Reactive transport modelling the oxalate-carbonate pathway of the Iroko tree; Investigation of calcium and carbon sinks and sources

Abstract

The oxalate carbonate pathway (OCP) of the Iroko tree has been shown to function as a carbon sink, due to the precipitation of calcium (Ca) carbonate in the tree and the soil around its roots, a soil developed on a carbonate-free bedrock. A 1D reactive transport (RT) model of the Iroko OCP system was constructed and used in a sensitivity analysis to replicate the geochemical pathway of the OCP and carbon sequestration, constrained by the Ca mass balance. The sensitivity analysis examined ranges of contributions for Ca sources to the system, including rainfall, dust, plant decomposition, mineral weathering, and optionally groundwater Ca sources. The study focused on identifying possible scenarios by which the observed magnitude of accumulated soil calcite could be reproduced, related to the OCP geochemical pathway. In line with field observations of the Iroko, all model results showed a soil transition from acidic (pH 5.1) to mildly alkaline (maximum pH 8.1) conditions, as well as precipitation of calcite. The location of calcite precipitation remained within the upper reach of the soil column, and between 29% and 63% of carbon was retained as calcite. The magnitude of calcite precipitation was limited both by the availability of Ca from the various sources, and by the tree’s rate of root water and solute uptake. The sensitivity analysis confirms that surficial sources such as rainfall and dust deposition contribute only a small fraction of Ca, and indicate that mineral weathering and the extent of the root system, must play a dominant role in supplying Ca to facilitate carbon sequestration through calcium carbonate formation.