Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> 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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">C</mi><mo>/</mo><mi mathvariant="normal">N</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="f135772273124e8de131c1d3d27c70de"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="soil-7-453-2021-ie00001.svg" width="24pt" height="14pt" src="soil-7-453-2021-ie00001.png"/></svg:svg></span></span> ratios and depletion of N. FOC constitutes up to 52.0â<span class="inline-formula">±</span>â13.2â% of total SOC stock in the C-depleted subsoil. Interestingly, total SOC stocks for these soils did not exceed those of sites without FOC. Additionally, FOC decreased strongly towards more shallow soil depths, indicating decomposability of FOC by microbial communities under more fertile conditions. Regression models, considering depth intervals of 0â10, 30â40 and 60â70â<span class="inline-formula">cm</span>, showed that variables affiliated with soil weathering, parent material geochemistry and soil fertility, together with soil depth, explained up to 75â% of the variability of SOC stocks and <span class="inline-formula">Î<sup>14</sup>C</span>. Furthermore, the same variables explain 44â% of the variability in the relative abundance of C associated with microaggregates vs. free-silt- and-clay-associated C fractions. However, geochemical variables gained or retained importance for explaining SOC target variables when controlling for soil depth. We conclude that despite long-lasting weathering, geochemical properties of soil parent material leave a footprint in tropical soils that affects SOC stocks and mineral-related C stabilization mechanisms. While identified stabilization mechanisms and controls are similar to less weathered soils in other climate zones, their relative importance is markedly different in the tropical soils investigated.
Highlights
IntroductionThe 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
40 1.1 soil organic carbon (SOC) research in the tropicsThe 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
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 with higher SOC stocks in valley positions compared to non valley positions
Summary
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 understudied (Schimel et al, 2015). Studies analyzing the effect of soil geochemistry on SOC dynamics and stabilization are 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 modelling 50 approaches (Vereecken et al, 2016) Most of these geochemical effect studies focus on mid latitudes 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). 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)
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