Evolvement characteristics and coupling relationship of soil organic carbon and total nitrogen in subtropical paddy field ecosystem under different fertilization practices

Soil samples were collected from long-term monitoring plots of paddy field ecosystem in Hunan Province to study the evolvement characteristics and coupling relationship of soil organic carbon and total nitrogen under different fertilization practices. The results showed that in 1986-2003, the contents of soil organic carbon and total nitrogen had a slight decrease under no fertilization (CK), basically remained stable under chemical fertilization (NPK), and increased under the combined application of organic manure and chemical fertilizers. Compared with those in CK, the contents of soil organic carbon and total nitrogen in treatments NPK, low application rate organic manure, and high application rate organic manure were increased by 13% and 18%, 54% and 45%, and 89% and 67% respectively. There was a significant positive correlation (P < 0.01) between soil organic carbon and total nitrogen. The soil C/N was around 10, with the majority ranged from 8.5 to 12.9. It was suggested that a combined application of o