Judith K. Turk and Robert C. Graham. Dept. of Environmental Sciences, Univ. of California, Riverside, CA 92521
Forest soils in the mountains of southern California may represent a regional sink for atmospheric carbon. Soil development, at a site in the San Bernardino Mountains, was studied using a chronosequence approach. Debris flow deposits, ranging from <1 to 244 years old, represented stages of soil formation over time. Ab horizons were commonly encountered because the debris flows bury soils formed in older debris flows. We evaluated rates of soil carbon and nitrogen accumulation and examined the influence of buried organic matter on carbon storage. Organic carbon accumulation occurred at a rate of 24.8 g m-2 yr-1 and was stored predominantly in the organic horizons. Nitrogen was depleted from the soils for approximately 75 years before a net accumulation took place. From 75 years onward, nitrogen accumulation occurred at a rate of 1.5 g m-2 yr-1. Nitrogen storage was nearly evenly divided between the organic and mineral soil horizons. The influence of buried soils on carbon storage in this system was explored by considering the influence of burial on decomposition rates. Decomposition rates were evaluated by mass loss from leaf litter in mesh bags placed at the soil surface or buried at 35 cm depth. Buried litter had decay constants that were approximately three times greater than decay constants for litter at the soil surface (k=0.13 for oak leaves at the surface; k=0.44 for buried oak leaves; k=0.12 for pine needles at the surface, k=0.34 for buried pine needles). Building of the carbon and nitrogen pools in fresh debris flow deposits and rapid decay of litter associated with buried deposits are two important processes in soils formed in debris flows. The combined influence of these processes suggests that the carbon pool in these soils has a cyclic nature, undergoing repeated phases of carbon accretion, burial, and rapid loss through decomposition.
Audio File