Mineralization of soil organic nitrogen in upland fields as determined by a ¹⁵nh₄ ⁺ dilution technique, and absorption of nitrogen by maize

Mineralization rates of soil N in Hokkaido upland fields, Japan, were determined in the laboratory using ¹⁵NH₄ ⁺ dilution technique at ambient temperatures. The soil samples were collected every month from fields of a Brown Andosol and an Ordinary Andosol from 21 May until 22 September 1984. The gross mineralization rates obtained ranged from 1.15 to 2.21 μg N g^-1 day^-1 for the Brown Andosol, and from 1.60 to 3.77 for the Ordinary Andosol. The rates were highest in July for both the soils. Assuming a topsoil thickness of 35 cm, the total amount of soil N mineralized during the growing season of maize was calculated to be 521 kg N ha ^-1 in the Brown Andosol and 808 in the Ordinary Andosol. Amounts of soil N absorbed by maize in the Brown Andosol were determined in the presence or absence of applied (¹⁵NH₄)₂SO₄. The total amount of soil N absorbed was 56 kg N ha^-1 in the plot applied with N-fertilizer, and 46 kg N ha ^-1 in the plot without N-fertilizer. The values corresponded to 11 and 9%, respectively, of the total inorganic N mineralized during the growing season for maize.