Evaluation of slag application to decrease methane emission from paddy soil and fate of iron

Organic carbon in paddy soil is oxidized to carbon dioxide by reducing electron acceptors for a certain period after submerging. Methane production commences after the reduction of iron oxide which is the most important electron acceptor in the soil. We aimed to study the long-term suppression of the methane emission from the paddy soil by single application of iron slag. A revolving furnace slag (RFS; 248 g Fe kg(-1)) was applied to the potted soil at the rate of 0 (control) or 20 ton ha(-1) in 2000. Rice plants were successively cultivated on the potted soils for 3 years without further application of the RFS. Methane emissions from the potted soils with rice plants were measured by the closed chamber method during these cultivation periods. Total flux of CH4 emission from the pot applied with RFS decreased by 5-30% compared with the control. The RFS supplied free iron oxide to the potted soil, and its iron acted as the oxidizing agent as evidenced by the increase in ferrous iron content in the soil. The amount of iron lost from leaching at the bottom of the pots was estimated as 54-59 kg Fe ha(-1) year(-1) at the percolation rate of 20 mm d(-1). Accordingly, half-life of the iron in the applied RFS was calculated as 42-46 years. Therefore, there is a possibility that the suppressing effect of RFS on CH4 emission is sustained for a half-century. Contents of heavy metals (Cd, Cu, and Zn) in the brown rice harvested from the pot applied with RFS were not significantly different with those from the control pot.