【Objective】To provide a basis for an advanced study on the impacts of biochar on soil carbon and nitrogen cycling, the effects of residence time after isotope labeling on the δ¹³C and δ¹⁵N values of rice aboveground and underground tissues and biochars derived from these two tissues were studied.【Method】Rice plants were labeled with ¹³Cand ¹⁵N by pulse labeling of ¹³C-CO₂ and foliar fertilization of ¹⁵N-urea, respectively. The residence time of 4 h, 6 h and 24 h was set after ¹⁵N isotope labeling. The labeled rice plants were divided into aboveground and underground tissues and four biochars were produced by pyrolyzing these two tissues at 300℃ and 500℃, respectively. The δ¹³C and δ¹⁵N values of rice tissues and their biochars were determined by isotope mass spectrometry.【Result】With the increase in residence time, δ¹³C values of the aboveground tissues of rice plants gradually decreased from 872‰ to 578‰, while the δ¹³C values of the underground tissues gradually increased from 226‰ to 869‰. Unlike with the δ¹³C, the δ¹⁵N values in the aboveground tissues of rice plants first increased then decreased, and the maximum(1 764‰)occurred at the residence time of 6 h, while δ¹⁵N values in the underground tissues first decreased and then increased. Overall, compared to rice tissues, the δ¹³C and δ¹⁵N values of the biochars decreased by 52.1% and 15.9%, respectively. Moreover, both the δ¹³C and δ¹⁵N values of the biochars were highest at the residence time of 24 h, especially for the 300℃ biochar. With the increase in residence time, the reduced proportion of the δ¹³C of hot water extractable dissolved organic carbon when compared with that of the residual solid increased from 4.14% to 11.0% for the 300℃ biochar, while it decreased from 32.3% to 18.9% for the 50 ℃ biochar. This indicates that increase in the residence time decreased and increased the uniformity of ¹³C of the 300℃ and 500℃ biochars, respectively.【Conclusion】Our results demonstrate that the effects of residence time after labelling on δ¹³C and δ¹⁵N values of the rice plants were different, and the biochars did not retain the isotopic signature of the raw rice tissues. Residence time and pyrolysis temperature together affected the uniformity of ¹³C in rice biochars.