Long‐Term Chinese Milk Vetch Application Alters the Molecular Composition of Dissolved Organic Matter Through Reshaping Microbial Ecological Strategists

ABSTRACT A deep understanding of the impacts of long‐term application of Chinese milk vetch (CMV) on soil microbial community, dissolved organic matter (DOM) composition, and soil enzyme activities in paddy field is still lacking. A long‐term fertilization experiment has been conducted for 6 years (2016–2022) with four treatments in paddy field: no fertilizer (CK), chemical NPK fertilizers (NPK), CMV alone (CMV), and combined NPK with CMV (NPKM). We investigated the molecular fingerprinting of DOM using ultrahigh‐resolution FT‐ICR mass spectrometry under these treatments. Microbial communities and life history strategies (r/K‐strategy) were characterized through high‐throughput sequencing approaches. Results showed that NPKM treatment significantly increased DOM molecular richness and proportion of labile DOM (LDOM) components (such as carbohydrates and proteins/amino sugars‐like compounds), whereas NPK treatment increased recalcitrant DOM (RDOM) proportions. Bacterial diversity and richness were enhanced by NPKM, while NPK promoted fungal diversity. Notably, CMV application reduced polyphenol oxidase (PPO) activity while increasing β‐xylanase (BX), β‐glucosidase (BG), and cellobiohydrolase (CBH) activities. Partial least squares path modeling (PLS‐PM) revealed significant interactions between DOM composition and microbial communities, where LDOM favored r‐strategists that enhanced enzyme activities for labile organic matter decomposition (BX, BG, and CBH), while RDOM promoted K‐strategists that increased PPO activity.