemistry of the local parent material and topography, and the effect this has on C cycling between plants, soils, and the atmosphere remains unclear. This gap in knowledge obstructs the mechanistic understanding of the controls of C cycling in tropical forest systems. Furthermore, data for African tropical forests are scarce, as most research has focused mainly on Amazon and South Asia. This thesis tried to answer these questions and provided directions on where future research can focus.
This thesis is based on both experimental (field and laboratory) and observational studies at different sites in the Eastern Congo Basin and along the Albertine Rift Valley System. It has three major parts: (a) nutrient uptake and distribution in the canopy of African tropical forests, (b) C stocks, Net Primary Productivity (NPP), and NPP C allocation between plant compartments, and (c) soil potential heterotrophic respiration (SPR) and soil organic carbon (SOC) turnover rate in forests developed along geochemical and topographic gradients. Specifically, the thesis focused on three contrasting geochemical regions (mafic magmatic, felsic metamorphic, and a mixture of sedimentary rock but distinct from mafic and felsic. Throughout the thesis, the three regions are referred to as “mafic”, “felsic”, and “sedimentary).
Chapter 2 assessed canopy chemistry of 344 samples collected from different tree species growing on different parent materials and topographic positions. The data shows that tropical forest canopy chemistry shifts significantly when local soils and parent material geochemistry indicate fertility constraints, mainly due to low amounts of rock-derived nutrients. In contrast, topography did not affect canopy chemistry in the three investigated geochemical regions.
Chapter 3 assessed the effects that soil parent material and topography as drivers of soil fertility have on forest NPP, C allocation, and biomass C stocks and how they relate to SOC stocks. Here a combination of two years monitoring of vegetation growth and soil geochemical properties measurements were used. The thesis found that soil fertility parameters reflecting the local parent material are the main drivers of NPP and C allocation patterns in tropical montane forests, resulting in significant differences in below to aboveground biomass ratio across geochemical regions. Topography did not constrain the variability in C allocation and NPP. Furthermore, SOC stocks showed no relation to C input in tropical forests. Instead, plant C input seemingly exceeded the maximum potential of these soils to stabilize C.
Chapter 4 assessed potential heterotrophic soil respiration and SOC turnover via lab-based incubation experiments. Here, depth explicit SPR and Δ14C of samples originating from the three geochemical regions and topographic positions were measured under constant temperature and moisture conditions. The results revealed distinct patterns in soil respiration with soil depth and parent material geochemistry. The topographic origin of the samples was not the main determinant of the observed respiration rates and Δ14C. However, in situ soil hydrological conditions likely influence soil C turnover by inhibiting decomposition in valley subsoils.
Overall, the results of this thesis demonstrate that, even in deeply weathered tropical soils, parent material has a long-lasting effect on soil geochemistry that can affect (1) nutrient availability, and uptake, (2) NPP, and C allocation, ultimately affecting differently above and belowground biomass, (3) microbial activity, the size of subsoil C stocks and the turnover rate of C in soil. Therefore, soil parent material and its control on soil chemistry need to be taken into account to predict C fluxes and to understand C cycling in African old-growth tropical forest systems
