Organic carbon composition and carbon sequestration function of cinnamon soil as affected by fertilization modes

Objectives We studied the evolution of organic carbon content and composition of cinnamon soil based on a long-term positioning experiment, to deeply understand the mechanism 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 was 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 N amounts for N₀, N₁, N₂, N₃ and N₄ were 0, 60, 120, 180 and 240 kg/hm²; the P₂O₅ amounts in P₀, P₁, P₂, P₃ and P₄ were 0, 37.5, 75, 112.5 and 150 kg/hm² ; the organic fertilizer dosage in M₀, M₁, M₂, M₃ and M₆ were 0, 22.5, 45, 67.5 and 135 t/hm², respectively. After harvesting maize in 1996, 2001, 2006, 2011, 2016 and 2020, soil samples in 0−20 cm soil layer were collected to determine the content of total and individual component of organic carbon. The relationship between carbon input and the content of total and individual organic carbon component, and the response of carbon sequestration rate and efficiency of treatment soils were assessed.

Results After 29 years of continuous experiments, N₂P₁M₁, N₃P₂M₃, N₄P₂M₂ and N₀P₀M₆ treatments increased the total 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 the respective initial values, N₀P₀M₀, N₁P₁M₀, N₃P₃M₀ and N₄P₄M₀ treatments decreased soil particulate organic carbon content by 0.41, 0.89, and 0.41 g/kg, and drastically decreased the readily oxidizable carbon content by 40.3%, 26.4% and 9.6%, respectively. 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 exhibited 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∙a), which were much higher than those in N₁P₁M₀, N₂P₂M₀, N₃P₃M₀, and N₄P₄M₀ treatments. The increment of all the organic carbon components showed 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 increases the total and various components of organic carbon contents, and increases the carbon sequestration rate and potential of the cinnamon soil. High organic fertilizer application rate is conducive to soil carbon sequestration and fertility maintenance, therefore, is considered an effective fertilization measurement to improve the fertility and function of carbon sink of cinnamon soil.