Soil carbon pool structure and temperature sensitivity inferred using CO₂ and ¹³CO₂ incubation fluxes from five Hawaiian soils

We measured respiration and δ¹³C values of respired and soil carbon in long-term incubations of soils from two forests and three pastures along an altitudinal gradient in Hawaii. CO₂ fluxes early in the incubations decreased rapidly, and then stabilized at approximately 20% of initial values for seven months. We suggest that the rapid drop and subsequent stabilization of respiration reflects a change in the dominant source of the CO₂ from labile (active) to much more recalcitrant pools of soil organic matter (SOM). Estimates of active SOM were made by integrating all of the carbon respired in excess of that attributable to respiration of the intermediate SOM pool; these values ranged from 0.7-4.3% of total soil C. δ¹³C values for carbon respired from the pasture soils showed that older, forest-derived C contributed an increasing fraction of total soil respiration with time. Initial and late-stage respiration responded similarly to changes in temperature, suggesting that intermediate SOM is as sensitive to temperature as the active fraction.