Abstract: Soil inherent and exogenous organic carbon are the carbon sources of soil respiration, but rarely distinguished in the previous studies about the effects of different fertilization modes on the carbon emission from paddy soils. In this study, three original treatments in a 30-year fertilization experiment of paddy soil were adjusted to seven treatments, i.e., original high organic material treatment (HOM), high organic material treatment changed from original normal organic material treatment (N-H), original normal organic material treatment (NOM), normal organic material treatment changed from chemical fertilizer (C-N), chemical fertilizer treatment changed from high organic material treatment (H-C), chemical fertilizer treatment changed from normal organic material treatment (N-C), and original chemical fertilizer treatment (CF). The CO₂ fluxes of the three original and four changed treatments were measured to study the effects of different fertilization modes on the CO₂ emission and to illuminate the effects of soil inherent and exogenous organic carbon on the CO₂ fluxes. The average CO₂ fluxes of the seven different fertilization treatments were 85.34, 69.10, 51.27, 49.15, 14.89, 12.92, and 11.59 mg C·m^-2·h^-1, respectively. In the treatments of inorganic fertilization and normal organic fertilization, the level of soil inherent organic carbon had no significant effects on the average CO₂ flux; while in the treatments of high organic fertilization, high level of soil inherent organic carbon promoted the average CO₂ flux. The relationship between the CO₂ flux (Y) and the exogenous carbon content (x) met the exponential equation Y=13.33e^1.719x (R²=0.967, n=21), indicating that the exogenous organic carbon had significant effects on the soil CO₂ flux. About 14% of the exogenous organic carbon was decomposed and released as CO₂-C during rice growth season, and this decomposition rate was less affected by soil organic carbon content and the added amount of organic material.

Key words: soil enzyme activities, inorganic nitrogen., rhizosphere, nitrogen application rate