Variations in the Process of Nitrification and Nitrate Reduction in Submerged Paddy Soils as Measured by¹⁵N Isotope Dilution Technique

Temporal variations in the process of nitrification and nitrate reduction in paddy fields after flooding were investigated by applying the 15N isotope dilution technique in laboratory model experiments using wet or air-dried soil. When the sample of wet paddy soil from the Hokuriku National Agricultural Experiment Station was submerged and kept at 25°C, an oxidative surface layer (1 cm thick) and an underlying reductive layer were differentiated after 20 days. Nitrifying activity in the oxidative surface layer increased from 100 ng-atoms N per g dry soil per day to 250 for 30 days, and declined later. Nitrate reducing activity was hardly detected. In the underlying reductive layer, the nitrifying activity did not vary appreciably after submergence but the nitrate reducing activity increased after 30 days and attained a level of 250 ng-atoms per g dry soil per day after 60 days. When the air dried soil was submerged, the two layers were distinguished within 5 days. The thickness of the oxidative surface layer was only 1 mm or less. Nitrifying activity in the surface layer (0–1 cm) which could not be detected during the first 15 days. increased up to 50 ng-atoms per g dry soil per day subsequently. Nitrate reducing activity in the surface layer increased almost linearly with time and attained a level of 200 ng-atoms N per g drY soil per day after 60 days.