Abstract: 【Objective】 Biochar are found having efficacy of retaining ions and reducing nutrient loss when applied into soils. The differences of their beneficial effects of biochar made in different temperatures were studied to provide a base for efficient biochar manufacture and reasonable use in cotton production. And the effect of three N fertilizer rates on cotton growth, N use efficiency and N leaching was studied using 15N tracer techniques. 【Methods】 Cotton straw was thermodynamic decomposed to make biochar at 450℃(450BC), 600℃(600BC) and 750℃(750BC), respectively. A two factor pot experiment was designed using the three kinds of biochar and three levels of nitrogen. The three kinds of biochar recorded as 450BC, 600BC and 750BC, with no biochar addition as control(CK). In each kind of biochar treatment, three levels of 15N labeled nitrogen fertilizer: 0(N0), 2.1(N1) and 4.2 g/pot(N2) were added. The biomass and yield of cotton and the N use efficiency were measured. 【Results】 1) The cotton dry weights were generally increased in order of 750BC600BC、450BC CK,the increase in cotton dry weights was averaged 9.2% in the 450BC treatments, 12.6% in 600BC treatments and 17.3% in the 750BC treatments. The cotton dry weights increased with the increased rate of N application. 2) The N uptake in cotton was generally increased,and the increase in the 750BC treatments was higher than in 600BC treatments, and also higher than in 450BC treatments regardless of the N application rates. When the N rate was N0, the nitrogen uptakes in the cotton in both the 750BC and the 600BC treatments were significantly higher than in CK,not significantly higher than in 450BC treatment. The input of N increased the N uptake in cotton in all the biochar treatments. The biochar addition averagely increased the N uptake by 29.5% in 450BC treatments, 37.1% in 600BC treatments, and 48.8% in 750BC treatments,respectively. 3) The 15N tracer data showed that compared with the CK treatment, plant 15N recovery in the 450BC treatment increased by 27.46% and plant 15N recovery in the 600BC treatment increased by 36.44%, and plant 15N recovery in the 750BC treatment increased by 42.87%. But in two N rates(N1, N2), there were no significant difference among the three biochar treatments. 4) Soil 15N recovery in the 450BC treatment increased by 101.4%, and 147.3% in the 600BC treatment, and 200.7% in the 750BC treatment relative to the CK treatment. In case of lower N rates(N1), soil 15N recovery increased as the biochar pyrolysis temperature increased. In N2 treatments, the soil 15N recovery in the 750BC was significantly higher than in both the 450BC and the 600BC treatments, but the difference between the 600BC treatment and the 450BC treatment was not significant. Soil 15N recovery decreased as N application rate increased in all three biochar amended treatments. 5) The 15N leaching in the biochar amended treatments was significantly lower than that in the CK treatment. Furthermore, 15N leaching decreased as the biochar pyrolysis temperature increased. 【Conclusions】 The beneficial effects of biochar increase as the pyrolysis temperature increased, and decrease as the N application rate increased. Therefore, cotton straws are recommended to be treated at high temperature, and the N input should be reduced for the maximum positive effects of biochar on crop growth and N use efficiency.