Anaerobic Microsites Have an Unaccounted Role in Soil Carbon Stabilization.

Soils represent the largest carbon reservoir within terrestrial ecosystems. Mechanisms controlling the amount of carbon stored and its feedbacks to the climate system, however, remain poorly resolved. Global carbon models assume that carbon cycling in upland soils is entirely driven by aerobic heterotrophic respiration; the impact of anaerobic microsites prevalent even within well-drained upland soils is missed within this conception. In this presentation, we show that anaerobic microsites, abundant in most soils, are important regulators of soil carbon persistence, shifting microbial metabolism to less efficient anaerobic respiration and protecting otherwise bioavailable, reduced organic compounds from decomposition. Our results explain the selective preservation of aliphatic compounds such as lipids and waxes observed in upland soils globally. Further, shifting from anaerobic to aerobic conditions leads to a 10-fold increase in volume-specific mineralization rate, illustrating the sensitivity of anaerobically protected carbon to disturbance. Vulnerability of anaerobically protected soil carbon to future climate or land use change thus constitutes a yet unrecognized positive soil carbon-climate feedback mechanism that should be quantitatively incorporated into terrestrial ecosystem climate models.