The $N{H}_4^{+}$-N, microbial biomass-N, humus-N, and extractable organic N derived from the added ¹⁵N-labelled ryegrass and soil indigenous pool were measured separately with ¹⁵N tracing techniques. Based on the recovery of $N{H}_4^{+}$-¹⁵N and lost-¹⁵N (mainly as NH₃), more than 30% of the added ryegrass ¹⁵N was mineralized in 15 d. The amount of mineralized N increased with time up to 90 d for all soils except for the upland soil in which it decreased slightly. The mineralization of ryegrass N and incorporation of ryegrass-¹⁵N into microbial biomass was greatest in upland soil. The transformation of ryegrass ¹⁵N into humus ¹⁵N occurred rapidly in 15 d, with higher humus ¹⁵N occurring in the upland or tea-garden soil than the paddy and unarable soil. The addition of ryegrass caused additional mineralization of soil indigenous organic N and enhanced the turnover of both microbial biomass N and stable organic N in soils.