Biomineralisation of inorganic carbon by agroforestry species in East Africa: The oxalate carbonate pathway of fig trees in Samburu County, Kenya

Soils store both organic (SOC) and inorganic carbon (SIC), yet biogenic processes driving SIC formation remain poorly quantified. One such process is the oxalate-carbonate pathway (OCP) where plant-derived calcium oxalate is transformed by oxalotrophic microorganisms into SIC, which can sequester atmospheric CO₂ in non-calcareous soils. Yet, the OCP has rarely been investigated in connection to trees with significant agroforestry potential. To further investigate the OCP and its connection to species with agroforestry potential, we investigated three East African fig species (Ficus glumosa, F. natalensis, and F. wakefieldii) in semi-arid Samburu County, Kenya.

Across contrasting parent materials devoid of primary carbonates, soils adjacent to fig trees exhibited significantly higher pH, exchangeable Ca, SOC, and SIC content compared to control soils, indicating the trees maintained hotspots of distinct biogeochemical conditions. Fig biomass samples contained substantial calcium oxalate contents (4.9±0.5 % dry weight), predominantly as prismatic whewellite crystals (CaC₂O₄.H₂O). Calcium carbonate coatings were observed on trunks and roots of all three species, which reacted strongly to hydrochloric acid. Synchrotron-based μ-X-ray Fluorescence coupled with μ-X-ray absorption near-edge structure spectroscopy (Ca K-edge) revealed that CaCO₃ had precipitated deeply into woody tissues, providing direct evidence for aboveground OCP. Amplicon-based sequencing showed diverse and abundant microbial communities on the aboveground biomass, litter, roots, and adjacent soils. In addition, a co-occurrence analysis of fungal and bacterial communities showed specific fungal genera and fungal oxalate-producers are tightly linked to known baterial oxalotrophs, indicating that bacterial-fungal interactions could be essential for oxalotrophy. Combined these results demonstrate an active OCP both above and belowground in connection to the food-providing fig trees (Ficus spp.) of semi-arid East Africa.

Our findings identify East African fig trees as previously unrecognised drivers of biogenic SIC sequestration. Integrating specific fig species into agroforestry systems could therefore represent a novel nature-based solution that couples food production with SOC and long-term SIC storage in dryland landscapes.