Objectives Cherry tomato stalk is a typical waste characterized by a recalcitrant carbon pool, with high lignocellulose content. Its direct return to the field leads to slow degradation and low humification efficiency. To break through the bottleneck of its resource utilization, this study attempted to regulate and drive its humification process by adding exogenous carbon sources. The aim was to enhance the humification efficiency of stalk composting by optimizing the structure of exogenous carbon components and to clarify the underlying mechanism.

Methods In this study, within the cherry tomato stalk composting system, treatments with the addition of labile carbon, corn flour (CSM), and a control without addition (SM) were established. Changes in temperature during the composting process, as well as the physicochemical properties and humification progress of the compost, were investigated. Three-dimensional fluorescence spectroscopy (EEM), two-dimensional correlation Fourier transform infrared spectroscopy (2D-COS-FTIR), and humus precursor marker analysis were employed to analyze the evolution of the molecular structure of key functional groups in humified substances within the stalk compost.

Results The high-temperature period lasted for 14 days and 22 days in the SM and CSM treatments, respectively. Compared to the start of composting, total organic carbon decreased by 5.40% and 27.32%, and total nitrogen increased by 28.46% and 46.75% in the SM and CSM treatments, respectively. Compared with the SM treatment, at the end of composting, the total phosphorus and total potassium concentrations in the CSM treatment increased by 59.59% and 32.01%, and the humification rate and humification index increased by 11.95% and 3.40%, respectively. The results of humus precursor component analysis indicated that during the 7～14 days of composting, the peak intensity of humic acid-like components in the CSM treatment increased faster compared to the SM treatment. Subsequently, the variation in peak intensity was smaller than in SM, indicating that the CSM treatment accelerated the humification process in the mid-stage of composting and maintained the stability of humic substance content. Compared to the SM treatment, the CSM treatment group showed a greater decrease in amino acid-like substances and higher contents of fulvic acid-like and humic acid-like substances. The absorption peak intensity in the range of 1600-1645 cm⁻¹ in the CSM treatment compost was stronger than in the SM treatment, proving that the compost contained more humic acid precursor components with C=C, C=O, and O-H bonds than SM. The absorption peak near 1100 cm⁻¹ represents lignocellulose content, and the degree of peak weakening in the CSM treatment was higher than in SM, indicating that stalk decomposition under the CSM treatment was more thorough.

Conclusions The key to promoting humification by adding labile carbon lies in the significantly accelerated consumption speed of compost precursor substances—polysaccharides, polyphenols, and amino acids, with faster reaction speeds of polysaccharides and polyphenols. In the CSM treatment group, polysaccharide-like substances were decomposed in large quantities, polyphenols condensed, and polymerized with amino groups to rapidly form humic acid-like substances, leading to an increase in aromatic substance content and the degree of humification. Therefore, the addition of labile carbon is an effective measure to promote the composting utilization of cherry tomato stalk.