Linking Variation in Active Soil Microbial Communities with Edaphic Properties in Mediterranean Grasslands Along a Precipitation Gradient

Microbial processes that mediate soil organic matter (SOM) formation and stabilization, and their interaction with edaphic factors that also shape the persistence of SOM, are largely unknown. In three Mediterranean grassland sites, that span a natural precipitation gradient in CA, we used ¹⁸O-H₂O quantitative stable isotope probing to connect microbial ecophysiology to chemical characterizations of soil and SOM. To assess microbial activity at each site, triplicate soil samples were incubated for 8 days with either natural abundance ¹⁶O-H₂O or 97% atom-fraction excess ¹⁸O-H₂O. In each sample, we fractionated isotopically-enriched microbial community DNA via density gradient ultracentrifugation, and sequenced 16S rRNA gene amplicons in 9 fractions per sample. Across the gradient, soil mineral composition is relatively similar, but microbial community composition (measured by both amplicon and shotgun metagenomic sequencing) is significantly different between sites. Using the quantitative stable isotope probing (qSIP) approach, we modeled taxon specific population growth and turnover, using the degree of incorporation of the heavy isotope during the incubation period. We then tested for relationships between taxon-specific relative growth and edaphic properties measured across the precipitation gradient. Radiocarbon analysis of soil C indicates that bulk soil C age increases with mean annual precipitation, however, the age of C metabolized and respired by microbes (¹⁴CO₂) is close to modern at all three sites. ¹³C nuclear magnetic resonance spectroscopy (NMR) analysis indicates that soil organic carbon has high levels of aromatic compounds (likely plant-derived) at the highest-precipitation site, and higher lipid and carbohydrate signatures (likely microbial) at the moderate-precipitation site. This work investigates geographic variation in active microbial communities and explores the connection between microbial ecophysiology and soil organic matter.