Characteristics of Dissolved Organic Carbon Components and Their Responses to Carbon Degradation Genes in Black Soil Under Long-Term Fertilization

Dissolved organic carbon (DOC) represents the most readily available and crucial carbon source for soil microorganisms, influencing their community structure, nutrient cycling, and metabolic functions. However, the interplay between functional genes and the organic components of DOC remains poorly understood. In this study, a 33-year fertilization experiment on black soil was carried out, setting up five fertilization treatments: unfertilized control (CK), nitrogen and potassium (NK), nitrogen, P and potassium (NPK), NPK plus straw (NPKS), and NPK plus manure (NPKM). The variation characteristics of soil DOC composition and carbon-degrading functional gene abundance under different fertilization treatments were systematically analyzed. The study found that applying chemical fertilizers combined with organic materials significantly increased soil organic carbon (SOC) and DOC contents in the thin-layer black soil of Gongzhuling. The soil DOC in this region is primarily derived from external inputs (Fresh plant-derived materials). Parallel factor analysis identified four fluorescent components: C1 as visible fulvic acid-like substances, C2 as humic acid-like substances, C3 as ultraviolet fulvic acid-like substances, and C4 as long-wavelength humic-like substances. Among these, NPK plus straw significantly enhanced the fluorescence intensity of the humic acid-like component (C2) and the total fluorescence intensity. The fluorescence intensity of the humic acid-like component increased by 36.0–208.9%, and the total fluorescence intensity increased by 23.8–270.9% compared to the CK. Moreover, the study found that the phylum composition of carbon-degrading microorganisms remained stable under different fertilization treatments. However, NPK plus straw significantly reduced the total abundance of carbon-degrading genes and influenced the composition and transformation of DOC by regulating the expression of key carbon-degrading genes ICL and abfA. These results offer new insights into the mechanisms by which fertilizer management affects the composition and stability of DOC in black soils via microbial functional gene pathways.