Impacts de la Voie Oxalate-Carbonate sur les caractéristiques édapliiques et organiques des sols forestiers tropicaux (Forêt de Kirindy, Madagascar)

The Oxalate-Carbonate Pathway (OCP) is a process by which oxalate, a photosynthetic product, is oxidized by soil oxalotrophic bacteria in interaction with Fungi. This process results in a local soil alkalinization, calcium carbonate precipitation, and possibly can lead to a long- term soil carbon sequestration. The present research, carried out in Madagascar, aimed to assess the impacts of OCP on edaphic and organic characteristics of tropical forest soils.

An investigation of tree species associated with OCP, and called "oxalogenic-oxalotrophic ecosystems", was first undertaken in Malagasy forests. This inventory allowed Tamarindus indica (Fabaceae) or tamarind tree to be identified. It is located in the dry deciduous forest of Kirindy, and used as a tree-model to be studied. Tamarind is also known to produce a large amount of oxalate.

The OCP was investigated around twelve large-sized Tamarinds. Soil profiles were dug around each tree. In addition, one profile was sampled 15 m away as a reference soil. Samples from the first 15 cm of all the different soils were taken and analyzed in the lab. The organic matter component was studied thermally using Rock-Eval pyrolysis. The organic matter and soil physicochemical characteristics (pH, soil texture, mineralogy, exchangeable basic cations, cation exchange capacity) under the influence and out of influence of tamarinds were compared.

Tamarind has been shown to modify the soil chemistry, starting with an increase in pH up to 2.5 units compared to distant soils which are neutral (pH = 6.5 -7.5) or acidic (pH = 5.8-6.5). This change in pH is accompanied by an increase in exchangeable alkaline cations (Ca2+, Mg2+ and K+) and cation exchange capacity (CEC). The OCP effects are however variable. The role of soil pH before OCP settlement is particularly emphasized as an important factor accounting for the extent of pH increase and the amount of carbonate accumulation. Moreover, the changes induced by the tamarind oxalogenic-oxalotrophic ecosystem seems to be at the origin of the accumulation of some refractory carbon in soils, through a biogeochemical stabilization favored by the presence of a calcium pool (Ca2+, CaCO3).

The results of this research provide a better understanding of the OCP and confirm the drastic effects of this process on soil properties. The integration of oxalogenic-oxalotrophic ecosystems in agroforestry systems for counteracting acidity and nutrient depletion in tropical soils is recommended. However, this valorization requires assessing the agronomic, socio- economic and environmental aspects for best adoption of these practices.

Finally, the natural formation of such calcium carbonate accumulations in soils, with a residence time in soil in the order of magnitude of 104 - 106 years, can provide a promising long-term solution for CO2 sequestration in the global context of tackling climate change.

Trees associated to OCP could thus be recommended for large-scale reforestation programs.