Reconstructing soil exchangeable calcium and magnesium in central Africa (DRC) during the Late Holocene Rainforest Crisis using a new biomarker lipid tool

Soil organic carbon (SOC) sequestration is widely promoted as a nature-based climate solution, but whether soils will continue to act as a net carbon sink depends on the future sensitivity of SOC to ongoing environmental changes. Studies show that a large fraction of SOC is chemically or physically associated with minerals. In addition, minerals provide essential nutrients (e.g. K⁺, Ca²⁺, Mg²⁺). Soil mineral properties, such as exchangeable base and acid cations and cation exchange capacity (CEC) change on millennial timescales and as a response to climate change. The impact of climate change on mineral-mediated SOC stabilization and mineral fertility parameters remains poorly understood because there is no method to directly quantify soil mineral fertility changes through time in paleosol archives.

Recent work on branched glycerol dialkyl glycerol tetraethers (brGDGTs), biomarker lipids that record past environmental changes, indicates that their distribution is influenced by concentrations (cmolc/kg) of mineral fertility properties (exchangeable Ca²⁺, Mg²⁺ and the sum of base cations). In this study we a) quantify how soil mineral fertility controls GDGT distributions in central African soils, b) develop and calibrate GDGT-based proxy ratios for soil mineral fertility for this region, and c) apply the proxy to a geological record.

To capture a large soil geochemical and climatic gradient in our dataset for proxy calibration, we selected 69 sites (topsoils) across tropical forests and savannah grasslands in central Africa that span different parent bedrocks, mean annual air temperatures (6-30°C) and precipitation regimes (580-2800 mm/yr) with contrasting soil pH (2.7-7.6), exchangeable basic (Ca²⁺, Mg²⁺) and acid cation (Al³⁺, Fe²⁺) concentration values. Exchangeable base cations, CEC and summed bases co-varied strongly with fractional abundance of specific brGDGTs in soils with medium (4.5<pH<6.5) to high pH (pH>6.5) soils. Multivariate analyses show that exchangeable Ca²⁺ and Mg²⁺, together with mean monthly potential evapotranspiration (mm) and mean annual precipitation (mm), explain a significant variance in brGDGT composition in the dataset. Based on these empirical relationships, we derived a novel proxy that correlates strongly with exchangeable Ca²⁺ (r = 0.94, p < 0.001) and Mg²⁺ (r = 0.85, p < 0.001). The performance and caveats of our brGDGT-derived fertility proxy for central Africa (for instance: influence of precipitation and evapotranspiration on the proxy ratio, limitations in low pH and high total organic carbon soils) will be discussed.

We then applied the proxy to a 2 m peat soil from Yangambi (Democratic Republic of the Congo), which, based on bulk ¹⁴C dating, covers two major Late Holocene Rainforest Crisis intervals (LHRC; 2500 and 4000 cal yr BP). We reconstructed soil exchangeable calcium and magnesium content (cmolc/kg) by using the calibrated proxy and found that the amount of both cation concentrations decreased between 4000 and 1900 cal yr BP. This preliminary cation concentration reconstructions for LHRC periods in Yangambi delivers new insights by comparing brGDGT signals from bulk fraction and oxidation-resistant mineral associated organic matter (MAOM) fraction.