Objectives We studied the evolution of organic carbon content and composition of cinnamon soil based on a long-term positioning experiment, to deeper understand the machanism of organic fertilizer on the carbon sequestration of soil, and to provide nutrient management basis for improving the function and productivity of cinnamon soil in maintaining ecological environment stability.

Methods The long-term fertilization experiment started since 1992, and located in the National Field Scientific Observation and Research Station of the Dry Land Agricultural Ecosystem in Shouyang, Shanxi Province, with planting system of one crop a year. An orthogonal design of three factors and four levels was used for the experiment, and nine of the 18 treatments were chosen for this research, including one blank control (N₀P₀M₀), four chemical fertilizer treatments (N₁P₁M₀, N₂P₂M₀, N₃P₃M₀, and N₄P₄M₀), and four chemical and manure combination treatments (N₂P₁M₁, N₃P₂M₃, N₄P₂M₂, and N₀P₀M₆). The four N rate were 60, 120, 180, 240, and 300 kg/hm²; the four P rate were 37.5, 75, 112.5, and 150 kg/hm²; and the organic fertilizer rate for M1, M2, M3, and M6 were 22.5, 67.5, 45, and 135 t/hm². Soils (-20 cm) were sampled in frequency of 5-years, i.e. at harvest of maize in 1996, 2001, 2006, 2011, 2016, and 2020, for the determination of total organic matter and organic carbon component contents. The relationship between carbon input and the content of soil organic carbon and each carbon component, and the response of carbon sequestration rate and efficiency of treatment soils were assessed.

Results After 29 years of continuous experiments, N₃P₂M₃, N₄P₂M₂ and N₀P₀M₆ treatment increased soil organic carbon content by 25.7%, 72.4%, 30.3% and 220%, respectively, and significantly increased the contents of various carbon components, compared with N₀P₀M₀; N₁P₁M₀, N₃P₃M₀ and N₄P₄M₀ treatments decreased soil particulate organic carbon content by 0.41, 0.89, and 0.41 g/kg, respectively, and drastically decreased the readily oxidizable carbon content by 40.3%, 26.4% and 9.6%, respectively, compared with the initial test value. During the 29 years period, the soil carbon pool management indices of all the treatments decreased sharply at first and then gradually increased, N₀P₀M₆ treatment was tested the highest carbon pool management index, which was 6.6 times higher than that of N₀P₀M₀. The total soil organic carbon sequestration rates in N₂P₁M₁, N₃P₂M₃, and N₄P₂M₂ treatments were 0.22～0.58 t/(hm²∙yr), which was much higher than those of N₁P₁M₀, N₂P₂M₀, N₃P₃M₀, and N₄P₄M₀ treatments. The increment of all the organic carbon components had a positive and linear relationship with the cumulative carbon input, and did not reach platform within the tested organic carbon input levels yet.

Conclusions Long-term combined application of organic and inorganic fertilizers significantly increased the total and each component of organic carbon contents, and increased the carbon sequestration rate and potential of the cinnamon soil. The application of high amounts of organic fertilizers is the most conducive to soil carbon sequestration and fertility maintenance. Therefore, long-term and high rate of organic fertilizer input is an effective fertilization measurement to improve the fertility and function of carbon sink of cinnamon soil.