Comparison of fate of nitrogen applied to 4 different kinds of soils with particular reference to denitrification

We determined the in situ denitrification rate of four different kinds of soils during the growing season of maize by the combination of an intact core method and an acetylene inhibition technique. The soils used were Low-humic Andosols, Cumulic Andosols, Gray Lowland Soils, and Yellow Soils to which 20 Mg ha^-1 of manure was applied every year. In addition, we simultaneously investigated the N balance sheet of the same maize fields using ¹⁵N-labelled fertilizer. The average denitrification rate during the growing season of maize (from May 21 to August 24, depth: 0-20 cm) was 2.0 g N ha^-1 d^-1 for Low-humic Andosols, 16.1 g N ha^-1 d^-1 for Cumulic Andosols, 20.3 g N ha^-1 d^-1 for Gray Lowland Soils, and 192.4 g N ha^-1 d^-1 for Yellow Soils, respectively. The denitrification rates in the subsurface layer (10-20 cm) were higher than those in the surface layer (0-10 cm) irrespective of the soils. Logarithm to the average denitrification rate throughout the growing season was highly correlated with the solid-phase ratio of the soils. Total amount of denitrified N from Yellow Soils during the growing season of maize was estimated to be 12% of applied N, whereas the amount from the other soils was lower than 2%. A considerable amount of nitrate remained in the subsurface layer (30-45 cm) of the Low-humic Andosols and Cumulic Andosols at harvest time. The major part of the remaining ¹⁵N-labelled fertilizer in the Gray Lowland Soils and Yellow Soils was distributed in the surface layer, which mainly occurred in an immobilized form. At harvest time, the ratio of ¹⁵N-labelled fertilizer which was unaccounted for was 19.2 ± 11.1% of the total for Low-humic Andosols, 44.4 ± 10.6% for Cumulic Andosols, 47.4 ± 9.8% for Gray Lowland Soils, 68.8 ± 12.0% for Yellow Soils, respectively. The discrepancy between the amount of denitrification and unaccounted loss indicates that leaching of nitrate was mainly responsible for the loss of N fertilizer. However, in the Yellow Soils, denitrification was considered to be a significant cause of N loss as well as leaching of nitrate.