A carbon-sink in a sacred forest: Biologically-driven calcite formation in highly weathered soils in Northern Togo (West Africa)

Soil organic matter (OM) is a source of fertility for food provisioning and a means for climate mitigation. A high pedodiversity observed over very short distances due to both past and present land use, such as in Western Africa (Northern Togo), makes this a crucial issue. In particular, some small spots covered by sacred forests (forêts sacrées) of high cultural value in this area are extremely biodiverse, calcareous and high in OM in contrast with the non-calcareous, poor OM, overgrazed woody savannah and agricultural surroundings. The objectives of our work were to understand this pedodiversity and the process of CaCO₃ formation in the soil under the highly biodiverse forest using morphological, micromorphological, chemical and mineralogical soil analyses. The results show a high degree of weathering of the gneiss parent material, presence of plinthite gravels, dominance of 1:1 clays, formation of swelling clays in the imperfectly drained soils, and low OM content. The presence of palygorskite, although in traces, suggests drier intervals and is consistent in this context with the extreme wet and dry climate conditions in the region. In the OM rich soils under forest, bio-calcification takes place in the form of CaCO₃ needles, micrite hypocoatings around biopores and calcified cells. Oribatid excrements are associated with calcite and organic matter in the sacred forest soil, indicating that litter recycling has played an important role in the formation of calcite. We hypothesize that the high biological activity releasing CO₂, formation of HCO₃^- and precipitation of CaCO₃ due to the Ca²⁺ released by the recycled organic matter and weathering of plagioclases, lead to different forms of secondary calcium carbonate in the sacred forest soils. The high oxalate content of the vegetation in the sacred forest suggests that calcium carbonate formation, possibly via the oxalate-carbonate pathway, may also have played a role in calcite precipitation in these in organic matter rich soils. The parent material of these soils is not calcareous, meaning that these are not lithogenic carbonates, thus making them an important carbon sink. The soil characteristics indicate a high potential for development of the soils of the area in both agricultural yields and in potential carbon sequestration relevant to global change policies.