Effects of Long-term Fertilization on the combined forms of humus

Longterm application of organic manure, or combined application of inorganic fertilizer and organic manure could significantly increase the level of loosely combined humus, stably combined humus and tightly combined humus in the topsoil of fluvoaquic soil, arid red soil and paddy red soil. The combined application was the most effective. Combined application of organic manure and inorganic fertilizer could increase the ratio of stably combined humus to organic carbon in heavy groups in the topsoil of the three soils, and the ratio of loosely combined humus to organic carbon in heavy groups in the topsoil of fluvoaquic soil, while it could decrease the ratio of tightly combined humus to organic carbon in heavy groups in the topsoil of the three soils. Single application of organic manure or chemical fertilizer could also increase the ratio of the three combined forms of humus to organic carbon in heavy groups in the topsoil of fluvoaquic soil, while it could decrease the ratio of tightly combined humus to organic carbon in heavy groups in the topsoil of arid red soil and paddy red soil. Thus the ratio of loosely combined humus to tightly combined humus could be increased in the topsoil of fluvoaquic soil, arid red soil and paddy red soil under longtime fertilization.