Objectives The contents of particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) in soil respectively represent the activity and stability of soil organic carbon. Exploring the variation characteristics of POC and MAOC in reclaimed soil of coal mining areas under different fertilization measures and their key driving factors can provide a scientific basis for formulating scientific and effective field management strategies to enhance soil organic carbon sequestration.

Methods The reclamation fertilization positioning experiment began in 2008, and three fertilization treatments were included: no fertilization (NF), application of inorganic NPK fertilizers (NPK), and combined application of matured organic nutrients and NPK fertilizers (NPKM). A nearby normal farmland was used as the control (NL), and the cropping system was continuous corn cultivation. After the corn harvest in 2023, soil samples (0-20 cm) were collected to determine the physical and chemical properties, POC and MAOC contents, and the total phospholipid fatty acids (PLFA) and PLFA contents of various microorganisms in the soil were measured by gas chromatography.

Results Both the NPK and NPKM treatments significantly increased the contents of soil organic carbon (SOC), POC and MAOC in the reclaimed soil, relative to NF, however, the SOC contents were still significantly lower than the normal farmland (NL), being 62.0% and 67.3% of the normal farmland, respectively. Moreover, the ratio of POC/MAOC in the reclaimed soil was significantly lower than that in NL, and the ratio of POC/SOC was lower than that in NL, while the ratio of MAOC/SOC was higher than that in NL. There were no significant differences in total PLFA between the NPK and NPKM treatments and compared with NL, and no significant differences in PLFA contents of Gram-positive bacteria (G⁺), Gram-negative bacteria (G⁻), aerobic bacteria, and fungi, while both treatments were significantly lower than NL. Compared with NF, NPK and NPKM treatments significantly increased the contents of TN, TP, AN, AP, microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) in the reclaimed soil, decreased soil pH, and the benefit effects of the two treatments were not significant different, except that the MBN of NPK was higher than MNPK. The contents of POC and MAOC in the soil were negatively correlated with pH (R²=0.85, P<0.001; R²=0.73, P<0.001), and positively correlated with SOC, TN, AN, AP, AK, and MBC (R²=0.50−0.88, P<0.05), and positively correlated with total PLFA, G⁺, G⁻, aerobic bacteria, and fungi PLFA. The contents of soil organic carbon components were closely related to the basic physical and chemical properties and the composition of the soil microbial community. The linear slope of POC and various microbial PLFA was greater than that of MAOC, indicating that POC was more sensitive to soil properties and microbial community changes. The random forest model showed that SOC, TN, and AN were the main soil factors affecting POC in the reclaimed soil, and pH, SOC, and TN were the main influencing factors for MAOC.

Conclusions For 15 consecutive years of application of mineral NPK fertilizers alone or in combination with organic fertilizers effectively increased the total SOC, POC and MAOC contents, enhanced the total and available nitrogen and phosphorus contents, but decreased pH of the reclaimed soil, thus leading to the increased number of various microorganisms. Although the pH is still higher, SOC lower than the normal farmland soil, the stability of organic carbon in the reclaimed soil is improved. The high soil quality and the enhanced activity of soil microorganisms brought by fertilization promots the turnover and accumulation of soil organic carbon, so fertilization management are effective strategies for increasing the organic carbon storage sequestration and rapid recover of function of reclaimed soil.