Soil carbon losses due to priming moderated by adaptation and legacy effects

Increasing soil organic carbon contents contributes to global climate change mitigation. However, new plant inputs can enhance the mineralization of native soil organic carbon by the positive priming effect, which may counterbalance the sequestration of new carbon. Here we use soils from a 20 year chronosequence of inverted pasture soils (reciprocal translocation of topsoil and subsoil to >1 m) to study the dynamics of soil organic carbon in topsoils and subsoils. We evaluated the root-induced priming effect by differentiating native soil organic carbon from ¹³C root-derived carbon in a 6 month incubation experiment. We found that the addition of fresh root-derived carbon caused positive priming of native soil organic carbon in new topsoils (109 ± 27% additional respiration compared with controls without roots) and subsoils (331 ± 84%) after inversion. This effect was temporary for new topsoils as they accumulated soil organic carbon and adapted to high carbon inputs within a few years, leading to no priming in the long term. In contrast, buried topsoils became more sensitive to root carbon inputs over time, demonstrating how the legacy of high carbon inputs mediates the magnitude of priming (50% to 390% after 20 years of inversion). Overall, carbon losses with priming never exceeded new root-derived carbon inputs. We conclude that priming is a temporary reaction to additional carbon, which attenuates when soils adapt to high carbon inputs within a few years to decades.