The role of geochemistry in organic carbon stabilization in tropical rainforest soils

Abstract

Abstract. Stabilization of organic carbon in soils (SOC) 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 changed hydrological conditions had a secondary control on SOC dynamics in valley positions, leading to higher SOC stocks there than at the non-valley positions. We also detected fossil organic carbon (FOC) at several sites, constituting 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 analysis 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 versus 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 investigated tropical soils.

Assessing soil redistribution of forest and cropland sites in wet tropical Africa using <sup>239+240</sup>Pu fallout radionuclides

“…cassava, sweet potato, groundnuts). In temperate regions, harvest erosion rates up to 12 Mg ha −1 yr −1 have been reported for different crop types (potato: 2.5-6 Mg ha −1 yr −1 ; Auerswald and Schmidt, 1986;Belotserkovsky and Larinovo, 1988;Ruysschaert et al, 2007) et al, 2001). With cassava and sweet potato being the main food crops within the NiCo region (cassava has a higher proportion on the less fertile soils in the DR Congo, while more sweet potato is cultivated in Uganda), this is a likely source of reduction of 239+240 Pu inventories.…”

Controls on heterotrophic soil respiration and carbon cycling in geochemically distinct African tropical forest soils

“…step, we used partial correlation analysis following (Doetterl et al, 2015b), to interpret the explanatory power of independent variables in our model, when controlling for soil depth and discuss our findings with respect to microbial (extracellular enzyme activity, see Kidinda et al, 2020 in review), mineralogical (pedogenic oxides, Reichenbach et al, 2021 in review) and soil fertility parameters (available nutrients and exchangeable base cations, see Doetterl et al, 2021 in review) collected for the soils investigated here in other studies. For all statistical tests, due to the relatively small sample size and to avoid Type II statistical errors, a threshold of p<0.1 was used to indicate significant difference.…”

“…This may be due to the presence of labile C that is inaccessible to microbial decomposers due to unfavourable environmental conditions in valley subsoils. In the laboratory, however, if not stabilized by other mechanisms such as aggregation or organo-mineral complexation (Reichenbach et al, 2021 in review), these C sources become available to decomposers once environmental constraints, such as water saturation, are removed ( Fig. 2b,d).…”

Comment on soil-2020-96

“…We did this in order to avoid interpreting variables as being important for the model when they were instead just auto-correlated to soil depth. In a final step, we used partial correlation analysis following (Doetterl et al, 2015b), to interpret the explanatory power of independent variables in our model, when controlling for soil depth and discuss our findings with respect to microbial (extracellular enzyme activity, see Kidinda et al, 2020 in review), mineralogical (pedogenic oxides, Reichenbach et al, 2021 in review) and soil fertility parameters (available nutrients and exchangeable base cations, see Doetterl et al, 2021 in review) collected for the soils investigated here in other studies. For all statistical tests, due to the relatively small sample size and to avoid Type II statistical errors, a threshold of p<0.1 was used to indicate significant difference.…”

“…While erosion rates at annual or decadal timescales are negligible for the investigated tropical forests (Drake et al, 2019;Wilken et al, 2020 in review), underlying geological erosion rates estimated for tropical mountain forests globally (Morgan, 2005) range between 0.03-0.2 t.ha -1 .y -1 . Assuming an average bulk density in our study area's topsoil roughly at 1.3 g.cm -3 (Doetterl et al, 2021 in review), 6.8-45.3 k years are required to erode the top 10 cm of soil. Thus, slow erosion of soil at millennial timescales may explain the residual content of fossil organic C in topsoil.…”