Dissolved Organic Matter Drives Soil Carbon Mineralization Efficiency and Indirectly Affects Stocks After Converting Forest Swamps to Cropland

Forest swamps, unique ecosystems with large amounts of organic matter, are seriously threatened by agricultural cultivation. As the most active component of soil, dissolved organic matter (DOM) is susceptible to environmental changes. However, the carbon dynamics of forest swamps after agricultural cultivation and the role of DOM in this process remain unclear. We collected topsoil (0–20 cm) and subsoil (20–40 cm) across forest swamps and post‐cultivation paddy and maize fields to investigate the DOM characteristics and carbon mineralization efficiency (CME). The results indicated that CME increased (from 0.08 ± 0.004 to 0.15 ± 0.004 g CO 2 ‐C/g SOC in topsoil; from 0.03 ± 0.001 to 0.11 ± 0.002 g CO 2 ‐C/g SOC in subsoil), and carbon stocks significantly decreased (from 87.71 ± 1.98 to 36.21 ± 0.65 Mg/ha in topsoil; from 57.38 ± 1.44 to 25.26 ± 0.72 Mg/ha in subsoil) after the conversion of forest swamps to cropland. The conversion significantly reduced the relative proportion of humic‐like components and humification degree ( p < 0.01), while significantly increasing the ligninase:cellulase ratio ( p < 0.01). Linear regression analyses revealed that microbial enzymatic strategies reduced the relative proportions of recalcitrant DOM components ( R 2 = 0.67, p < 0.01), with changes in DOM composition being the major driver of increased CME ( R 2 = 0.88, p < 0.01), resulting in carbon loss indirectly after cultivation. Our study emphasizes the vital role of DOM in the soil carbon dynamics and provides new insights into carbon cycling under agricultural encroachment.