Effect of silicate iron slag containing electron acceptors on reducing methane emission from paddy soil during rice cultivation

Pil Joo Kim^1,2), Muhammad Aslam Ali³⁾, Chang Hoon Lee¹⁾, and Sang Yoon Kim¹⁾

¹⁾  Division of Applied Life Science, Graduate School, Gyeongsang National University, Jinju 660-701, South Korea

²⁾ Institute of Agriculture and Life Sciences, Gyeongsang National University, Jinju 660-701, South Korea

³⁾ Environmental Science, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh

Abstract

Application of electron acceptors such as ferric iron oxides and hydroxides for controlling methane (CH₄) emission from wetland rice fields deserves special attention due to its dominant role over all other redox species in wetland soils. Three industrial by-products such as fly ash, phosphogypsum and blast furnace slag (hereafter, silicate iron slag), being potential source of electron acceptors viz. ferric oxide, manganese oxide and sulfate were selected as soil amendments for reducing methane (CH₄) emission during rice cultivation. In the incubation test, CH₄ production rates in anoxic soil slurries significantly decreased with 0.5, 1, 2 and 5% (wt wt^-1) of soil amendments applied especially for phosphogypsum followed by silicate iron slag and fly ash. In the greenhouse pot experiment, CH₄ emission rates from the rice planted potted soils significantly decreased with the increasing levels (2-20 Mg ha^-1) of the selected amendments applied, while rice yield simultaneously increased over the control. Rice grain yield was most effectively increased by silicate iron slag among the selected amendments, and seasonal CH₄ emission was most highly decreased by phosphogypsum and silicate iron slag application. The suppression of CH₄ production rates as well as total seasonal CH₄ flux could be due to the increased concentrations of active iron, free iron, manganese oxides, and sulfate in the amended soil, which acted as electron acceptors and controlled methanogens' activity by limiting substrates availability. Conclusively, silicate iron slag among the selected amendments would be a suitable soil amendment for reducing CH₄ emissions as well as sustaining rice productivity.

Silicate iron slag, a byproduct of steel industry containing electron acceptors, was applied in paddy field at the rate of 0, 1, 2 and 4 Mg ha^-1 to investigate their effects on reducing CH₄ emissions from flooded paddy soil during rice cultivation. CH₄ emission rates measured by closed chamber method decreased significantly (p<0.05) with increasing levels of silicate iron slag application during rice cultivation. Soil redox potential (Eh) showed a contrasting response to CH₄ emission rates. The concentrations of dissolved iron materials in percolated water, and the active and free iron oxides in soil significantly increased with the applications of silicate iron slag, which acted as oxidizing agents and electron acceptors, and eventually suppressed CH₄ emissions during the rice growing seasons. Total CH₄ emission was decreased by 16-20 % with 4 Mg ha^-1 silicate iron slag application and simultaneously rice grain yield was increased by 13-18 %. Silicate iron slag application significantly stimulated rice plant growth, especially root biomass, root volume and porosity, which might have improved rhizosphere oxygen concentration, and then partially contributed to reduce CH₄ emission through enhancing methane oxidation. Therefore, silicate iron slag could be a good soil amendment for reducing CH₄ emission as well as increasing rice productivity in wetland paddy field.

Keywords: Methane emission, silicate iron slag, electron acceptor, paddy soil, rice.