<p>Biogas slurry topdressing (BSTD) is a promising strategy for efficient liquid waste utilization and sustainable agricultural production. However, the field-scale regulation of soil dissolved organic carbon (DOC) by BSTD remains insufficiently understood across crop growth stages and soil depths. Therefore, this field experiment investigated the temporal and spatial responses of soil DOC to BSTD during the middle and late stages of maize growth. The results showed that, compared with pure chemical fertilizer topdressing (CF), BSTD increased soil DOC content at the tasseling stage (VT) but decreased it at the milk stage (R3), while the ultraviolet–visible spectrum slope ratio increased at both stages and decreased at the maturity stage (R6). The fluorescence index (FI), freshness index (β:α), and biological index (BIX) increased with soil depth but decreased with maize growth stage. Overall, BSTD promoted increases in FI (2.08%), β:α (3.80%), humification index (2.83%), and BIX (3.98%), and altered soil DOC composition by increasing the proportions of humic acid-like substances (5.86%) and soluble microbial by-products (4.97%). Additionally, partial least squares path modeling indicated that soil depth was the most direct and significant factor regulating excitation-emission matrix indices, organic components, and soil DOC content. By linking BSTD-derived inputs with crop-stage dynamics and soil-profile heterogeneity, this study clarifies how biogas slurry reshapes DOC transformation and partial stabilization under field conditions, providing a scientific basis for the sustainable agricultural use of biogas slurry in dryland agriculture.</p> Graphical Abstract <p></p>