Nutrient-induced acidification is associated with reduced dissolved organic matter transformation potential in green manure-amended paddy soils

Dissolved organic matter (DOM) plays a crucial role in soil carbon sequestration, yet the molecular mechanisms by which nutrient enrichment influences DOM transformation remain poorly understood. Here, we employed high-resolution mass spectrometry and high-throughput sequencing to examine the effects of nutrient gradients on DOM transformation, defined as pairwise biochemical conversions among DOM molecules, and the associated bacterial community in green manure-amended paddy soils. Our results showcased that nutrient addition increased DOM content by 7.1%–24.2% compared to unfertilized control. Nutrient enrichment enhances DOM stability by shifting its molecular composition (toward more recalcitrant condensed aromatic- and tannin-like compounds and less lignin-like compounds) and reducing molecular diversity (i.e., the number of molecular formulas). These molecular compositional changes were accompanied by a decline in DOM transformation potential (i.e., lower reaction levels; 1.1–1.5 fold decrease) and by shifts in DOM-bacteria association patterns. Importantly, soil pH was significantly associated with these patterns, suggesting that nutrient-induced acidification may contribute to reduced DOM transformation potential. Our findings suggest a coupled nutrient–pH–microbe response that offers insights into optimizing nutrient management strategies to balance soil carbon sequestration and ecosystem resilience in paddy soils.