Cross-biome antibiotic resistance decays after millions of years of soil development.

Soils harbor the most diverse naturally evolved antibiotic resistance genes (ARGs) on Earth, with implications for human health and ecosystem functioning. How ARGs evolve as soils develop over centuries, to millennia (i.e., pedogenesis), remains poorly understood, which introduces uncertainty in predictions of the dynamics of ARGs under changing environmental conditions. Here we investigated changes in the soil resistome by analyzing 16 globally distributed soil chronosequences, from centuries to millennia, spanning a wide range of ecosystem types and substrate age ranges. We show that ARG abundance and diversity decline only after millions of years of soil development as observed in very old chronosequences. Moreover, our data show increases in soil organic carbon content and microbial biomass as soil develops that were negatively correlated with the abundance and diversity of soil ARGs. This work reveals natural dynamics of soil ARGs during pedogenesis and suggests that such ecological patterns are predictable, which together advances our understanding of the environmental drivers of ARGs in terrestrial environments.