Soil carbon is a crucial component of the global carbon cycle, and carbon mineralization is influenced by various factors. However, there is a lack of systematic analyses on the responses of carbon mineralization in different soil types to the addition of exogenous organic matter. This study investigates the effects of compost addition on the mineralization and kinetic characteristics of soil carbon across three typical agricultural soils: paddy soil, black soil, and cinnamon soil. A 210-day incubation study was conducted with four treatments: Control (un-amended soil), R (soil + straw), R1M (soil + straw + low compost application rate), R2M (soil + straw + high compost application rate). The results showed that the CO₂ emission rates of the three soils were higher during the early stage (1–37 days) and decreased afterward. The CO₂ emission rates of the paddy soil and the black soil were significantly higher than those of the cinnamon soil. The addition of compost significantly increased both the CO₂ emission rate and the cumulative release of CO₂, especially in the R2M treatment. At the end of the incubation, the SOC contents were higher in the R2M treatment than in the Control for all three soils (p < 0.05), with the most notable increase in the cinnamon soil (60.93%). Compost addition significantly enhanced the active carbon pool (C_a), slow carbon pool (C_s), and potentially mineralizable carbon pool (C_p), while decreasing the mineralization rate (k_a) of the C_a, but the effect on the mineralization rate (k_s) of the C_s and mineralization entropy (C_m) varied by soil types. The k_s of the paddy soil was significantly reduced by 23.08% under the R1M and R2M treatments compared with the Control and R treatment. The k_s of the black soil was significantly increased by 59.52% under the R2M treatment compared with the Control. The k_s of the cinnamon soil was elevated considerably by 79.31% under the R2M treatment compared with the Control, R, and R1M treatments (averaging 0.29 × 10⁻² d), and the k_s of the paddy soil and black soil were significantly higher than those of the cinnamon soil under the R2M treatment. The C_m was significantly higher in the organic material added treatments than in the Control for the black soil and the paddy soil, but showed a higher value in the R treatment than in the R2M and Control for the cinnamon soil. In conclusion, compost addition stimulated soil carbon mineralization and improved the SOC content, especially in the cinnamon soil, while reducing the mineralization rate of the active carbon pool across the three soils. The mineralization rate of the slow carbon pool and the changes in mineralization entropy were dependent on soil types, primarily related to the initial soil nutrient contents, pH, and particle compositions. These findings offer valuable insights for managing the soil carbon pool in agricultural ecosystems.

Iron redox cycling in paddy soils drives the release and mineralisation of dissolved organic carbon (DOC), influencing the emission of CO₂ and CH₄. Light irradiation exerts an inhibitory effect on the mineralisation of soil organic carbon, but the responses to light intensity of iron redox processes coupled with organic carbon transformation and greenhouse gas emissions remain underexplored. Here, we conducted a slurry incubation experiment with paddy soil at varying light intensities. The dynamics of soil ferrous iron [Fe(II)], DOC, dissolved inorganic carbon (DIC), and chlorophyll a, as well as headspace CO₂ and CH₄, were monitored over a 40-day period. The results demonstrated that light irradiation inhibited iron reduction, leading to a 58.1–74.7% decrease in soil Fe(II) concentration compared to dark incubation. The oxidation of Fe(II) generated from iron reduction was enhanced under light incubation (3.12–3.53 mg g⁻¹), and the oxidation rate constant trended higher with increasing light intensity. Light irradiation reduced CO₂ and CH₄ emissions to 8.8–76.9% and 2.3–6.7% of those under dark incubation, respectively. With the extension of incubation time, soil DIC concentration showed an increase followed by a decrease under light incubation, and the earlier DIC decrease occurred at higher light intensities. The DOC decrease rate constant was greater under light incubation (0.024–0.042 d⁻¹) than under dark incubation (0.012 d⁻¹). Light irradiation activated phototrophic microorganisms producing chlorophyll a (4.71–6.46 mg g⁻¹), whereas this pigment was undetectable under dark incubation. Organic carbon mineralisation was positively correlated with Fe(II) concentration, and Fe(II) oxidation was positively correlated with chlorophyll a concentration and DOC decrease (p < 0.05). Agricultural practices optimizing light exposure, such as shallow flooding or reducing plant density, are promising approaches to bolster DOC sequestration and mitigate CO₂ and CH₄ emissions in paddy fields.

To discriminate the transport characteristics of residue-derived carbon (C_res) from soil native carbon (C_soil) in black soil with split nitrogen application, a 540-day incubation study was conducted with four treatments: Control (unamended soil), R (soil + residue), RN1 (soil + residue + one-time application of nitrogen fertilizer), and RN3 (soil + residue + three-time application of nitrogen fertilizer). The total soil organic carbon (TOC) of the incubated soil was separated into three fractions: light fraction (LF), occluded-particulate organic matter fraction (OPOM), and heavy fraction (HF). The results showed that the TOC content was significantly higher in the RN1 and RN3 (averaging 20.77 g/kg) than in the R (18.43 g/kg) and Control (19.03 g/kg) after 540 days. Nitrogen fertilization significantly increased the residual rate of HF−C_res by 11.75% (p < 0.05), and the RN3 treatment significantly increased the residual rate of OPOM−C_res by 18.84% (p < 0.05) and reduced the loss rate of LF−C_soil by 77.01% (p < 0.05) compared with the R treatment. The soil catalase activity declined continuously along with incubation and was higher in the RN3 treatment than in the RN1 treatment after 180 days. The correlation analysis showed that the LF−C_soil and −C_res, as well as the HF−C_soil and catalase activity, were the main contributors to the TOC. Conclusively, nitrogen application, especially split nitrogen application, could stimulate the ability of soil to retain exogenous carbon and preserve native carbon.