Hideki Kawamata and Shinjiro Sato, Department of Environmental Engineering for Symbiosis, Soka University, Hachioji-shi, Tokyo, Japan
Sewage sludge is expected to be recycled as alternative phosphorus resources in agriculture. Especially, pyrolytic conversion of sewage sludge into biochar could be one of the sustainable management options for agricultural soils. The pyrolysis temperature, however, is one of the critical deciding factors for properties of biochar derived from sewage sludge (SSB). In this study, two biochars pyrolysed at 800°C (SSB800) and 300°C (SSB300) were separately mixed with two different types of Japanese soils for a pot study, and nutrient, in particular, P cycling was investigated. Before the pot study, total P, available P was determined using 2% citric acid extraction, and P sequential extraction was performed on two different types of soil: an Andisol in Tokyo and Kunigami mahji in Okinawa. Each SSB was mixed with soils at 4 different application rates (SSB800: 0%, 0.8%, 1.6%, and 3.2%; SSB300: 0%, 0.6%, 1.2%, and 2.3% by volume) in 1 L pots to grow Japanese mustard spinach (Brassica rapa). After a month of cultivation, dry weight, total P, total C, and total N in the plant, and total P, available P, inorganic N was analyzed. SB800 and SSB300 contain 43 and 32 g kg-1 of total P, respectively. Labile P fraction by sequential extraction was significantly lower in SSB800 containing 1% compared with 30% of the total P in SSB300. Seventy percent of the total P in SSB800 was in the form of 1 M HCl extractable. Citric acid extractable P in SSB800 and SSB300 were 1.7 and 2.2 g kg-1, respectively. In terms of P bioavailability of SSB, SSB300 is expected to be more useful than SSB800 as SSB300 contains more relatively available forms of P than SSB800. However, tar and other heavy metal in SSB300 may have negative effects on the plant growth.