Decomposition and transformations of added organic substrates (¹⁴C labeled glucose and rice straw) in upland soil and paddy soil and their influences on mineralization of native soil organic C with soil samples collected from the red earth region of China were investigated under incubation at 25℃ and 45% water holding capacity (WHC) for 100 days.During this period,49.6% and 46.7% of glucose-C were mineralized in the upland soil and in the paddy soil,respectively,and the correspon sive values for straw-C were 25.2% and 21.8%.The influences of these amendments on soil microbial biomass C (B_C) and on the mineralization of soil native organic C were larger in the upland soil than in the paddy soil.The maximum percentages of glu cose-C assimilated by the microbial biomass were 23.5% and 21.6% for the upland soil and the paddy soil,respectively,and the corresponsive values for straw-C were 10.4%and 11.3%.Under the conditions,the turnover time of B_C,calculated from the gross turnover rate of ¹⁴C B_C in the treatment with glucose,was 329 days and 127 days for the upland soil and the paddy soil,re spectively.These results demonstrated that under soil moisture of 45% WHC,the decomposition rate of glucose and rice straw was faster in the upland soil than in the paddy soil,but the turnover rate of B_C was in reverse.