Nitrate reduction and denitrification in flooded soils

Abstract

It has been well known that the utilization rate of ammonium sulfate fertilizer by lowland rice was as low as 40 percent of the applied ammonium. This low utilization rate was due to nitrogen loss from the paddy field. There are problems as to whether the loss of nitrogen from the flooded soil was caused by the nitrification of ammonium nitrogen and its subsequent denitrification, by the evaporation of ammonium, or by the leaching of ammonium-nitrogen with percolated water. Shioiri and his associatesll clarified that this loss of nitrogen resulted largely from denitrification through nitrate reduction in 1942. After the paddy soil is flooded with water, the oxygen in furrow slice is consumed by aerobic microorganisms, and then the soil becomes reductive. Conversely the oxigen is supplied to the soil through the water from the air, and from various kinds of algae and duckweeds, which produce oxygen through photosynthesis. At the early stage the reduction by oxygen consumption is superior to the oxidation by oxygen supply, the furrow slice is reductive, and is bluish gray in color due to the presence of certain ferrous compounds. After months flooding, the oxygen supply becomes superior to oxygen consumption and the uppermost layer of furrow slice becomes brown in color due to the presence of ferric compounds. This layer corresponds to an “oxidised layer” where microorganisms live aerobically. In this oxidized layer the nitrifying bacteria converts ammonium nitrogen into nitrate nitrogen which is percolated into the reduced layer, and lost through denitrification. A large amount of ammonium sulfate fertilizer is then dressed at the uppermost layer, after flooding, the loss of nitrogen through denitrification is serious.