Decomposition of hybrid elephant straw ¹⁴C in situ in the field was studied using the ¹⁴C-tracer technique in yellow-brown earth under a wheat-rice rotation system for one year.Results indicated that the amount of straw added had little effect on its decaying rate.After one year, straw ¹⁴C decomposed by about 72% with a decaying rate constant of 2.7×10^-3 d^-1.However, straw addition was closely related to decomposition of native soil C and balance of soil carbon.Soil native carbon decomposed by 5.45% ~ 6.07% annually with a decaying rate constant from 1.04×10^-4 d^-1 to 1.18×10^-4 d^-1.With straw addition, decomposition of straw ¹⁴C increased in both amount and rate, while the net loss of soil organic C decreased.Biomass ¹⁴C accounted for 3.79%~10.63% of input ¹⁴C and 12.27%~17.43% of soil residual ¹⁴C.Its variat ion was much significant than that of biomass ¹²C.The ratio of biomass ¹²C to biomass ¹⁴C varied from 0.74 to 3.85, which indicated that under most situations, native soil C was still the major source of energy and nutrients for microorganisms.Turnover rates of soil biomass ¹⁴C and ¹²C were 1.10~1.18 a^-1 and 0.97~1.06 a^-1 respectively.Straw addition could accelerate turnover of soil organisms, and in turn increase in the turnover rate of soil organisms accelerate the decaying rate of straw C and soil native C.Decomposition processes of soil native C and straw C showed similar trends to dynamics of soil biomass ¹²C and soil biomass ¹⁴C, which indicated that decomposition rates of organic C were the apparent indication of activity of soil organisms.