Fig. P1.

The contribution of carbon fixed more than 10 y ago (blue, >10 y) and carbon fixed more recently (green, <10 y) to soil carbon stocks (pie charts) and decomposition-derived CO₂ fluxes under three temperature treatments at two sites (Aspen FACE and Duke FACE). Decades-old (>10 y) carbon made up 70% of carbon stored in the soil and 30% of CO₂ flux at all temperatures. The proportional increase in CO₂ flux of both age classes with warming suggests that they have similar vulnerability to increased decomposition as temperatures rise.