Characteristics of readily oxidizable organic carbon and soil organic carbon pool under long-term fertilization in cinnamon soils

The effects of 24 years of long-term fertilization on total organic carbon (TOC), total organic carbon storage (TOCs), net carbon sequestration efficiency (NCSE) and carbon pool management index (CPMI) of cinnamon soils were analyzed in order to provide theoretical basis for the evaluation of soil carbon pool change and quality, and to guide scientific application mode of fertilizers in cinnamon soils. Using data from long-term experiment on soil fertility and fertilizer use history, TOC and ROOC (readily oxidizable organic carbon) contents were analyzed and the relative indexes for TOCS, NCSE and CPMI were calculated for 9 different treatment conditions. The treatments included Group Ano fertilization treatment (N₀P₀, CK), Group Bsingle application of inorganic fertilizer treatments (N₁P₁, N₂P₂, N₃P₃ and N₄P₄), Group Cmixed organic and inorganic fertilizers treatments (N₂P₁M₁, N₃P₂M₃ and N₄P₂M₂) and Group Dsingle application of high amount of organic fertilizer treatment (M₆). Results showed that the application of medium and high organic manure (N₃P₂M₃ and N₄P₂M₂) increased the contents of TOC and ROOC in different soil layers and in different periods, but the promotion effect decreased with increasing soil depth. TOCS, NCSE and TOC had similar trends in both time and space. TOCS increased with increasing application of organic fertilizer (Group C, Group D), with 76.77% and 17.36% lower for average of Group A and Group B than that for Group C and Group D, respectively. Long-term fertilization improved net carbon fixation efficiency and the application of organic fertilizer significantly improved NCSE. NCSE decreased in the order of Group D > Group C > Group A > Group B. NCSE for Group D was 1 152.27 kg·hm^-2·a^-1, which was 2.51 times that of Group C, and 16.20 times that of Group B. Compared with CMPI before the experiment, there was no significant difference in CPMIs of Group C and Group D; while CPMIs of Group A and Group B decreased by 16.38-40.02. Also compared with control, the application of low-level inorganic fertilizer treatment (N₁P₁) and application of organic fertilizer of Group C and Group D significantly increased CPMI by 23.30-45.67. There was a significant positive correlation between CPMI and ROOC in the 0-40 cm soil layer, which implied that CPMI was a good indicator for organic carbon change. As noted, the application of high amount of organic manure or high amounts of organic and inorganic fertilizers (N₃P₂M₃ and N₄P₂M₂) significantly improved soil TOCS, NCSE and CPMI. In other words, application of high amounts of organic fertilizer increased organic carbon sequestration, decreased the application of inorganic fertilizer and enhanced better soil development and soil fertility.