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Abstract
Root exudation of organic acids and acid phosphatase (APase) is important for the utilization of unavailable phosphorus (P) such as organic and sparingly soluble inorganic phosphate compounds in sewage sludge, which could become an important P source in the future, in view of the shortage of rock phosphate. Therefore, we studied a model of rhizosphere regulation, focused on the extraction of P from sewage sludge by applying APase and citric acid. By applying APase secreted from lupin roots and citric acid to sewage sludge, the amounts of organic and sparingly soluble inorganic phosphate compounds decreased and the amounts of soluble inorganic phosphates increased. These results indicate that APase and citric acid can release available inorganic phosphates from sewage sludge.