Spectroscopic Evidence of Soil Carbon and DOM Transformation Across an 8–63-Year Paddy Chronosequence in Western Jilin, China

Understanding the long-term evolution of soil carbon pools and dissolved organic matter (DOM) is crucial for evaluating carbon cycling and soil fertility in paddy ecosystems. This study investigated the changes in soil organic carbon (SOC), dissolved organic carbon (DOC), and DOM optical characteristics across an 8–63-year rice cultivation chronosequence in the western Jilin irrigation district of northeastern China. Soil samples were collected from five depth intervals (0–10, 10–20, 20–30, 30–40, and 40–50 cm) to assess physicochemical properties, ultraviolet–visible (UV-Vis) absorption, and three-dimensional excitation–emission matrix (EEM) fluorescence features. The results showed that long-term rice cultivation reduced soil salinity and alkalinity while significantly increasing SOC and DOC contents. The UV–Vis indices (SUVA254, SUVA260, SUVA300) increased with cultivation duration, whereas E2/E3, E4/E6, and SR decreased, indicating enhanced aromaticity, humification, and molecular weight of DOM. Fluorescence analysis revealed a gradual transformation from protein-like to humic-like components, supported by PARAFAC modeling that identified four dominant components (two humic-like and two protein-like). Correlation and PLS-SEM analyses demonstrated that cultivation duration positively influenced soil carbon accumulation and DOM humification, while soil depth exerted a negative effect. Soil carbon acted as the core mediator linking UV–Vis and EEM indices, explaining more than half of the observed variance. Overall, long-term rice cultivation promoted carbon stabilization and humic substance formation, improving soil quality and carbon sequestration potential in saline–alkaline paddy soils. These findings provide valuable insights into the spectroscopic mechanisms of DOM transformation and the sustainable management of carbon processes in temperate agroecosystems.