The vulnerability of whole-profile soil organic carbon to in situ warming and root carbon inputs

Subsurface soils (>30 cm) store more than half of global soil organic carbon (SOC) and the processes governing soil C turnover vary with depth. However, most research has focused on surface soils, and controls on subsoil C dynamics are poorly understood. We have developed a whole-profile (to 1.3 m) soil warming experiment in a coniferous forest to study the effects of warming and root inputs on SOC dynamics throughout the profile. In this prototype experiment, we installed 2.4-m deep resistance heaters around the circumference of 3-m diameter plots (24 heaters per plot) to heat intact soil profiles to 4°C above ambient while maintaining natural temperature gradients. Highly 13C-enriched root-derived C substrates were added to three depths (10, 50, and 90 cm) within heated and control plots. Soil temperature, moisture, and oxygen concentrations are monitored throughout the profile in both heated and control plots. We are measuring CO2 fluxes and C isotopes from surface chambers and gas wells monthly to quantify the effect of warming and root inputs on soil C dynamics. We will also quantify C pools and isotopes of bulk soils and density fractions after 0.5, 1, and 2 years to trace the fate of root-derived C substrates. Specifically, this prototype experiment aims to investigate (1) the temperature sensitivity of native SOC and added root substrate decomposition at three different depths; (2) the priming effect of root C inputs on native SOC decomposition at three different depths; and (3) interactions between warming and root C inputs on native SOC decomposition. This study is one of the first to study responses of whole-profile SOC to in situ warming and root C inputs. It is designed to enhance our understanding of deep SOC stabilization mechanisms and to improve predictions of SOC's fate in a changing climate.