Short-term effects of a sewage sludge biochar amendment on total and available heavy metal content of a tropical soil

Abstract

The production of biochar is a technological alternative to transform sewage sludge (SS) into a useful product for agro-environmental purposes. Despite an increased knowledge on the role of sewage sludge biochar (SSB) for improving soil quality and crop productivity, some concerns remain regarding the accumulation of total and available heavy metals (HM) in soils amended with SSB. Particularly, there is a dearth of studies under field conditions and in tropical latitudes. Therefore, the objective of this study was to evaluate the effect of applying SS biochars, prepared at different pyrolysis temperatures, on the accumulation and availability of heavy metals (HM). To do so, 15Mgha−1 of SS biochar produced at 300°C (BC300) and 500°C (BC500) were applied to a soil cultivated with corn. Total and available levels of HM in SS, biochars and post-harvest soil were determined. Pyrolysis concentrated total HM in the biochars in relation to the SS. However, HM availability was reduced with increasing pyrolysis temperature due to an increase in pH, pore volume, specific surface area, P and K content, and reduction of the H/C ratio. Biochar did not alter the total HM contents, with the exception of Mn. Available levels of all HM in the soil were <1.2% of the total contents. In addition, the available levels of Zn and Mn, when evaluated as micronutrients, were considered low. Therefore, the results of the present study indicate that biochar produced from SS of a sewage treatment plant processing predominantly domestic sewage can be used in agriculture, without risk of soil contamination by HM. Moreover, in some tropical soils, with a low value of micronutrients, biochar can provide a source of these elements.