Application of biochar to soils changes soil physicochemical properties and stimulates the activities of soil microorganisms that influence soil quality and plant performance. Studying the response of soil microbial communities to biochar amendments is important for better understanding interactions of biochar with soil, as well as plants. However, the effect of biochar on soil microorganisms has received less attention than its influences on soil physicochemical properties. In this review, the following key questions are discussed: (i) how does biochar affect soil microbial activities, in particular soil carbon (C) mineralization, nutrient cycling, and enzyme activities? (ii) how do microorganisms respond to biochar amendment in contaminated soils? and (iii) what is the role of biochar as a growth promoter for soil microorganisms? Many studies have demonstrated that biochar-soil application enhances the soil microbial biomass with substantial changes in microbial community composition. Biochar amendment changes microbial habitats, directly or indirectly affects microbial metabolic activities, and modifies the soil microbial community in terms of their diversity and abundance. However, chemical properties of biochar, (especially pH and nutrient content), and physical properties such as pore size, pore volume, and specific surface area play significant roles in determining the efficacy of biochar on microbial performance as biochar provides suitable habitats for microorganisms. The mode of action of biochar leading to stimulation of microbial activities is complex and is influenced by the nature of biochar as well as soil conditions.

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While Cr(III) is strongly retained onto soil particles, Cr(VI) is very weakly adsorbed and is readily available In this study, seven organic amendments (biosolid compost, farm yard manure, fish manure, horse manure, spent mushroom, pig ma-for plant uptake and leaching to ground water (James nure, and poultry manure) were investigated for their effects on the and Bartlett, 1983). Leaching studies have indicated that reduction of hexavalent chromium [chromate, Cr(VI)] in a mineral Cr(VI) is readily leached compared with Cr(III) and soil (Manawatu sandy soil) low in organic matter content. Addition anions, such as arsenate (Carey et al., 1996; Bolan and of organic amendments enhanced the rate of reduction of Cr(VI) to Thiyagarajan, 2001). Chromium(VI) can be reduced to Cr(III) in the soil. At the same level of total organic carbon addition, Cr(III) in the environments where a ready source of there was a significant difference in the extent of Cr(VI) reduction electrons is available. Suitable conditions for Cr(VI) among the soils treated with organic amendments. There was, however, a significant positive linear relationship between the extent of reduction occur where organic matter is present to act Cr(VI) reduction and the amount of dissolved organic carbon in the as an electron donor, and Cr(VI) reduction is enhanced soil. The effect of biosolid compost on the uptake of Cr(VI) from the in acid rather than alkaline soils (Bartlett and Kimble soil, treated with various levels of Cr(VI) (0-1200 mg Cr kg Ϫ1 soil), 1976; Cary et al., 1977). Reduction of Cr(VI) to Cr(III), was examined with mustard (Brassica juncea L.) plants. Increasing and subsequent hydroxide precipitation of Cr(III) ion, addition of Cr(VI) increased Cr concentration in plants, resulting in is the most common method of treating Cr(VI)-contamidecreased plant growth (i.e., phytotoxicity). Addition of the biosolid nated industrial effluents (Besseliever, 1969). Various compost was effective in reducing the phytotoxicity of Cr(VI). The redistribution of Cr(VI) in various soil components was evaluated organic materials, such as powdered leaves (Suseela et by a sequential fractionation scheme. In the unamended soil, the al., 1987) and Scotch pine (Pinus sylvestris L.) bark concentration of Cr was higher in the organic-bound, oxide-bound, (Alves et al., 1993) have been used to remove Cr(VI) and residual fractions than in the soluble and exchangeable fractions.from industrial effluents. Addition of organic amendments also decreased the concentration of

Copper (Cu) is bound strongly to clay minerals and organic matter in soils, and forms both insoluble and soluble organic complexes with organic carbon. In this experiment, the effect of five manure composts (biosolid, farmyard manure, spent mushroom, pig manure, and poultry manure) on the adsorption and complexation of Cu in a mineral soil (Manawatu sandy soil, Palmerston North, New Zealand) low in organic matter content was examined. The effect of biosolid on the uptake of Cu from the soil, treated with various levels of Cu (0-400 mg/kg soil), was examined by using mustard (Brassica juncea L.) plants. The redistribution of the added Cu in soil was evaluated by a chemical fractionation scheme. Addition of manure compost increased the adsorption and complexation of Cu by the soil. At the same level of total organic carbon addition, a significant difference was found in the extent of Cu adsorption among the manure-amended soils. However, less difference was found in the amount of Cu complexed among the manure-amended soils. A significant inverse relationship was found between the extent of Cu adsorption and the dissolved organic carbon (DOC) in the manure-amended samples, indicating that DOC formed soluble complexes with Cu. Increasing addition of Cu increased Cu concentration in plants, resulting in decreased plant growth at high levels of Cu (i.e., phytotoxicity). Addition of biosolid was found to be effective in reducing the phytotoxicity of Cu at high levels of Cu addition. Significant relationships were found between dry matter yield and total Cu or free Cu2+ concentration in soil solution. Addition of biosolid decreased the concentration of the soluble and exchangeable Cu fraction but increased the concentration of the organic-bound Cu fraction in soil.