The role of geochemistry in organic carbon stabilization against microbial decomposition in tropical rainforest soils
Authors: Reichenbach Mario , Fiener Peter , Garland Gina , Griepentrog Marco , Six Johan , Doetterl Sebastian 
Published: August 2021

Abstract. 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 C/N ratios and depletion of N. FOC constitutes up to 52.0 ± 13.2 % 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 0–10, 30–40 and 60–70 cm, showed that variables affiliated with soil weathering, parent material
geochemistry and soil fertility, together with soil depth, explained up to 75 % of the variability of SOC stocks and
Δ14C. Furthermore, the same variables explain 44 % 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.