Estimate of nitrogen cycling in ¹⁵N-amended soil during long-term submergence

We investigated temporal changes in the content of NH⁺₄ and ¹⁵N-NH⁺₄ in ¹⁵N-amended soil during long-term submergence. We also investigated changes in the gross rates of mineralization and immobilization of N in the submerged soils with and without the addition of rice straw. Although both the rates increased with time in the soils without rice straw, the ratio of the immobilization rate to the mineralization rate ranged between 88-97%. In the soils with rice straw, enhancement of immobilization during the initial stage and later increase of the mineralization rate were observed. The remineralization rate of immobilized ¹⁵N was estimated by applying a mathematical model to the changes with time in the content of NH⁺₄ and ¹⁵N-NH⁺₄. The rate constants of remineralization were calculated to be 0.023 day^-1 at 0.25 mm of applied (¹⁵NH₄)₂SO₂ and 0.019 day^-1 at 0.5 mm. Additionally, the rate constants of transformation to less readily-decomposable organic matter were estimated to be 0.015 day^-1 at 0.25 mm and 0.007 day^-1 at 0.05mm.