The mineralization and transformation of both added organic nitrogen and native soil N in red soils from four different ecological conditions

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The NH 4 +-N, microbial biomass-N, humus-N, and extractable organicN derived from the added 15 N labelled ryegrass and soil indigenous pool were measured separately with 15 N tracing techniques. Based on the recovery of NH 4 +- 15 N and lost 15 N (mainly as NH 3), more than 30% of the added ryegrass 15 N 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 15 N into microbial biomass was greatest in upland soil. The transformation of ryegrass 15 N into humus 15 N occurred rapidly in 15 d, with higher humus 15 N 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.