Linking dissolved organic matter chemistry to soil organic carbon sequestration and nematode food web energetics under long-term organic amendments

Dissolved organic matter (DOM) serves as both an energy source for soil biota and a precursor for stable soil organic carbon (SOC), yet how amendment-induced changes in DOM chemistry bridge SOC sequestration and soil food web energetics remains underexplored. Here, we examined four fertilization treatments in a long-term field experiment including chemical fertilizer alone (CF), and chemical fertilizer combined with crop straw (ST), organic manure (OM), or biochar (BC). DOM chemistry was characterized by three-dimensional excitation-emission matrix fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry; SOC was fractionated into particulate organic carbon (POC) and mineral-associated organic carbon (MAOC); and nematode community structure and energy flux were determined through morphological identification and metabolic footprint calculation. ST and OM increased dissolved organic carbon concentration and protein-like and humic-like fluorescence intensities, and altered UV–visible absorption properties as indicated by higher chromophoric DOM concentration ( a (355)) and aromaticity (SUVA 254 ) and a lower spectral slope ratio ( S R ); in contrast, BC did not significantly alter DOC concentration or fluorescence peak intensities, but exhibited a higher S R compared with ST and OM characteristic of a low-molecular-weight, less aromatic DOM pool. Although most molecular formulas did not differ significantly among treatments, the modified aromaticity index (AI mod ) and nominal oxidation state of carbon (NOSC) were higher in OM and lower in BC; functional diversity based on AI mod ( FD (AImod) ) was highest in ST and lowest in BC. ST primarily increased POC, OM increased MAOC, and BC increased both POC and MAOC. The MAOC/SOC ratio was highest under OM and lowest under BC. Microbial necromass carbon (MNC) content was highest under BC, whereas the MNC/MAOC ratio was lowest under OM. Nematode total energy flux and flow uniformity were highest under ST, intermediate under OM, and lowest under CF and BC. DOM chemical indicators including S R , NOSC, ( FD (AImod) ) and fluorescence intensities of Peaks T, B, and M were the strongest predictors of nematode energy metrics. Partial least squares path modeling revealed that DOM quality exhibited a negative direct effect on nematode food web energy flux and uniformity, consistent with the preferential consumption of labile DOM constituents leaving behind recalcitrant residues. Overall, straw stimulated the nematode food web more strongly than organic manure but did not promote MAOC accumulation; in contrast, biochar enhanced MAOC accumulation while suppressing food web activity. Balancing labile and recalcitrant DOM fractions through targeted organic amendment strategies offers a pathway toward reconciling SOC sequestration with soil biological functioning.