Elevated CO₂ stimulates soil respiration in a FACE wheat field

Understanding the response of soil carbon (C) dynamics to higher atmospheric CO₂ concentrations is critical for evaluating the potential for soil C sequestration on time scales of decades to centuries. Here, we report on changes in soil respiration under Free-Air CO₂ Enrichment (FACE) where spring wheat was grown in an open field at two CO₂ concentrations (ambient and ambient+200 μmol mol^-1), under natural meteorological conditions. FACE increased soil respiration rates by 40-70% during the peak of wheat growth. On the FACE plots, stable C isotopic composition of soil CO₂ was used to partition the soil CO₂ flux into C from rhizosphere respiration and decomposition of pre-existing C. Decomposition contributed 100% of the soil CO₂ flux before crop growth commenced, and only 35-45% of the flux at the peak of the growing season. Decomposition rates were not correlated with soil temperature, but rhizosphere respiration rates were strongly correlated with green leaf area index.