Heterotrophic soil respiration and carbon cycling in geochemically distinct African tropical forest soils

Abstract

Abstract. Heterotrophic soil respiration is an important component of the global terrestrial carbon (C) cycle, driven by environmental factors acting from local to continental scales. For tropical Africa, these factors and their interactions remain largely unknown. Here, using samples collected along topographic and geochemical gradients in the East African Rift Valley, we study how soil chemistry and fertility drive soil respiration of soils developed from different parent materials even after many millennia of weathering. To address the drivers of soil respiration, we incubated soils from three regions with contrasting geochemistry (mafic, felsic and mixed sediment) sampled along slope gradients. For three soil depths, we measured the potential maximum heterotrophic respiration under stable environmental conditions and the radiocarbon content (Δ14C) of the bulk soil and respired CO2. Our study shows that soil fertility conditions are the main determinant of C stability in tropical forest soils. We found that soil microorganisms were able to mineralize soil C from a variety of sources and with variable C quality under laboratory conditions representative of tropical topsoil. However, in the presence of organic carbon sources of poor quality or the presence of strong mineral-related C stabilization, microorganisms tend to discriminate against these energy sources in favour of more accessible forms of soil organic matter, resulting in a slower rate of C cycling. Furthermore, despite similarities in climate and vegetation, soil respiration showed distinct patterns with soil depth and parent material geochemistry. The topographic origin of our samples was not a main determinant of the observed respiration rates and Δ14C. In situ, however, soil hydrological conditions likely influence soil C stability by inhibiting decomposition in valley subsoils. Our results demonstrate that, even in deeply weathered tropical soils, parent material has a long-lasting effect on soil chemistry that can influence and control microbial activity, the size of subsoil C stocks and the turnover of C in soil. Soil parent material and its control on soil chemistry need to be taken into account to understand and predict C stabilization and rates of C cycling in tropical forest soils.

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

“…Similarly, our data on the chemical composition (C : N ratios) of living canopy leaves (Fig. S5) and the findings of Bukombe et al (2021) provide further support for the notion that nutrient-limited systems tend to develop plant traits that are typically signs of resource conservation strategies (Grau et al, 2017;Urbina et al, 2021) while accumulating comparably thick litter layers (Fig. 2a,b) and thick O-horizons (Fig.…”

“…Soil C stocks in tropical African forests have been shown to be determined predominantly by the potential of soils to stabilize C by these organo‐mineral associations (Kirsten et al ., 2021; Reichenbach et al ., 2021). Recent studies from tropical African montane forests, for example, have demonstrated that geochemical soil properties related to the local parent material explain up to 75% of variability in SOC stocks (Reichenbach et al ., 2021) and were significant in explaining soil C turnover under stable, warm‐humid atmospheric conditions (Bukombe et al ., 2021). Thus, drivers of NPP and the associated C fluxes in tropical forest systems remain unclear, as they crucially rely on the complex interplay of soil formation and nutrient availability, topography, climate, and biology (Yoo & Mudd, 2008).…”

Microbial properties in tropical montane forest soils developed from contrasting parent material—An incubation experiment

“…Using the same soils as used in this study, Bukombe et al. (2021) showed that soil fertility constraints (e.g., pH KCl , base saturation, exchangeable acidity) are important drivers of C decomposition. In our study, regression analysis showed that soil acidity and exchangeable cations were the most important controls on microbial C acquisition as indicated by vector length (Table 2) although explained variance was relatively small ( R 2 = 0.12, RMSE = 1.32).…”

“…Soils were incubated in the dark at 20°C, which is a common temperature for incubation studies and for studies dealing with enzyme activity over larger environmental gradients (e.g., Craine et al., 2010; Schädel et al., 2020). Thereafter, all jars were closed periodically after every 2 (for topsoils) and 7 days (for subsoils) to allow for CO 2 accumulation over a period of 120 days (Bukombe et al., 2021). The end of the 4‐day pre‐incubation period is referred to in this study as ‘pre‐incubation’, while the end of the 120‐day incubation (without counting the 4 days of pre‐incubation) is referred to as ‘post‐incubation’.…”

“…to allow for CO 2 accumulation over a period of 120 days (Bukombe et al, 2021). The end of the 4-day pre-incubation period is referred to in this study as 'pre-incubation' , while the end of the 120-day incubation (without counting the 4 days of pre-incubation) is referred to as 'post-incubation' .…”