The energetic and chemical signatures of persistent soil organic matter

A large fraction of soil organic matter (OM) resists decomposition over decades to centuries as indicated by long radiocarbon residence times, but the mechanisms responsible for the long-term (multi-decadal) persistence are debated. The current lack of mechanistic understanding limits our ability to accurately predict soil OM stock evolution under climate and land-use changes. Using a unique set of historic soil samples from five long-term (27-79 years) bare fallow experiments, we demonstrate that despite wide pedo-climatic diversity, persistent OM shows specific energetic signatures, but no uniform chemical composition. From an energetic point of view, thermal analyses revealed that combustion of persistent OM occurred at higher temperature and provided less energy than combustion of more labile OM. In terms of chemical composition, persistent OM was H-depleted compared to OM present at the start of bare fallow, but spectroscopic analyses of OM functional groups did not reflect a consistent chemical composition of OM across sites, nor substantial modifications with bare fallow duration. The low energy content of persistent OM may be attributed to a combination of reduced content of energetic C-H bonds or stronger interactions between OM and the mineral matrix. Soil microorganisms thus appear to preferentially mineralize high-energy OM, leaving behind material with low energy content. This study provides the first direct link between long-term persistence of OM in soil and the energetic barriers experienced by the decomposer community.