Effects of fertilization on particulate and mineral-associated organic carbon and their key drivers in reclaimed coal mining soils

Objectives The particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) represent the active and stable fractions of soil organic carbon (SOC), respectively. Exploring the variation of POC and MAOC in reclaimed soil of coal mining areas under different fertilization regimes and identifying their key driving factors can provide a scientific basis for formulating scientific and effective field management strategies to enhance soil organic carbon sequestration.

Methods A long-term fertilization experiment was established in 2008, and three fertilization treatments were included: no fertilization (NF), application of inorganic NPK fertilizers (NPK), and combined application of matured organic fertilizer and NPK fertilizers (NPKM). a nearby normal farmland was used as the control (NL) was used as the control, and the cropping system was continuous corn cultivation. After the corn harvest in 2023, soil samples (0−20 cm) were collected to determine 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 using gas chromatography.

Results Both NPK and NPKM treatments significantly increased SOC, POC and MAOC contents in the reclaimed soil compared with NF. however, 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 the total PLFA, as well as the PLFA content of Gram-positive bacteria (G⁺), Gram-negative bacteria (G⁻), aerobic bacteria and fungi between the NPK and NPKM treatments, the PLFA content of G+ and fungi in the NPK treatment was significantly lower than NL. Compared with NF, NPK and NPKM treatments significantly increased the contents of total N (TN), total P (TP), available N (AN), available P (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 significantly 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=0.73, P<0.0001; R²=0.85, P<0.0001), and positively correlated with SOC, TN, AN, AP, available K (AK), and MBC (R²=0.42−0.95, 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 Continuous application of mineral NPK fertilizers alone or combined with organic fertilizers over 15 years effectively increased SOC, POC, and MAOC contents, enhanced total and available nitrogen and phosphorus contents, and decreased pH in a reclaimed soil, thus stimulated microbial abundance. Although pH is still higher than that of normal farmland soil, and SOC content remained lower than that of normal farmland soil, the stability of organic carbon in the reclaimed soil was improved. Fertilization thus enhances soil quality and microbial activity, promoting organic carbon turnover and accumulation, and represents an effective strategy to increase soil carbon sequestration and accelerate the functional recovery of reclaimed soils.