Soil Organic Matter Dynamics Along Gradients in Temperature and Land Use on the Island of Hawaii

We studied soil organic matter (SOM) dynamics in allophanic soils (Udands) along independent gradients of temperature (altitude) and land use (forest—pasture) on the island of Hawaii. Using an integrated 1 3 C signal derived from land conversion along with measurements of soil respiration and soil carbon, we separated rapid, intermediate, and very slow turnover SOM pools, and estimated turnover times for the large intermediate pool. These estimates were compared to independent estimates using either bomb—derived soil 1 4 C or the Century soil organic matter model. All calculations based on a three—pool SOM structure yield rates of turnover that are 3 times slower than those produced by a single pool model. Accordingly, analyses of potential feedbacks between changes in climate, atmospheric CO 2 , and soil carbon should incorporate the heterogeneous nature of soil organic matter. We estimate that roughly three—quarters of the carbon in the top 20 cm of these soils has turnover times less than 30 yr. Turnover times for intermediate SOM double with a 10°C change in mean annual temperature, suggesting that recalcitrant pools of SOM may be as sensitive to changes in temperature as the smaller labile pools.