Soils were incubated continuously in an atmosphere of (CO2)-C-14 and the distribution of labeled C into soil organic carbon (C-14-SOC) was determined at 0-1, 1-5, and 5-17 cm down the profile. Significant amounts of C-14-SOC were measured in paddy soils with a mean of 1,180.6 +/- 105.2 mg kg(-1) at 0-1 cm and 135.3 +/- 47.1 mg kg(-1) at 1-5 cm. This accounted for 5.9 +/- 0.7 % and 0.7 +/- 0.2 %, respectively, of the total soil organic carbon at these depths. In the upland soils, the mean C-14-SOC concentrations were 43 times (0-1 cm) and 11 times (1-5 cm) lower, respectively, than those in the paddy soils. The amounts of C-14 incorporated into the microbial biomass (MBC) were also much lower in upland soils (5.0 +/- 3.6 % and 2.9 +/- 1.9 % at 0-1 and 1-5 cm, respectively) than in paddy soils (34.1 +/- 12.4 % and 10.2 +/- 2.1 % at 0-1 and 1-5 cm, respectively). Similarly, the amount of C-14 incorporated into the dissolved organic carbon (DOC) was considerably higher in paddy soils (26.1 +/- 6.9 % and 6.9 +/- 1.3 % at 0-1 and 1-5 cm, respectively) than in upland soils (6.0 +/- 2.7 % and 4.3 +/- 2.2 %, respectively). The observation that the majority of the fixed C-14-SOC, RubisCO activity and cbbL gene abundance were concentrated at 0-1 cm depth and the fact that light is restricted to the top few millimeters of the soil profiles highlighted the importance of phototrophs in CO2 fixation in surface soils. Phylogenetic analysis of the cbbL genes showed that the potential for CO2 fixation was evident throughout the profile and distributed between both photoautotrophic and chemoautotrophic bacteria such as Rhodopseudomonas palustris, Bradyrhizobium japonicum, Rubrivivax gelatinosus and Ralstonia eutropha.