Microbial and plant-derived compounds both contribute to persistent soil organic carbon in temperate soils

Our study tests the emerging paradigm that biochemical recalcitrance does not affect substantially long-term (50 years) SOC persistence. We analyzed the molecular composition of SOC in archived soils originating from four European long-term bare fallow experiments (Askov, Rothamsted, Versailles and Ultuna). The soils had been collected after various periods (up to 53 years) under bare fallow. With increasing duration of bare fallow without new organic inputs, the relative abundance of cutin- and suberin-derived compounds declined substantially, and the abundance of lignin-derived compounds was close to zero. Conversely, the relative abundance of plant-derived long-chain alkanes remained almost constant or increased during the bare fallow period. The relative abundance of N-containing compounds, considered to be abundant in SOC derived from microbial activity, increased consistently illustrating that microbial turnover of SOC continues even when plant inputs are stopped. The persistence of the different families of plant-derived compounds differed markedly over the scale of half a century, which may be ascribed to their contrasting chemical characteristics and recalcitrance, or to differences in their interactions with the soil mineral matrix, and likely some combination since chemical composition drives the degree of mineral association. Using soil from this unique set of long-term bare fallow experiments, we provide direct evidence that multi-decadal scale persistent SOC is enriched in microbe-derived compounds but also includes a substantial fraction of plant-derived compounds.