Abstract: To study the decomposition and transformation patterns of exogenous organic carbon after returning to fields and investigate the effects on the constitution,transformation and characteristics of organic matters in the soil of tobacco-rice rotation fields,¹⁴C-labelled ryegrass roots were added into tobacco planting soils at the rates of 0,0.3% and 0.6% and cultured with carborundum tubes.The results showed that:1) The decomposition of exogenous organic ¹⁴C proceeded rapidly at the early period and then slowed down,its mineralization rate was over 50% during whole flood-drought cultivation,between 20%-30% during drought cultivation and about 30% during flood cultivation,respectively.2) Due to continuous tobacco cropping,the content and quality of soil organic matter(SOM) decreased.Exogenous organic ¹⁴C did not significantly affect the content of SOM,however,the composition and characteristics of organic matter changed greatly,for example,the proportion of humic acid(HA) increased.3) The quantities of HA and fulvic acid(FA) transformed by endogenous organic C and exogenous organic ¹⁴C were in dynamic equilibrium during drought cultivation.While the quantity of FA transformed by exogenous organic ¹⁴C and the humification coefficient of SOM increased during flood cultivation.These results suggest that straw returning and tobacco-rice rotation are the effective cultural measures for improving the quality of SOM in tobacco fields.