Abstract: With reddish yellow soil (RYS) and alluvial purple soil (APS), the two typical paddy soils in the Dongting Lake floodplain of China as test soils, an incubation test was conducted at 25 ℃ to study the dynamic changes of soil microbial biomass and dissolved organic carbon and nitrogen under flooded condition. Three treatments were installed,i . e . , control (CK), ammonium sulfate (N), and rice straw powder plus ammonium sulfate (S-N). The results showed that during incubation , soil microbial biomass carbon (SMBC), soil microbial biomass nitrogen (SMBN), soil dissolved organic carbon (SDOC), and soil dissolved organic nitrogen (SDON) reached their maximum initially, decreased thereafter, and tended to be stable. After amending the substrates to the two soils, the averages of SMBC to soil total carbon,SMBN to soil total nitrogen, SDOC to soil total carbon, and SDON to soil total nitrogen were 2%～3%, 2%～3%, 1% or so, and 5%～6%,respectively.In the two soils, the peak values of SMBC in treatment N and those of SMBN, SDOC and SDON in treatment S-N were the highest, while thaose of SMBC in treatments N and S-N had no significant difference. The peak values of SMBN, SDOC and SDON in RYS were significantly different between treatments N and S-N,while significant difference was observed between the peak values of SMBN and SDOC in APS, because the fertility of RYS was lower than that of APS. In the first 7 days of incubation, SMBC/SMBN ratio was ＜10, while after 14 days of incubation, this ratio was higher in treatment N than in treatment S-N at the same time in the same soil. The SDOC/SDON ratio in all treatments was the highest at the 3rd d, and the lowest at the 28th d of incubation.

Key words: elevated atmospheric CO₂ concentration, agricultural product, quality