Continental-scale controls on soil organic carbon across sub-Saharan Africa

Abstract

Abstract. Earlier studies have demonstrated that soil texture and geochemistry strongly affect soil organic carbon (SOC) content. However, those findings primarily rely on data from temperate regions with soil mineralogy, weathering status and climatic conditions that generally differ from tropical and sub-tropical regions. We investigated soil properties and climate variables influencing SOC concentrations across sub-Saharan Africa. A total of 1,601 samples were analyzed, collected from two depths (0–20 cm and 20–50 cm) at 45 sentinel sites from 17 countries as part of the Africa Soil Information Service (AfSIS) project. The dataset spans climatic conditions from arid to humid and includes soils with a wide range of pHH20 values, weathering status, soil texture, exchangeable cations, extractable metals and a variety of important land cover types. The most important SOC predictors were identified by linear mixed effects models, regression trees and random forest models. Our results indicate that SOC is primarily controlled by aridity index (PET/MAP), exchangeable calcium (Caex) and oxalate-extractable aluminum (Alox); this was found across both depth intervals. Oxalate-extractable iron (Feox) emerged as the most important predictor for both depth intervals in the regression tree and random forest analyses. However, its influence on SOC concentrations was strong only below Feox concentrations of 0.25 wt %. This suggests that Feox can act as a pedogenic threshold – even on a continental scale. Across model-ling approaches, clay and fine silt content (

Similar importance of edaphic and climatic factors for controlling soil organic carbon stocks of the world

“…5). This result is in line with findings from a continental-scale study across sub-Saharan Africa where climate and geochemistry are more important predictors of SOC content than aboveground carbon inputs (von Fromm et al, 2020). The importance of NPP may largely depend on how much NPP ends up in the soil and how it is translocated to different depths (Wang et al, 2021).…”

“…More importantly, soil carbon can be physically protected from decomposition via occlusion with soil aggregates and binding with minerals (Lehmann and Kleber, 2015;Dungait et al, 2012;Schmidt et al, 2011), while the protection capacity is largely determined by soil physicochemical properties (Six et al, 2000). These physical protection processes may lead to soil-dependent stabilization/destabilization of different soil carbon substrates (Waldrop and Firestone, 2004;Keiluweit et al, 2015;Six et al, 2002). However, it should be noted that complex interplays of various soil properties are involved in SOC stabilization and destabilization processes.…”

Rock weathering controls the potential for soil carbon storage at a continental scale

“…Local studies have shown that PCMs are correlated with SOC stocks across soils of different ages, climates, and parent materials (Dahlgren et al 1997;Torn et al 1997;Chadwick et al 2003;Masiello et al 2004;Heckman et al 2009;Rasmussen et al 2018b). More expansive, continental scale data syntheses indicate that PCMs play a major role in SOC storage in humid climates (Rasmussen et al 2018a;Kramer and Chadwick 2018;von Fromm et al 2020). These studies relate PCMs to SOC; however, they have not specifically evaluated the role of primary mineral weathering in controlling PCM abundance.…”

New uses for old tools: Reviving Holdridge Life Zones in soil carbon persistence research

“…Similarly, von Fromm et al. (2020) pointed to the ratio of the annual PET to the MAP as the best predictor for SOC distribution. The mean age of SOC also relates to global climate ( Shi et al., 2020).…”

“…These relations decisively increase our understanding of general patterns contributing to stabilizing C in the soil at scales relevant for C management. By definition, the HLZ classification cannot go beyond climate control, but the latest literature ( Kramer and Chadwick , 2018; Rasmussen et al., 2018; Hein et al., 2020; von Fromm et al., 2020) indicates that multiple controls on SOC persistence vary systematically with climate. HLZ is a good fit to separate SOC stocks into clearly distinguishable and process‐oriented zones, varying in their mechanisms to mediate SOC persistence.…”