Effects of the application of heated sewage sludge on soil nutrient supply to plants

We previously reported that heating sewage sludge significantly changes the rate of N mineralization. The present study was undertaken to examine the extent to which these changes affect plant growth and nutrient supply after application to different soils. A pot experiment in which komatsuna plants (Brassica campestris L. var. rapa) were grown in a Fluvisol or an Arenosol indicated that the amount of N taken up by the plants increased significantly by heating air-dried sludge at 120 degrees C and decreased significantly by heating at 180 degrees C. Heat-drying of the sludge at 120 degrees C or 180 degrees C also increased the N uptake significantly. These plant responses could be explained by the heat-induced changes in the release of inorganic N from the sludge. In contrast to N, the sludge materials containing Fe as a coagulant immobilized soil soluble P. When the sludge was applied to an Andosol, the N-supplying effects on plant growth were offset by P adsorption onto the sludge and soil. In a successive pot experiment conducted without additional sludge application, it was further suggested that frequent sludge application is required to maintain plant growth. However, it will also lead to the accumulation of sludge components in the soil because less than 40% and 15% of the sludge N and P, respectively, were apparently recovered in two harvests of the plants. In conclusion, heated sludge materials can act as an effective organic N fertilizer provided that they are applied to a suitable soil and that the short-term effects on soil productivity are balanced with the long-term effects on environmental quality.