Temporal responses of DOM molecular characteristics to conversion from dryland to paddy field: Insights from soil aggregate size fractionation

Dissolved organic matter (DOM) is predominantly present within soil aggregates and its structure critically influence the environmental behavior of pollutants. However, the long-term evolution of DOM structure across aggregate sizes following dryland-to-paddy conversion remains unclear. This study examined the structure characteristics of DOM in paddy soils at land use change sequence (5, 12, 21, 29, 41, and 53 years) across different aggregate size fractions (<2, 2–53, 53–250 and 250–2000 μm). Three-dimensional fluorescence spectroscopy, UV–visible spectroscopy, and elemental composition analysis were employed. The results showed that < 2 μm aggregates contained protein-like, tryptophan-like, LVA-like humic substances (Longer wavelength humic-like component), humic acid-like, and LVC-like humic substances (Lower wavelength humic-like component), with < 5 % variation throughout the conversion; 2–53 μm aggregates exhibited relatively uniform DOM component distribution; 53–250 μm aggregates were dominated by LVC-like humic substances and humic acid-like components (collectively 60 %); 250–2000 μm aggregates primarily contained humic acid-like and LVA-like humic substances. Spectral parameter analysis (SUVA₂₅₄, E₄/E₆, and A_240–400, S_R and HIX) elucidated the co-evolutionary dynamics of DOM structure with aggregate size fractions and land-use conversion chronology. The results demonstrated that DOM in smaller aggregates (<2 μm and 2–53 μm) exhibited elevated aromaticity and stabilized humification features, while the 250–2000 μm fraction was dominated by high-molecular-weight DOM with pronounced aromaticity fluctuations across conversion stages. Two-way ANOVA confirmed significant time-size interactive effects (P < 0.001). These findings offer new insights into DOM structure dynamics during paddy soil evolution, with implications for carbon pool management in agroecosystems.