Solid Phase Molecular Diversity Enhances Soil Organic Matter Persistence

Molecular diversity has been proposed as a critical factor controlling soil organic matter (SOM) persistence. However, in contrast to dissolved organic matter, molecular diversity of solid phase organic matter remains largely unexplored. Here, we show the molecular diversity features of solid phase organic matter through a direct mass spectrometric scan of particulate and mineral associated organic matter (POM and MAOM) that show a strong relationship with carbon turnover rates, collected from a long-term grassland recovery experiment after 0, 23, and 43 years. We found that the highest molecular diversity (Hill Number = 1603±124) existed in SOM that had the slowest carbon turnover rate. Molecular diversity of MAOM exhibits greater correlation (R² = 0.95, p < 0.001) with SOM persistence than that of POM. Molecular diversity became increasingly enriched from top to subsoil horizons (360% increase), consistent with a breaking down of large molecules into a range of low- to high-molecular-weight molecules. Soil depth and total iron content were main factors impacting the diversity change of POM and MAOM, highlighting the combined control of microbial decomposition and mineral interaction in shaping molecular features of solid phase organic matter. Together, these results suggest that molecular diversity may not operate as a limiting factor for carbon utilization by decomposers but as an ecosystem property that incorporates organo-mineral interactions.