The transformation of nitrogen was studied in a well-drained paddy soil by adding ¹⁵N labelled cratalaria,¹⁵N labelled wheat straw,(¹⁵NH₄)₂SO₄ and (¹⁵NH₄)₂SO₄ together with cellulos so as to adjust the C/N ratio of applied fertilizer from zero to 50 at the same N level.Rice was planted under three soil water regimes,i.e.submerged condition,submerged and percolated condition (1.5 cm/day),and dryland condition kept with 60% W.H.C.