Interactions between Photochemical and Microbial Degradation on the Pyrogenic Soil-Derived Dissolved Organic Matter

Abstract Photochemical and microbial processes act as primary pathways regulating the transformation and fate of pyrogenic dissolved organic matter (pyDOM) in natural environments, yet their interactions remain poorly understood under different heating conditions. Here, we combined optical spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry to elucidate how photochemical and microbial degradation (applied individually, concurrently, or sequentially) altered pyDOM carbon loss and molecular composition. Microbial degradation caused 38–67% dissolved organic carbon (DOC) reduction and preferentially removed labile aliphatic and peptide-like compounds, whereas photochemical degradation yielded smaller DOC losses (28–62%) but extensive molecular diversification (20–68% newly formed formulas). Concurrent degradation led to intermediate DOC loss but the highest molecular diversity, likely due to transient microbial suppression by reactive oxygen species. Sequential degradation revealed strong process-order dependence. Photochemical pretreatment enhanced subsequent microbial degradation, particularly for high-temperature pyDOM enriched in aromatic molecules (up to 20% extra DOC degradation), while microbial pretreatment produced microbially processed residues that were only slightly further photodegraded. Despite distinct pathways, both sequences converged toward similar optical and molecular signatures, indicating a persistent pyDOM fraction dominated by photochemical transformation products. These findings reveal that not only the fire conditions but also the temporal coupling of photochemical and microbial reactions. not only the fire conditions but also the temporal coupling of photochemical and microbial reactions; refers to two independent factors that jointly regulate the degradation efficiency, compositional trajectory, and persistence of pyDOM..