Heavy Metal Fluxes in Tropical Urban Forest Soil in Abidjan District (Côte d’Ivoire)

Abstract

In Western Africa, the growth of cities has led to natural resource pollution, especially air pollution. Urban forests play a key role in filtering atmospheric particles and pollutants through the canopy before reaching the soil. This study aims to quantify heavy metal fluxes in an urban forest in the district of Abidjan in order to assess its role in the protection of natural resources. A monitoring of wet deposition (throughfall and open field rain) and litterfall was carried out for six months in the urban forest of the National Floristic Center located in Abidjan, C?te d’Ivoire. The results show that the soil of this urban forest is a ferralsol type characterized by a sandy-clay texture and a low load of coarse elements. The annual litterfall is estimated to 12.16 ± 0.71 t·ha-1·yr-1, similar to other tropical forests. Annual quantities of rain and throughfall are in the range of the rainfall recorded in the district of Abidjan (2013 ± 152 and 1773 ± 51 mm). Chemical analyses showed that litter and rainfall contain Mn, Zn, Ni, Cr, Cd and Hg. Manganese and Zn are the most abundant elements and Hg the least abundant in both rainfall and litter. The main source of input of the heavy metals into the urban forest soil is associated with biological recycling through the litter. The litterfall contributes to metal fluxes in soil 109 times greater than metal fluxes carry by wet depositions (open field rain and throughfall). However, a detailed study of rainfall showed that the forest canopy constitutes a barrier for the transfer of heavy metal to urban soil. This is indicated by a decrease in heavy metal content from open field rain to throughfall. Consequently, this study recommends the creation and maintenance of urban forests to increase biomass canopy and improve atmospheric air quality for West African cities undergoing constant change and development.