We examined the suitability of waste fluid from a methane fermentation plant, the Yagi Bio–Ecology Center in Nantan city, Kyoto prefecture, as a nitrogen fertilizer for paddy rice (Oryza sativa L. ‘Kinuhikari’). The plant processes 55 m3 of dairy and pig excreta per day to produce methane for electricity generation. Waste fluid contains about 3.6 g kg-1 of nitrogen, of which 2.2 g is ammonium–N and 1.4 g is organic–N. A 5–year trial in a 1–m2 container revealed that the waste fluid could support rice growth as well as ammonium sulfate with the same ammonium–N content. The ammonium–N in the waste fluid was prone to volatilization, but a quarter of the initial amount of organic–N was mineralized, so the waste fluid might have similar potential as ammonium sulfate when applied at the same rate of ammonium–N as inorganic fertilizer. However, the rice plants took up more N than plants that received ammonium sulfate during successive applications over 5 years. This suggests that successive applications of the waste fluid would build up soil organic–N in paddy fields.
