Uniformly ¹⁵N -labelled ryegrass was used to investigate$NH\frac{4}{+}$ -production, microbial transformation and humification of organic N in two types of red soils by incubating the soils amended with labelled material. The results showed that there was little significant difference in biomass N transformation in the tested soils between ¹⁵N tracer method and conventional method, but the amount of$NH\frac{4}{+}$ -N from ryegrass measured with ¹⁵N method was significantly higher than that measured with conventional method. The results also indicated that there was more$NH\frac{4}{+}$ -N released from the ryegrass in the clayey soil than in the sandy soil at all sampling time. By 120 d of incubation, humified N was less than 10% of the amount of the applied N in two types of red soils and the amount of residual N in the clayey red soil was obviously higher than that in the sandy red soil.