The formation of fulvic acid (FA), an aromatic compound, is affected by the compost amendment. This study aimed to assess the extent of the humification of FA in soil amended with seven different composts. Results showed that composts improved the FA concentration in soil. Parallel factor (PARAFAC) analysis, combined with hetero-two-dimensional correlation spectroscopy (hetero-2DCOS), indicated that the inner changes in FA components determined the evolution of mineralization. The diversity in the composts used and the dominant microbes present might be responsible for the evolution of different mechanisms of FA transformation. Structural equation models (SEMs) demonstrated that the FA components were transformed directly by microbes, or indirectly via changes in the total organic carbon (TOC) and total nitrogen (TN) contents, C:N ratio, humic substance (HS) levels, and humic acid (HA): FA ratio, which regulate the microbial community structure. Our results will be useful for improving the bioavailability of compost products and realizing sustainable utilization of the soil.

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Microbes are the drivers for disposing of organic solid waste (OSW) during composting. Moreover, humus is a multipurpose biological macromolecule synthesized by microorganisms during composting. Here, we investigated the humification degree (HD), and the humus content was assessed in terms of the bacterial community. Microbial sequencing and bioinformatics approaches were combined to analyze the biological characteristics of 105 composting samples. The bacterial communities were useful indicators for making predictions and even correctly determined the categories of OSWs with 94% accuracy. Furthermore, the bacterial genera screened were designated as a bacterial code. The bacterial codes can also provide a better prediction of HD. Our results suggested that the bacteria code is a reliable biological method to assess HD effectively. Bacterial codes can be used as ecological and biological indicators to evaluate the quality of composting of different materials.