The formation of calcite in otherwise carbonate-free acidic soils through the biological degradation of oxalate is a mechanism termed oxalate-carbonate pathway, which occurs during interaction between biological and geological systems. In this pathway, atmospheric CO2 is fixed by the photosynthetic activity of plants, part of which is destined to the production of oxalate to control the intracellular Ca2+ concentrations. An additional source of calcium oxalate is fungi, which are able to produce this organic acid to cope with elevated concentrations of metals. The decay of plant material results in a source of calcium oxalate for other trophic levels. In spite of its abundance as a substrate, oxalate is a very stable organic anion that can be metabolized only by a group of bacteria that use it as a carbon and energy source. These bacteria close the biological cycle by degrading calcium oxalate, releasing Ca2+ and changing the local soil pH. If the conditions are adequate, the geological part of the pathway begins because this biological process will indirectly lead to the precipitation of secondary calcium carbonate (calcite) under unexpected geological settings.

The activity of the oxalate-carbonate pathway has now been demonstrated in several places around the world. Furthermore, it can constitute an important, although underestimated, soil mineral carbon sink. This is because due to the initial acidic soil conditions and the absence of geological carbonate in the basement, it is unexpected to find C in the form of calcite. By its global scale and its stability through a long period of time, this terrestrial C sink is of a crucial interest as the sustainability of other C sequestering processes (e.g. sinking of CO2 in the ocean) is under question.

The study of the oxalate-carbonate pathway constitutes a multidisciplinary research that brings together competences in biology (botany, physiology, microbiology) and geology (geochemistry, mineralogy, soil science). Thus, from its inception, this research has been carried out by a multidisciplinary team and by combining two crucial aspects: field expeditions and laboratory work using several tropical soils as models. Our most recent results show that biological interactions between bacteria and fungi, that have been underestimated, are essential for reproducing the pathway in vitro. Also, we have observed that Ca budget/availability has a direct impact on pedogenic carbonate accumulations.

Therefore, the aims of this proposal are twofold: first, we aim at elucidating the nature of the interaction between bacteria and fungi and its importance for the oxalate-carbonate pathway. Second, we aim at understanding how the Ca cycle (pools, fluxes and limitations in the concentration of Ca2+) can drive an “ecosystem induced-C sink”, in our case the oxalate-carbonate pathway system.

By pursuing these two objectives, we expect to contribute essential information within two of the current “black boxes” of the system, which will allow the establishment of a model of the dynamics of carbon accumulation associated with the oxalate-carbonate pathway. This may have an enormous impact due to the potential importance of this ecosystem (or equivalent ecosystems) in tackling the increasing atmospheric CO2 concentrations and their direct effect over global climate.