Friday, 14 July 2006

Soil Organic Carbon Dynamics in a Forested Debris-Flow Chronosequence in Southern California.

J.K. Turk, University of California - Riverside, Dept. of Environmental Sciences, Riverside, CA 92521 and Robert Graham, University of California, Riverside, Dept. of Environmental Sciences, Riverside, CA 92521-0424.

The anthropogenic increase in atmospheric carbon is partially mitigated through afforestation. However, the role of soils in the forest sink is poorly understood. This study was conducted to determine the rates of soil carbon accumulation and transformation during early stages of pedogenesis under semi-arid coniferous forest. The chronosequence used for this study consists of a series of debris flows in the San Bernardino Mountains. Conifer stands, of mostly ponderosa pine and incense cedar, are established in the first or second year after a debris flow and were used to date the flow deposits using dendrochronology. Surfaces younger than 100-years age were studied at an approximately 20-year resolution and three older surfaces, up to 400-years old, were also included. Pedon description and sampling according to morphologic horizons was conducted at each stage of the chronosequence. Pedogenesis in this chronosequence is driven mainly by organic carbon from the growing forest biota. Accumulation of organic matter at the mineral surface was reflected in melanization and rubification of the soil color. Many profiles contained buried O and A horizons, which may serve as a sink for atmospheric carbon. Carbon content of the soil decreased for the first 3-4 decades following debris flow deposition and increased from 40 to 100 years, but at a slower rate [0.03 kg/(sq.m*y)] than previously reported for ponderosa pine forest at a site 900 km NNW of our study site. Changes in C/N ratio are consistent with reports from previous chronosequence studies. This high-resolution chronosequence provides a detailed record of the initial stages of soil development.

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