Organic fertilizer intensifies the vertical heterogeneity of DOM in paddy fields through interactions with soil minerals

Dissolved organic matter (DOM) is a complicated soluble organic composite in soils that is known to be linked to carbon turnover, nutrient cycling, and climate change. However, uncertainties exist about the potential influence of soil Fe and Mn oxides on DOM characteristics in paddy soil profiles following the application of inorganic and organic fertilizers. We investigated the DOM characteristics in a paddy soil profile (0–40 cm) treated with inorganic and organic fertilizers and the relationships between DOM and Fe and Mn oxides. The paddy soils were subjected to four fertilizer treatments: a control with no fertilizer (CK), inorganic fertilizer only (NPK), 50 % chemical N substituted with organic fertilizer (NPKM), and 100 % chemical N substituted with organic fertilizer (M). Ultravioletvisible (UVVis) spectroscopy and fluorescence excitation-emission matrix (EEM) spectrophotometry were integrated to explore the vertical variation in the DOM. There were no significant differences in the DOM content between the surface soil (0–20 cm) and the subsurface soil (20–40 cm) under the CK, NPK, and NPKM treatments. However, the DOM content significantly increased with soil depth under the M treatment. The application of organic fertilizer resulted in a significant reduction in the aromaticity and hydrophobic components of the DOM, as well as the fluorescence intensity of the humic-like and protein-like components with increasing soil depth; the content of easily reducible Fe significantly increased, whereas the total Mn and reducible Mn contents significantly decreased with increasing soil depth. The addition of organic fertilizer increased the vertical variation in the DOM characteristics of the paddy soil profiles. The consistency and correlation analysis of the DOM and Fe and Mn oxide contents in the profile indicated that Fe and Mn oxides might be the main influencing factors of DOM vertical heterogeneity, which was mediated by the weakening of mineral protection and the increase in the degree of mineral dissimilar reduction with increasing soil depth.