Effect of application of iron materials on methane and nitrous oxide emissions from two types of paddy soils

Iron oxide is the most important electron acceptor in paddy fields. We aimed to suppress the methane emission from paddy fields over the long term by single application of iron materials. A revolving furnace slag (RFS; 245 g Fe kg(-1)) and a spent disposable portable body warmer (PBW; 550 g Fe kg(-1)) were used as iron materials. Samples of a soil: with a low iron level (18.5 g Fe kg(-1)), hearafter referred to as a low-iron soil and of a soil with a high iron level (28.5 g Fe kg(-1)), hearafter referred to as an iron-rich soil, were put into 3 L pots. At the beginning of the experiment, RFS was applied to the pots at the rate of 20 and 40 t ha(-1), while PBW was applied at the rate of 10 t ha(-1) only, and in the control both were not applied. Methane and nitrous oxide emissions from the potted soils with rice plants were measured by the closed chamber method in 2001 and 2002. When RFS was applied at the rates of 20 and 40 t ha(-1) to the low-iron soil, the total methane emission during the cultivation period significantly decreased by 25-50% without a loss of grain yield. Applied iron materials clearly acted as electron acceptors, based on the increase in the amount of ferrous iron in soil. However, the suppressive effect was not evident in the iron-rich soil treated with RFS or PBW. On the other hand, nitrous oxide emission increased by 30-95%. As a whole, when the total methane and nitrous oxide emissions in the low-iron soil were converted to. total greenhouse gas emissions expressed as CO2-C equivalents in line with the global warming potential, the total greenhouse gas emissions decreased by about 50% due to the application of RFS.