Effects of Coal Fly Ash Addition on the Carbon Mineralization of Agricultural Soil Under Different Moisture Conditions

Laboratory incubation experiments were conducted to investigate the effects of coal fly ash (FA) amendment (0%, 2.5%, 7.5%, and 15%) and moisture regimes (40%, 70%, and 100% water holding capacity (WHC)) on the mineralization of carbon (C) in an acidic agricultural soil. The results showed that the soil C mineralization intensity initially increased and subsequently decreased throughout the incubation period, with the mineralization dynamics well described by the first-order kinetic model (0.9633 ≤ R2 ≤ 0.9972). Carbon mineralization increased with the application rate of FA, while moisture effect followed the order 70% WHC > 100% WHC > 40% WHC. Indicators showing highly significant correlations with total C mineralization amount included FA application rate, pH, water-soluble organic carbon, (WSOC) and cellulase (CEL) activity. Specific bacterial (Acidobacteriota, Gemmatimonadota, Pseudomonadota, and Actinobacteriota) and fungal phyla (Chytridiomycota, Glomeromycota, and Olpidiomycota) exhibited stronger correlations with C mineralization. The microbial taxa exhibiting significant responses to FA and moisture conditions were not consistent. Although the addition of high proportions of FA, especially with adequate moisture conditions, can enhance soil microbial activity and C mineralization, the potential risks of soil C loss and the accumulation of toxic elements necessitate the prudent implementation of elevated FA application rates in practical scenarios.