Abstract: 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 ncreased the organic carbon content in top soil significantly, and there was a significantly negative exponential correlation between soil organic carbon content and soil depth (P<0.01). 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 (P<0.05). 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).

Key words: biomass allocation, photosynthetic capacity, cultivar, Chenopodium quinoa, growth trait, nitrogen application rate