Abstract:A field experiment was conducted at Key monitoring Experimental Station for Reddish Paddy Soil Eco-Environment in Wangcheng, Hunan to study the effect of applying controlled release nitrogen fertilizer (CRNF) on soil microbial biomass carbon (SMBC) and nitrogen (SMBN) contents. There were five treatments in this experiment: ① CK, non-N application; ② Urea, (N-source is urea); ③ CRNF, (N-source is CRNF, the same N rate as treatment ②); ④ 70%CRNF, (N-source is CRNF, 70% N rate of treatment ②); ⑤ 50%CRNF+M, (N-source are CRNF and pig manure, the same N rate as treatment ④, the nitrogen ratio of CRNF to pig manure is 5:2). Results showed that the highest contents of SMBC and SMBN in soil were detected in all treatments supplied with exogenous N at 10 d after fertilizers application, after that significantly decreased as growth stage progressed, finally a marked increase was observed at the late stage of rice growth. Results also revealed that the content of SMBN from CRNF treatment was 5.4% and 22.5% higher than those from Urea treatment; The contents of SMBC and SMBN in the CRNF treatment were decreased much more than those in the Urea treatment during rice middle and late growh satges, implying that, more nitrogen fertilizer was transferred to rice by using CRNF; In the treatments with CRNF application, the content of SMBN was positively related to the rate of N supply in most of sampling times, on contrary the content of SMBC decreased with an increase of the N rate; Compared to 70%CRNF, additional organic N had a remarkable impact on the contents of SMBN and SMBC. These findings suggest that a combination of CRNF and organic fertilizer can not only save the N usage but also increase the contents of SMBN and SMBC in soils.