Soil geochemistry – and not topography – as a major driver of carbon allocation, stocks, and dynamics in forests and soils of African tropical montane ecosystems

Abstract

Summary The lack of field‐based data in the tropics limits our mechanistic understanding of the drivers of net primary productivity (NPP) and allocation. Specifically, the role of local edaphic factors – such as soil parent material and topography controlling soil fertility as well as water and nutrient fluxes – remains unclear and introduces substantial uncertainty in understanding net ecosystem productivity and carbon (C) stocks. Using a combination of vegetation growth monitoring and soil geochemical properties, we 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 components across geochemical (soil) regions. Topography did not constrain the variability in C allocation and NPP. Soil organic C 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. We conclude that, even after many millennia of weathering and the presence of deeply developed soils, above‐ and belowground C allocation in tropical forests, as well as soil C stocks, vary substantially due to the geochemical properties that soils inherit from parent material.

Soil carbon stocks in stable tropical landforms are dominated by geochemical controls and not by land use

“…In our study across distinct geochemical regions and focusing on deeply weathered and developed (non‐eroded) soils, no effect of land conversion from forest to cropland on SOC stocks and soil Δ 14 C could be detected, neither in topsoil nor in subsoil layers (Figure 3). To explain this observation, we argue that our findings point toward a limitation of C storage in deeply weathered soils that is independent of C inputs, which differ significantly between tropical forests and cropland (Bukombe et al, 2022; Kaiser et al, 2016). Rather, soil C storage seems to depend more strongly on a soil's ability to stabilize C inputs with the mineral matrix.…”

“…First, despite differences in C input and rooting patterns across geochemical regions in our study (Bukombe et al, 2022;Doetterl, Asifiwe, et al, 2021), SOC depth curves are similar between forest and cropland soils (Figure 3a, Figure S2). Importantly, the identified relationships between predictor variables and SOC content or soil Δ 14 C do not change with soil depth or for separate depth increments (Table S2).…”

“…The region is characterized by tropical humid climate (Köppen Af‐Am) with monsoonal dynamics. The regional climate is subdivided into four seasons (weak dry in December–February; strong rains in March–May; strong dry in June–August; and weak rains in September–November) each covering three months (Bukombe et al, 2022; Doetterl, Asifiwe, et al, 2021). The mean annual temperature (MAT) varies between 15.3 and 19.2°C and mean annual precipitation (MAP) varies between 1697 and 1924 mm (Fick & Hijmans, 2017).…”

“…A specific feature of soils in this region is the presence of fossil, geogenic organic carbon free of radiocarbon in the parent material (dark clay-silt schists). For further details on the soil mineralogy of the study region, please refer to Bukombe et al (2021Bukombe et al ( , 2022, Doetterl, Asifiwe, et al (2021) and Reichenbach et al (2021).…”

“…The dominant natural vegetation in the area is tropical montane forest (Bukombe et al, 2022;Doetterl, Asifiwe, et al, 2021).…”