The mobility and bioavailability of trace metals in tropical soils: a review

In recent years, initiatives on critical loads of metals have resulted in the development of models to predict critical, i.e. mobile or bioavailable, concentrations of potentially toxic elements in soils. These models have been based on data from temperate soils and, considering the fundamentally different soil chemistry of the major tropical soil types, it is unlikely that such models could be applied successfully in the tropics. In particular, the major tropical soil types, particularly oxisols and ultisols, are distinguished from most temperate soils by low activity clays, low organic matter contents, low pH values and high levels of Fe oxides. The aim of this review paper is to collate existing information on the behaviour of trace metals in tropical soils, particularly in relation to metal mobility and bioavailability, and note differences, where these exist, with temperate soils. The main findings are as follows. The acidic nature of many tropical soils may not, of itself, lead to greater metal solubility. The relatively low organic matter content of tropical soils suggests that it cannot be relied upon to retain metals. However, when it is present, it appears to be important in the retention of copper, lead and zinc. The clay fractions of many tropical soils are dominated by kaolinite which is less able than 2: 1 layer silicate clays to adsorb metals. However, oxisols and ultisols in particular have relatively high Fe oxides contents and these may be effective in metal sorption. Overall though, soils such as oxisols and ultisols appear to sorb relatively low amounts of added metals, most likely explained by the low activity clay minerals they contain and the rapid decomposition of organic matter incorporated into them. Dominant retention mechanisms, where they occur, are inner and outer sphere adsorption but not precipitation. Cadmium, Cu, Zn and Ni appear to be mobile in tropical soils. Chromium may be mobile in the A horizon but retained in the B horizon. The evidence for Pb points to surface retention, as is often observed for this metal in temperate soils. Metals in contaminated tropical soils appear to be rather bioavailable, judging by the few studies on plant uptake. It should be noted that the above statements are based on a relatively small number of studies. Further work is required if we are to gain an improved understanding of the mobility and bioavailability of trace metals in tropical soils.