Hydrophobic neutral and basic fractions of organic fertilizer-derived dissolved organic matter promote phosphorus mobilization in paddy soils

Dissolved organic matter (DOM) is a highly active constituent of organic fertilizers, playing a key role in enhancing soil nutrient bioavailability and crop yields. However, the influence of the polar fractions and molecular diversity of organic fertilizer-derived DOM on soil phosphorus (P) mobilization remains insufficiently explored. This study integrated solid-phase extraction, spectroscopy, and mass spectrometry in both field and P sorption experiments to investigate the effects of DOM polar fractions on soil P availability. Long-term inorganic and organic fertilization increased the soil P activation coefficient and P use efficiency by 29 % and 30 %, and maintained crop P uptake and yield despite a 34 % reduction in chemical P input compared to chemical-only fertilization. This effect is significantly correlated to the concentration of hydrophobic neutral (HON) and hydrophobic basic (HOB) fractions. The P sorption experiments further confirmed that the hydrophobic fractions (HON and HOB) more effectively suppressed P sorption compared to hydrophilic and hydrophobic acidic fractions. This inhibition may stem from the competitive interactions at soil P sorption sites and the reduction of soil zeta potential driven by lignin-highly unsaturated and phenolic compounds, characterized as aromaticity values of 0.14–0.36, double-bond equivalents of 6.8–10.9, and hydrogen-to-carbon ratios of 1.0–1.4. In summary, this study highlights the influence of DOM polar fractions and molecular composition on soil P availability, providing valuable direction for developing organic fertilizers designed to promote P use efficiency and agricultural sustainability.