Organic carbon sequestration under chaparral and pine after four decades of soil development

Soils are the largest carbon reservoir of terrestrial ecosystems, and play a central role in the global carbon cycle. The larger lysimeter installation at the San Dimas Experimental Forest in southern California allowed quantification of carbon storage in a biosequence of soils under chamise (Adenostoma fasciculatum Hook. and Arn.), hoaryleaf ceanothus (Ceanothus crassifolius Torr.), scrub oak (Quercus dumosa Nutt.), and Coulter pine (Pinus coulteri B. Don). After four decades of soil development, carbon sequestration in the lysimeters ranged from 4552 to 17,561 g m(-2). Carbon accretin in the mineral soils (0-1 m) under chaparral represented a larger percentage of total above-ground biomass (23-27%) as compared to the pine (13%). Also, contribution of the A horizon to whole soil (0-1 m) OC sequestration was higher under chaparral than under pine. Carbon accretion in the surface horizons was related to earthworm activity, which was intense under scrub oak, but absent under pine. Soils sampled in 1987 and corresponding archived fill materials were fractioned according to density and mineral particle size fractions, and analyzed for OC and N by dry combustion. Carbon and nitrogen concentrations in all mineral soil fractions can be ranked from highest to lowest by plant species: ceanothus > chamise > scrub oak > Coulter pine. Under chaparral, a greater proportion of total soil carbon was recovered in the sand fraction as compared to the pine. The C/N ratio of this sand-sized organic matter was higher under chaparral than under pine. This is indicative of fresh plant residues that may not contribute to the long-term carbon storage in soils. (C) 1998 Elsevier Science B.V.