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.
Highlights
Tropical forests and the soils therein are one of the most important and largest global terrestrial carbon (C) pools and serve as important climate regulators (Cleveland et al, 2011; Kearsley et al, 2013; Lewis et al, 2009; Sayer et al, 2011)
Geology can control C dynamics as soils developed from felsic parent material provide less potential for C stabilization and a lower capacity to release rock-derived nutrients than soils developed from mafic parent material, limiting organic matter input
Some soils developed from sedimentary parent material can contain a large fraction of fossil organic carbon of generally poorer quality than fresh organic matter inputs, which can be resistant to decomposition under in situ environmental conditions (Kalks et al, 2021)
Summary
Tropical forests and the soils therein are one of the most important and largest global terrestrial carbon (C) pools and serve as important climate regulators (Cleveland et al, 2011; Kearsley et al, 2013; Lewis et al, 2009; Sayer et al, 2011). They contain about one-third (421 Pg C) of the global soil organic carbon (SOC) stock in the upper 1 m of soil (Köchy et al, 2015) and are characterized by high annual C turnover rates (Raich and Schlesinger, 1992). In order to explain SOC and its exchange between soil and the atmosphere, the interactions of geochemical, geomorphic and climatic drivers are central (Angst et al, 2018; Berhe et al, 2012; Doetterl et al, 2015b; von Fromm et al, 2021; Kramer and Chadwick, 2018; Luo et al, 2017)
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