Decomposition causes short-term increases in functional molecular diversity of dissolved organic matter.

The molecular diversity of dissolved organic matter (DOM) in soil depends on the stage of plant litter decomposition and microbial metabolism. Yet the contributions of catabolic and anabolic processes on DOM molecular diversity, and their consequences for organic carbon mineralization, remain unclear. To address this question, we used an 18O-H2O isotope-labelling approach to track microbial transformation of DOM during blue grama grass (Bouteloua gracilis) decomposition and determine how these processes alter molecular diversity. Here, we show that 18O-isotopically labeled compounds indicate that microbially produced DOM increases functional molecular diversity (recognizing compound dissimilarity) during early decomposition (days) but not at later stages (months). Furthermore, carbon mineralization from DOM is most strongly correlated with molecular weight, highlighting the role of chemical properties in regulating microbial decomposition. Our findings suggest that early microbial catabolic and anabolic metabolism enhances DOM molecular diversity, whereas later decomposition favors the accumulation of fewer, recycled microbial compounds.