Quantitative study on nitrification in flooded rice soil

Abstract

Abstract The nitrifier population and in vitro nitrifying activity in surface and reduced paddy soils of long-term fertility plots were measured. Plots without nitrogen, phosphorus, and potassium fertilizers, and those with fertilizers were compared. The results indicated that the nitrifier population and in vitro nitrifying activity were increased only in oxidized-surface soils in reponse to the application of nitrogen fertilizer. The 15N-nitrate dilution method was developed to assess the nitrification and the nitrate consumption activities in undisturbed soil core samples simultaneously. The core soil samples were taken from the paddy field using 1.7 cm diameter tubes, and the 1 cm surface layer was kept in the tubes. The tubes were incubated for 24 hr after adding 15N-labeled nitrate (25–40 μg N/tube). The nitrate was extracted with water, and the ammonium was excluded from the water extract using cation exchange resin. The nitrate was reduced to ammonia, and subjected to analysis of the 15N abundance by emission spectroscopy. N-serve depressed the nitrification on the surface of the paddy field, suggesting that the nitrification in the paddy soil was carried out by autotrophic nitrifiers. The nitrification rate was lower than the rate of nitrate disppearance. After the application of urea to the floodwater of the paddy field, the nitrifying activity of the surface soil was monitored by the 15N-NO3 dilution method. The highest nitrifying activity appeared 12 days after the urea application, amounting to 1.2 kg N/ha per day. These results provide quantitative data of nitrification in paddy soils.