摘要：

Soil samples were collected from an experimental paddy field with long-term(26 years)fertilization in Taihu Lake region of Jiangsu Province to study the effects of different fertilization on the organic carbon distribution and stable carbon isotope natural abundance(δ13C)in the soil profile,and on the humus composition.The results showed that long-term fertilization increased the organic carbon content in top soil significantly,and there was a significantly negative exponential correlation between soil organic carbon content and soil depth(P0.01).The organic carbon content in 10-30 cm soil layer under chemical fertilizations and in 20-40 cm soil layer under organic fertilizations was relatively stable.Soil δ13C increased gradually with soil depth,its variation range being from-24‰ to-28‰,and had a significantly negative linear correlation with soil organic carbon content(P0.05).In 0-20 cm soil layer,the δ13C in treatments organic manure(M),M+NP,M+NPK,M+straw(R)+N,and R+N decreased significantly;while in 30-50 cm soil layer,the δ13C in all organic fertilization treatments except R+N increased significantly.Tightly combined humus(humin)was the main humus composition in the soil,occupying 50% or more,and the rest were loosely and stably combined humus.Long-term fertilization increased the content of loosely combined humus and the ratio of humic acid(HA)to fulvic acid(FA).

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