Depth-dependent coupling of dissolved organic matter chemodiversity and microbial community traits reveals threshold effects of combined fertilization on soil carbon stabilization in arid agroecosystems

Background How fertilization influences the coupled dynamics of DOM composition and microbial communities across soil depths in arid agroecosystems remains unclear. Methods The molecular composition of DOM and microbial communities in surface (0–20 cm) and subsurface (20–40 cm) soil layers from a 7-year field experiment comprising six fertilization treatments: no fertilization (CK), chemical fertilizer (NPK), organic fertilizer (M), half-rate chemical fertilizer plus M (0.5NPKM), chemical fertilizer plus M (NPKM), and 1.5-fold chemical fertilizer plus M (1.5NPKM) was analysied using spectroscopic, mass spectrometric, and sequencing approaches. Results NPK and M treatments significantly reduced DOM content in the surface layer by 12.14% and 22.87%, respectively, relative to CK. However, combined fertilization increased surface DOM content compared with the single treatments, and this effect became more pronounced with higher proportions of NPK. Organic fertilizer enhanced DOM aromaticity and facilitated the transformation of humic-like into protein-like and fulvic-like components. The application of organic fertilizer promoted the conversion of oxygen-rich and labile compounds (aliphatic/peptide-like and lipids) into nitrogen- and sulfur-containing compounds as well as more aromatic structures. Combined organic-inorganic fertilization enriched Proteobacteria/Ascomycota and increased microbial network complexity. Actinobacteria consumed labile substrates, while Acidobacteria/Chloroflexi allowed persistence via slow utilization. Fungi driving both the synthesis of fresh microbial metabolites in surface soils and the formation of stable carbon pools in subsurface layers through humification and lipid accumulation. Ascomycota regulating DOM aromaticity in the surface layer and coupling with nitrogen cycling in the subsurface. Conclusion Organic-inorganic fertilization drives soil organic carbon stabilization through molecular-level DOM-microbe interactions.