Impact of high natural soilborne heavy metal concentrations on the mobility and phytoavailability of these elements for sugarcane

This study was designed to evaluate whether sewage sludge could be recycled in crop fields in Réunion (volcanic island in the Indian Ocean). We first quantified heavy metal (HM) concentrations (Cd, Cr, Cu, Hg, Ni, Pb and Zn) in the soils. Fifty-eight sampling sites were selected to account for the entire range of soils on the island. The Ni concentration in 86% of the studied soil samples was higher than the standard thresholds beyond which spreading is not authorized (75% for Cr and 14% for Cu). We demonstrated that the high soil Cr, Cu, Ni and Zn concentrations were mainly due to the natural pedogeochemical background. Despite these high concentrations, the mobility and phytoavailability assessed by chemical extraction were low and always below the thresholds. For these soils, we also inventoried HM concentrations in sugarcane (bagasse and juice). We demonstrated that the Cd, Cr, Cu, Ni, Pb and Zn concentrations were low and not proportional to the soilborne HM concentrations. Note that the lack of correlation between the sugarcane HM contents and the phytoavailable fraction determined by chemical extraction (1 mol L−1 NH4NO3) indicated that this was ineffective in determining the HM (Cd, Cr, Cu, Ni, Pb and Zn) phytoavailability for sugarcane. However, these overall findings could enable competent authorities to issue a waiver to authorize the use of sewage sludge in agriculture in Réunion.

Research Highlights►High soil Cr, Cu, Ni and Zn concentrations in Réunion are due to the pedogeochemical background. ►Phytoavailability and mobility of heavy metals (HM) were low. ►HM in sugarcane were low and not proportional to the soilborne HM concentrations.