Stabilization of soil organic carbon (SOC) against microbial decomposition depends on several soil properties, including the soil weathering stage and the mineralogy of parent material. As such, tropical SOC stabilization mechanisms likely differ from those in temperate soils due to
contrasting soil development. To better understand these mechanisms, we investigated SOC dynamics at three soil depths under pristine tropical
African mountain forest along a geochemical gradient from mafic to felsic and a topographic gradient covering plateau, slope and valley
positions. To do so, we conducted a series of soil C fractionation experiments in combination with an analysis of the geochemical composition of soil
and a sequential extraction of pedogenic oxides. Relationships between our target and predicting variables were investigated using a combination of
regression analyses and dimension reduction. Here, we show that reactive secondary mineral phases drive SOC properties and stabilization mechanisms
together with, and sometimes more strongly than, other mechanisms such as aggregation or C stabilization by clay content. Key mineral stabilization
mechanisms for SOC were strongly related to soil geochemistry, differing across the study regions. These findings were independent of topography in
the absence of detectable erosion processes. Instead, fluvial dynamics and changes in soil moisture conditions had a secondary control on SOC
dynamics in valley positions, leading to higher SOC stocks there than at the non-valley positions. At several sites, we also detected fossil organic
carbon (FOC), which is characterized by high
The tropics are considered potential tipping points for the climate–carbon (C) feedback due to their substantial C storage in the biosphere, fast C turnover and the associated potential C losses to the atmosphere. Despite this key relevance in the terrestrial C cycle and climate regulation, the tropics remain highly underrepresented in research (Schimel et al., 2015). This is especially true for tropical soils, which are estimated to contain approximately one-third of global soil organic carbon (SOC) (Köchy et al., 2015). Many interacting soil processes, both in temperate and tropical soils, are not adequately represented in C turnover models, such as the effect of soil aggregation on soil biota and SOC dynamics (van Keulen, 2001; Wood et al., 2012; Vereecken et al., 2016). Studies analyzing the effect of soil geochemistry on SOC dynamics and stabilization against microbial decomposition combined are also rare (Wattel-Koekkoek et al., 2003; Denef and Six, 2005; Zotarelli et al., 2005; Quesada et al., 2020), and such effects are not included in large-scale C cycle modeling approaches (Vereecken et al., 2016). Most of these geochemical effect studies focus on midlatitudes in the Northern Hemisphere, while the specific conditions under tropical conditions with highly weathered soils remain relatively unknown (Schimel et al., 2015) and can differ greatly compared to temperate soils (Denef and Six, 2006; Denef et al., 2007). Thus, findings from midlatitudes are not easily transferable to tropical soils, since the potential in stabilizing SOC depends on geochemical soil properties that differ fundamentally between geo-climatic zones as a function of pedogenesis. The lack of mechanistic understanding regarding SOC dynamics and their controlling factors creates substantial uncertainties when predicting the future of SOC stocks in the tropics (Schmidt et al., 2011; Shi et al., 2020).
SOC dynamics in tropical rainforests are characterized by high C input and fast C turnover rates (Pan et al., 2011; Wang et al., 2018). Carbon input to soils is mainly driven by root growth and litter production (Raich et al., 2006), both of which are often driven by climatic and hydrological variables that govern vegetation dynamics. Climatic factors such as temperature and precipitation are strong drivers of soil environmental conditions, which can greatly influence soil microbial activity and hence C mineralization and turnover (Davidson and Janssens, 2006; Zhang et al., 2011; Feng et al., 2017). For example, decomposition rates increase in general with temperature, but soil microbial communities adapted to high temperatures are less sensitive to warming (Blagodatskaya et al., 2016). However, climate-driven factors can also influence SOC dynamics indirectly through the interaction with soil factors (Doetterl et al., 2015b). For example, C-depleted tropical subsoils contain small but metabolically active microbial communities contributing to C cycling (Kidinda et al., 2020; Stone et al., 2014). Low soil pH in combination with high clay content dominated by pedogenic oxides can stabilize enzymes on mineral surfaces, which will affect microbial C acquisition (Dove et al., 2020; Allison and Vitousek, 2005; Liu et al., 2020). The accessibility of C for mineralization is predominantly driven by several interacting mechanisms that can stabilize C in soils against microbial decomposition on a decadal up to a millennial timescale (Trumbore 2000; Trumbore, 2009). For example, certain C compounds such as pyrogenic or aromatic C show biochemical resistance since the decomposition of its complex molecular structure is an energy-demanding process and microbes will preferentially consume more easily available organic C forms (Czimczik and Masiello, 2007; Knicker, 2011). Another C fraction that is characterized by long turnover times is fossil organic carbon (FOC), deposited during the formation of sedimentary rocks and often hard to decompose (van der Voort et al., 2019; Kalks et al., 2020). Carbon can also be protected physically against decomposition by encapsulation within soil aggregates. Minerals can also increase the energetic barrier for microorganisms to overcome by forming organo–mineral associations (Oades, 1984; Oades, 1988; von Lutzow et al., 2007; Lehmann et al., 2007; Cotrufo et al., 2013). In particular, it has been shown that the availability of reactive mineral surfaces influences the formation of organo–mineral associations as well (Eusterhues et al., 2003; Jagadamma et al., 2014; Angst et al., 2018). Furthermore, reactive and adsorptive surfaces not only contribute to chemical C stabilization, but also favor the formation of soil aggregates (Simpson et al., 2004; Six et al., 2004; Chenu and Plante, 2006; Lehmann et al., 2007).
While these general types of stabilization against microbial decomposition in the tropics are similar to those in temperate soils, their relative
importance and abundance differ greatly due to contrasting weathering history (Six et al., 2002; Denef et al., 2004). Most temperate soils have
developed from young (peri)glacial sediments and relatively unweathered bedrock (
In addition to larger-scale biogeochemical and climatic controls of C dynamics, in undulating landscapes soil redistribution processes can highly influence SOC dynamics (van Hemelryck et al., 2010; Doetterl et al., 2016; Wilken et al., 2017). Excessive erosion of topsoils on hillslopes often results in exposure of subsoils with low C contents, which on the one hand can lead to dynamic C replacement (Harden et al., 1999) and on the other hand may stimulate the decomposition of older SOC due to priming with fresh C inputs (Fontaine et al., 2004; Keiluweit et al., 2015). Thereby, removal of weathered topsoils brings new mineral surfaces in contact with fresh C input, which could favor C sequestration due to organo–mineral associations (Doetterl et al., 2016), especially in highly weathered tropical landscapes (Vitousek et al., 2003; Porder et al., 2005). These processes are of potentially great importance for tropical soil systems, as an erosional rejuvenation of land surfaces can bring an entirely different soil mineral composition in touch with the biological C cycle and provide a geochemically entirely different environment for C stabilization against microbial decomposition. Similarly, fossil organic carbon that is brought to the surface might become increasingly decomposed when brought in contact with more active microbial communities compared to subsoil environments. Parallel to these processes of soil denudation, at depositional sites in valleys and at footslopes, former topsoil SOC can become buried by colluvial and alluvial sediments, potentially greatly decreasing microbial decomposition. However, the fate of buried SOC depends greatly on the prevailing environmental conditions in the depositional sites and the sedimentation rates (Gregorich et al., 1998; Berhe et al., 2007; Berhe et al., 2012). Topography can control hydrological patterns in tropical rainforests (Silver et al., 1999; Detto et al., 2013). For example, high water tables lead to lower soil oxygen levels in valley positions, which in turn reduce microbial C decomposition and potentially result in the accumulation of labile SOC. Furthermore, changes in soil water content can cause reductive dissolution of iron oxides, which ultimately affects organo–mineral associations (Berhe et al., 2012). Thus, the interplay between environmental, geochemical and topographic conditions sets the stage for C stabilization and will most likely differ from temperate to tropical soils.
In summary, our current understanding of how geochemistry and topography in highly weathered tropical soils affects SOC stocks and stabilization
mechanisms against microbial decomposition is still limited. This study thus aimed to better understand the influence of topography and geochemical
properties of soils developed from different parent materials on (i) SOC stocks, (ii) SOC fractions and (iii) SOC stabilization mechanisms in tropical
forest soil systems. In addition, (iv) we assessed the contribution of FOC to SOC stocks in sedimentary-rock-derived soils using SOC stocks and geochemical soil properties sensitive to soil redistribution will vary as a function of a soil's topographic position. C stabilization mechanisms against microbial decomposition in highly weathered tropical soils, indicated by the amount of C associated with
minerals (stable microaggregates and free silt and clay), will be driven by geochemical soil properties as a function of parent material
composition. Fossil organic carbon content in C-bearing parent material will vary as a function of soil depth because it may become accessible for
microbial decomposition under surface conditions.
The study region is located in the eastern part of the Congo basin and the western part of the Blue Nile basin, with study sites located along the East
African Rift Mountain System. Vegetation at all sites is dominated by primary tropical mountain forests (Fig. 1b). Climate of the region is
characterized as humid tropical (Köppen Af–Am) with a short dry season (i.e., only 2 months per year with
The study region consisted of three main sites. The Kahuzi-Biega site (from here on called mafic site) is located in the South Kivu province of the
Democratic Republic of the Congo (DRC) (
The Nyungwe site (from here on called mixed sedimentary site) is situated in the southwestern part of Rwanda (
The Kibale site (further called felsic site) is located in western Uganda (0.46225
In the framework of the project TropSOC (Doetterl et al., 2021a, b), soil sampling took place from March to June 2018, applying a stratified random
sampling design with triplicate plots of 40
Each plot was subdivided into four 20
A wide range of soil physical and chemical parameters were analyzed in the framework of project TropSOC (Doetterl et al. 2021a, b), from which the
following were used in this study as potential covariates for controls on SOC: bulk density, total elements of base cations (Ca, Mg, K, Na), total
phosphorus, metal oxides with relevance to C stabilization against microbial decomposition (Al, Fe, Mn), elements where concentrations relate strongly
to weathering (Si, Ti, Zr) and additional soil properties that relate to soil fertility (texture, pH, effective and potential cation exchange
capacity, base saturation, bioavailable phosphorus). Generally, each analysis was performed with 20 % of the samples analyzed in triplicate to
assess analytical error. Prior to analyses, all samples were oven-dried at 30
Three soil size fractions representing different stabilization mechanisms against microbial decomposition associated with varying SOC turnover times
were isolated using a microaggregate isolator (Six et al. 2000a; Stewart et al., 2008; Doetterl et al., 2015b). These fractions consisted of
(i)
To assess the abundance of Al-, Fe- and Mn-bearing phases and their correlation with
The radioisotopic signature (
In a second step, the amount of FOC at the mixed sedimentary site was assessed as follows:
Based on results of the analyses of total element concentrations (see Doetterl et al., 2021b for details), the chemical index of alteration (CIA, in %)
(Fiantis et al., 2010) was calculated to assess the weathering stage of the soil as follows:
To illustrate gains and losses of nutrients in the soil column compared to the underlying parent material, the relative and absolute difference in
element concentration for key elements that enrich or deplete with weathering (
The significance level for all statistical analysis was set at
For dimension reduction of independent potential predictors of SOC, to illustrate the variance of these predictors across the dataset, and to minimize
multicollinearity in regression analyses, we performed a varimax-rotated principal component analysis (rPCA) (
Only RCs with an eigenvalue
The remaining RCs were used as explanatory variables in multiple linear stepwise regressions to the most important predictors explaining differences
in SOC variables. We focused our analyses on predicting
Note that we have pretested for correlations between SOC stocks, mean annual temperature (MAT) and mean annual precipitation (MAP) across our study sites. No significant correlations were found with the included climatic variables (Table A1), indicating no significant effect of climatic variation between sites on SOC dynamics. Hence, we focused our further analyses on the impact of local geochemistry and topography on SOC stocks and stabilization against microbial decomposition.
For all tested SOC variables, significant differences in the means of different topographic positions within each geochemical region were found
between valley and non-valley positions (plateaus and slopes) with higher SOC stocks in valley positions compared to non-valley positions. No
significant differences were found between plateau and slope positions (Table A2). Even though valley positions are of the same geochemistry as the
non-valley positions, geochemical soil properties in valleys were significantly different than at non-valley positions, as fluvial activity and
sedimentation unrelated to hillslope processes were dominant (see Supplement 1 for additional results and discussion on valley
positions). Consequently, for all follow-up analyses on differences with soil depth and geochemistry, the dataset was split into valley positions
vs. non-valley positions. Due to the limited sample size for the valley positions (
Parent material, from which soils in the three geochemical regions have developed, showed distinct differences in elemental composition (Doetterl
et al., 2021a, b), with generally low concentrations of Ca, Mg and Na base cations (0.01–0.58 wt %). Al and Fe concentrations were significantly higher in the mafic (Al: 6.27
Across geochemical regions, soils were highly weathered, as indicated by high CIA values of 78 %–99 % at all three soil depths (data not
shown). Soils developed from mafic parent material were depleted in Ca, Mg and Na base cations compared to parent material (Ca:
For non-valley positions, pyrophosphate-extractable oxides (0.02 to 1.93
At all sites and soil depths, fractions were dominated by microaggregate-associated C (
Proportion of biogenic- vs. fossil-derived organic carbon (OC) in soils developed from mixed sedimentary rocks in non-valley positions (
Soils from all geochemical regions at non-valley positions were significantly more depleted in
Biplots of the varimax-rotated principal component analysis.
Multiple linear stepwise regression analysis (beta coefficients) and relative importance analysis in brackets for non-valley soils.
Four rPCs were determined (rPC
Soil depth and rPC4
Partial correlation analysis between SOC variables (
When controlling for soil depth, correlations between
In contrast to our initial hypothesis that topography affects C stabilization in tropical forest soils through lateral material movements, we found no
indication of this in our analysis (see the Supplement and short discussion therein). Despite prolonged chemical weathering, parent material leaves
an identifiable, long-lasting footprint in the chemical properties of tropical forest soils (Fig. 5). Overall, the differences in elemental
composition between parent materials in each geochemical region, together with enrichment and depletion processes of elements during weathering, have
resulted in soils with specific properties and prerequisites for SOC stabilization against microbial decomposition (Table A3). In particular,
stabilization mechanisms related to pedogenic oxides (Fig. 3) and the formation of organo–mineral associations are relevant for SOC stabilization at our sites, as illustrated by the strong correlation of variables representing organo–mineral complexes with
While depth trends in
Our regression analyses revealed that a wide variety of soil variables contribute to predicting SOC and its turnover in a quantitative and
qualitative way (Tables 2 and A4). An exact mechanistic interpretation is difficult due to the relatively small number of observations compared to
potential predictor variables. However, in general a set of variables related to soil fertility and the chemistry of the solid phase and soil
solution contributed to predicting our three target variables: (1)
Overall, we found a minimal impact of topography on SOC variables as a function of soil fluxes along slopes but observed higher SOC stocks in valleys
compared to non-valley positions due to varying hydrological conditions and alluvial processes. Instead, chemical soil properties, derived from parent
material geochemistry, were identified as major explanatory factors. We argue that the strong role of geochemical variables in explaining SOC is a
function of reactive mineral surfaces dependent on the composition of the parent material and its weathering status. More available reactive surfaces
will favor sorptive C stabilization and the formation of stable aggregates, thus leading to higher
Partial correlation analysis between
SOC variables grouped by soil depth, topographic position and geochemical regions (section 1: mafic; section 2: felsic; section 3: mixed). Different capital letters indicate significant differences in means (
Asterisks indicate no significant differences in means (
Absolute and relative differences of mineral soil at plateau positions (30–40
Rotated principal components and variable loadings of varimax-rotated principal component analysis for non-valley positions (rPC) (
All data used in this study will be published in an open access project-specific database
All soil samples are logged and barcoded at the Department of Environmental Science at ETH Zurich, Switzerland.
The supplement related to this article is available online at:
SD and PF designed the research. MR conducted the sampling campaign and collected the data. All authors analyzed and interpreted the data. All authors contributed to the writing of the paper.
Sebastian Doetterl is a liaison editor of the special issue “Tropical biogeochemistry of soils in the Congo Basin and the African Great Lakes region”, and Johan Six is an executive editor and Peter Fiener a topical editor of
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
This article is part of the special issue “Tropical biogeochemistry of soils in the Congo Basin and the African Great Lakes region”. It is not associated with a conference.
This work is part of the DFG funded Emmy Noether Junior Research Group “Tropical soil organic carbon dynamics along erosional disturbance gradients in relation to soil geochemistry and land use” (TROPSOC; project no. 387472333). The authors like to thank following collaborators of this project: International Institute of Tropical Agriculture (IITA), Max Planck Institute for Biogeochemistry, Institute of Soil Science and Site Ecology at Technical University Dresden, Sustainable Agroecosystems Group and the Soil Resources Group both located at ETH Zurich and the Faculty of Agriculture at the Catholic University of Bukavu. The authors would like to thank the whole TROPSOC team, especially the student assistants, for their important work in the laboratory and all fieldwork helpers, who made the sampling campaign possible. The authors like to thanks the three anonymous referees for their valuable comments.
This research has been supported by the Deutsche Forschungsgemeinschaft (grant no. 387472333).
This paper was edited by Pauline Chivenge and reviewed by three anonymous referees.