Organic-Inorganic Fertilization Shifts Dissolved Organic Matter Chemodiversity in Runoff From Tropical Cropland: Roles of Soil Nutrient Stoichiometry

Dissolved organic matter (DOM) is a crucial quality indicator for water health. However, it remains unknown how organic-inorganic fertilization differently shifts the DOM chemodiversity in cropland-impacted runoff, especially the roles of terrestrial nutrient stoichiometry. Here, a field experiment subjected to 7-year history of four fertilization regimes, that is, unfertilized (CK), chemical fertilization (NPK), straw return (SR), and NPK + SR, was conducted to examine the quantity and chemodiversity of runoff DOM in tropical China. Moreover, the ecological relationships between runoff DOM properties and soil C:N:P stoichiometry were evaluated. Relative to the control treatment, the carbon content (DOC) of runoff DOM increased by 29.20% and 48.82% under SR and NPK + SR treatments, but remained unchanged in NPK treatment. Conversely, the DOM nitrogen content (TDN) only elevated in NPK treatment (262.58%). For DOM chemodiversity, SR treatment was rich in methyl carbon and protein-like substances, but lacked fulvic-/humic-like compounds. NPK treatment increased the hydrophilic compounds like amino acids and polysaccharides, while reducing aromaticity. NPK + SR treatment enhanced DOM's aromaticity, molecular weight and humification due to the enrichment of quinone, ketone and fulvic-like substances, possibly shaping an oligotrophic aquatic microbial community. Further study revealed that the occurrence of aromatic DOM was positively correlated with soil moisture content, TK (total potassium), total organic carbon and stoichiometric N:P ratio, indicating the contribution of terrestrial straw-decomposed products. Inversely, the accumulation of hydrophilic substances (e.g., TDN) correlated positively with soil NO3-N and C:P ratio, indicating the contribution of endogenous DOM.