Effect of recycling wood ash on microbiological and biochemical properties of soils

Wood ash production is increasing due to the increasing use of wood as a sustainable fuel. Wood ash can be recycled in crop soils as a source of nutrients and as a liming agent to correct soil acidity. However, the effect of wood ash on soil quality is poorly known. Here we studied the effect of addition of wood ash at 5 and 20 t ha(-1) on microbial and biochemical properties in samples from the 0-30 cm surface of three neutral and two alkaline agricultural soils. The samples were incubated for one year at 25 C and 60% water-holding capacity. The soil microbiological biomass (microbial biomass C and N), general microbial activity using fluorescein diacetate hydrolysis, and alkaline phosphatase, arylsulphatase, catalase and o-diphenoloxidase activities were determined periodically together with pH and electrical conductivity. Increases in microbial biomass C and fluorescein diacetate hydrolysis occurred at the lower wood-ash dose, whereas microbial biomass C decreased at the higher wood-ash dose over the entire period, and the fluorescein diacetate hydrolysis after 4 months. The microbial C/N ratios of treated samples were significantly different with respect to that of the control, suggesting changes in the structure of the microbial communities. The changes in microbial biomass and microbial activity were related to the increases in pH, which induce the development of bacteria to the detriment of fungi, to the electrical conductivity, and to the nutrient levels after wood-ash addition. Alkaline phosphatase and arylsulphatase activities were weakly inhibited at 20 t ha(-1), and catalase activity was stimulated in the neutral samples and inhibited in the alkaline samples. The highest o-diphenoloxidase activity observed throughout in all of the treated samples may suggest a decrease in the mineralisation process. Our findings reveal that the soil biochemical parameters investigated are significantly affected only in some cases at 20 t ha - 1, suggesting that a treatment exceeding 5 t ha(-1) is not recommended until the agro-chemical and physical functions of the soil are further studied.