Reducing capacity of water extracts of biochars and their solubilization of soil Mn and Fe

Biochar, being produced in an oxygen-restricted environment, is chemically more reduced than the original feedstock. Consequently, it was hypothesized that reduced biochar components could participate in redox-mediated reactions in the soil. This hypothesis was tested by measuring the reducing capacities of aqueous extracts of biochars and the reduction and solubilization of soil Mn and Fe oxides by the extracts. The reduction capacity of extracts from biochars produced from three feedstocks (eucalyptus wood, EUC; olive pomace, OP; and greenhouse waste, GHW) at different highest pyrolysis treatment temperatures (HTT; 350, 450, 600 and 800°C) was less for the EUC feedstock than the others, and was greater for biochars produced at lower HTTs. The organic fraction of the extracts apparently was responsible for the major part of the reducing capacity. Extracts of smaller-HTT biochars, having greater dissolved organic carbon (DOC) contents, had greater reducing capacities than extracts of larger-HTT biochars from the same feedstock. Extracts of two GHW biochars (GHW-450 and GHW-600) solubilized Mn and Fe from soils at pH values below 8. The extract with the greater reducing capacity (GHW-450) solubilized both metals to a significantly greater extent. Smaller-HTT biochars produced from agricultural wastes, having a greater variety and concentration of soluble reducing agents, are expected to have more impact on soil redox reactions than larger-HTT biochars. By participating in chemical and biological redox-mediated reactions in the soil, biochar could influence microbial electron shuttling, nutrient cycling, pollutant degradation, contaminant mobilization and abiotic formation of humic structures.