Whole-profile soil carbon responses to warming and root carbon inputs in a coniferous forest

Over half of global soil organic carbon (SOC) is stored in subsurface soils (>30 cm). This deep SOC may generate a positive feedback with climate change if warming increases its turnover. While most warming experiments have only warmed surface soils, we are warming (+4°C) a coniferous forest soil in situ down to 1 m. Our experiment is investigating: (1) how the mechanisms controlling SOC turnover differ with depth, (2) the temperature sensitivity of native SOC and added substrate decomposition at different depths, and (3) interactions between warming and carbon inputs on native SOC decomposition. Warming began in October 2013, and we will present the first year of results. Thus far, warming has increased surface soil respiration by 39% relative to the control and increased concentrations of dissolved organic carbon (DOC) in soil water collected from tension lysimeters at 30 and 70 cm depths. We have added highly 13C-enriched substrates to 15, 50, and 90 cm depths within the warming plots in two related experiments: a long-term (>2 year) incubation of root litter and a short-term (4 day) incubation of injected DOC. The labeled substrates allow us to trace how the transformation of organic carbon differs with depth and warming and to measure how native SOC decomposition at different depths is affected by carbon inputs. After six months, depth has not affected the decomposition of the root litter. This study is one of the first to test whole-profile SOC responses to warming and root carbon inputs, and will enhance our understanding of carbon cycling mechanisms throughout the soil profile to improve predictions of soil carbon's role in our changing climate.