SOIL CARBON POOL STRUCTURE AND TEMPERATURE SENSITIVITY INFERRED USINGCO2 AND (13)CO2 INCUBATION FLUXES FROM 5 HAWAIIAN SOILS

We measured respiration and delta(13)C values of respired and soil car bon in long-term incubations of soils from two forests and three pastu res along an altitudinal gradient in Hawaii. CO2 fluxes early in the i ncubations decreased rapidly, and then stabilized at approximately 20% of initial values for seven months. We suggest that the rapid drop an d subsequent stabilization of respiration reflects a change in the dom inant 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 attributa ble to respiration of the intermediate SOM pool; these values ranged f rom 0.7-4.3% of total soil C. delta(13)C values for carbon respired fr om the pasture soils showed that older, forest-derived C contributed a n 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 th e active fraction.