Promoting the recovery of soil health in As and Sb-polluted soils: new evidence from the biochar-compost option
The role of mixed compost and biochar in the recovery of As- and Sb-polluted soils is poorly investigated, as well as the influence of the respective application rates on soil health and quality. In this study, we therefore investigated the effectiveness over time (2, 4 and 6 months) of a municipal solid waste compost (MSWC), and a biochar (BC), both applied at 10 and 30% rates, and of selected mixtures (MIX applied at 10 and 30% total rates, 1:1 ratio of MSWC and BC), on labile As and Sb in a polluted soil from a dismissed Sb mining site (Djebel Hamimat, Algeria). At the same time-points, the amendment impact on soil chemistry was also monitored, while the activity and diversity of the resident microbial communities was investigated after 6 months. Significant changes in the dynamics of labile pollutants and soil chemical properties were recorded during the soil-amendment contact time. After 6 months, MSWC, BC and MIX applied at the higher rate (i.e., 30%) significantly increased soil pH (from 7.5 up to 8.2), while MSWC and MIX increased soil EC to worrying values. The soil DOC content was also greatly increased by MSWC and MIX at the higher rates (up to 50-fold), while BC showed a negligible impact. All the amendments reduced the concentration of labile Sb in soil, with BC 10% being the most effective treatment (i.e., reducing labile Sb from ~ 60 to 20 mg kg-1 soil). On the contrary, only BC and MIX applied at 10% significantly reduced labile As (e.g., from ~ 12 to 4 mg kg-1 soil in the case of BC). MSWC and MIX at both rates significantly increased soil dehydrogenase activity (up to 2000-fold), while BC showed a null impact. The Biolog community level physiological profile and sequencing of the partial 16S rRNA gene showed a reduction of catabolic activity and alpha-diversity, and a change of the community composition, of bacterial populations in treated soils. Overall, the results indicate that MIX treatment, especially at 10%, is the most promising option for the chemical and biological recovery of As- and Sb-polluted soils.