Objectives Cherry tomato stalk is a typical residue characterized by a recalcitrant carbon pool and high lignocellulose content. Direct field incorporation results in slow degradation and low humification efficiency. To break through the bottleneck of its resource utilization, this study investigated the regulaiton of the humification process through the addition of an exogenous labile carbon sources. The aim was to enhance humification efficiency of stalk composting by optimizing the structure of exogenous carbon components and to clarify the underlying mechanisms.

Methods A composting experiment was conducted using cherry tomato stalks with two treatments: addition of a labile carbon source (corn flour, CSM) and a control without addition (SM). Temperature dynamics, physicochemical properties, and humification progression were monitored throughout composting. Three-dimensional fluorescence spectroscopy (EEM), two-dimensional correlation Fourier transform infrared spectroscopy (2D-COS-FTIR), and humus precursor marker analysis were employed to characterize the evolution of molecular structures and key functional groups in humified substances.

Results The thermophilic period lasted for 14 and 22 days in the SM and CSM treatments, respectively. Compared with initial values, total organic carbon decreased by 5.40% and 31.82%, while total nitrogen increased by 28.46% and 29.95% in the SM and CSM treatments, respectively. Compared with the SM treatment, at the end of composting, total phosphorus and total potassium concentrations in the CSM treatment increased by 102.42% and 8.06%, and the humification rate and humification index increased by 118.42% and 56.06%, 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 enhancement of humification by labile carbon addition is primrily attributed to significantly accelerated consumption of compost precursor, including polysaccharides, polyphenols, and amino acids, with particularly rapid transformation of polysaccharides and polyphenols. In the CSM treatment, extensive polysaccharide degradation, polyphenol condensation, and polymerization with amino compounds promoted the rapid formation of humic acid-like substances, increasing aromaticity and the degree of humification. Therefore, the addition of labile carbon is an effective strategy for improving the composting utilization efficiency of cherry tomato stalk.