Abstract: Soil microbial biomass carbon (MBC) and dissolved organic carbon (DOC) are the main components of soil active organic carbon, and can rapidly respond to the changes of soil environment. In this study, ¹⁴C-labeled glucose and rice straw were added to an upland soil and a paddy soil sampled from the red earth region of China, and the effects of the amendment on the active organic carbon in the two soils were investigated with laboratory incubation under 45% water holding capacity (WHC). The results showed that the DOC derived from the exogenous organic substrates was assimilated immediately by soil microbes, and the MBC reached the maximum during the first 5 days of incubation. With the amendment of glucose and rice straw, the peak value of MBC in upland soil and paddy soil was 69.4% and 55.1%, and 10.2% and 10.5% higher than the control, respectively. In the upland soil added with glucose and straw, the average content of MBC in the whole incubation period (100 d) had a significant increase (P<0.05), being 82.1% and 41.5% higher than the control, respectively;while in the paddy soil added with these organic substrates, the MBC had an increase of 8.8% and 5.1% during 0-60 d, but was similar to the control thereafter. The amendment of glucose and rice straw had no significant effects on the total DOC content in upland soil except in the first 2 days of incubation, but increased it by 12.8%-26.0% in paddy soil. The percentages of ¹⁴C-labeled MBC to total MBC and of ¹⁴C-labeled DOC to the total DOC in the two soils were 4.5%-47.4% and 4.0%-12.7%, respectively. It was suggested that under the condition of 45% WHC, the effects of the amendment of organic substrates on MBC were larger in upland soil than in paddy soil, while those on DOC were in adverse. ¹⁴C tracer showed that soil organic matter itself was the main source of MBC and DOC.

Key words: Land use, Landscape pattern, Change, Taihu Lake Basin