Abstract: The effects of cropping changes front upland maize to flooded rice cultivation on soil organic carbon (SOC) were studied. Three treatments of field experiments; continuous maize (M treatment), continuous paddy rice (R treatment) and maize-rice rotation (RM treatment) were made. Cropping change from maize to flooded rice resulted in an increase in soil bulk density and soil organic carbon content when compared to that of maize-lice rotation and continuous maize. The total soil organic carbon after two cropping was 16.50, 20.88 and 19.35 ton c ha'1 in the M, R and RM treatments, respectively. The effects of these short-term cropping changes were observed at both the aggregate level and in the humic substances of soil organic matter. The majority of soil organic carbon (ca. 65%) was present in association with the macro-aggregate (>250 pm), of which the fraction size of 250-500 pm contained the highest carbon concentration. After two cropping, the 813c values of SOC and humic substances were shifted towards the 813c values of rice straw when soil was incorporated with rice straw. The shift of 813c values towards maize straw’s 813c values was also observed in cases when maize straw was incorporated into soil. The results demonstrate that a shift from upland maize to flooded rice could enhance soil carbon sequestration, and decomposition and incorporation of organic materials (maize and rice straw) into SOC and humic substances (hiinic acid, fulvic acid and humin) was detectable within a short time period.
Keywords: Soil organic carbon, cropping change, natural 13c abundance, soil aggregate, humic substances.