People used to focus on the relationship between the greenhouse gases in the atmosphere and the soil organic carbon (SOC) as mutual source or sink, and little has been done on effect of greenhouse gases on decomposition and transformation of SOC per se, and properties of its fractions.SOC can be divided into humic acid (HA), fulvic acid (FA), humin (HM) and non-humic substances.Formation, decomposition and transformation of SOC (HA, FA and HM) are all known mainly to be soil biochemical processes, which, however, belong to the issue of thermodynamic stability, if they are addressed from the aspect of energy changes from the initial to the final states of SOC.There are a number of factors that affect SOC stability in soil.However, as long as thermodynamic stability is concerned, only the following three,i.e.water activity, partial pressures of oxygen and carbon dioxide, may do, when the temperature is set to be 25℃.This is based on the understanding that any kind of organic matter is made of water, oxygen and carbon dioxide and can be decomposed eventually to the three substances.In order to explore effect of CO₂ concentration on formation and transformation of SOC, an indoor incubation experiment with organic material added the but no crop planted is conducted under controlled conditions varying in CO₂ concentration.Dynamic changes in amounts of total organic carbon (TOC), water soluble substances (WSS), extracted humic substances (HE), HA and HM in the soil were determined, and contents of water floating substances (WFS) and FA in the corn stalks under incubation for 180 days, calculated with the revised humus composition method.Results show that the "newly formed" net TOC decreased in amount gradually with corn stalk decomposing, and the net HE increased in the first 7 days, and then decreases.The absolute amount of HM, however,decreased gradually.The proportion of HA to HE (PQ) exhibited an increasing trend in the beginning, and then leveled off, indicating that the formation of FA is faster than that of HA in the initial stage of incubation.HA and FA transformed reciprocally for a period of time, and reached dynamic balance finally with the incubation going on.The contents of TOC, WSS, HE and HM in the treatment 30% in CO₂ concentration (V/V) were significantly higher than in the treatment 3% in CO₂ concentration and normal in CO₂ concentration (0.037 5%), and WSS and HM were more sensitive to CO₂ concentration, but the difference between treatments 3% and normal CO₂ concentration was not significant.The three treatments are in the order of 30% CO₂ > 3% CO₂ > normal CO₂, in terms of PQ of extractable humus, suggesting that high CO₂ concentration is beneficial to the formation and stability of FA rather than HA.