Background and Aims

The oxalate-carbonate pathway (OCP) has been observed in acidic tropical soils with low alkaline cation content where compartments are transient and fed by the rapid turnover of organic matter. By acting on edaphic parameters, the OCP may influence soil nutrient distribution. This study aims at assessing the influence of the OCP on soil nutrients within an agroforestry system associated to oxalogenic iroko trees.

Methods

Soil nutrient distribution was studied in a 30 m long and 1 m deep transect starting at the iroko tree towards the vegetation surrounding it.

Results

Processes controlling nutrient distributions varied with both distance and depth. The tree drastically impacted edaphic variables, in the first instance pH. Changes in pH generated gradients of calcium and magnesium, both of the exchangeable and the total fraction. In contrast, total phosphorus and potassium distribution were mostly influenced by depth.

Conclusions

This downward gradient fits the “plant cycling model” that explains the effect of vegetation on the recharge of soil nutrients. This is the first example of the effect of the OCP on soil nutrient distribution. Considering that the OCP is not restricted to the iroko tree, this study highlights a more general pattern of forest dynamics developed on highly weathered tropical soils.