Distribution, accumulation, and fluxes of soil carbon in four monoculture lysimeters at San Dimas Experimental Forest, California

Abstract

This research examines how vegetation type controls soil processes involving soil carbon fluxes, accumulation, and transport in a chaparral ecosystem. Carbon concentrations and δ 13C values were measured for soil samples collected in 1987 from 1-m depth profiles in four lysimeters in the San Dimas Experimental Forest, southern California, USA. Each lysimeter has sustained a single vegetation type since 1946, the species being Quercus dumosa, Ceanothus crassifolius, Adenostoma fasciculatum, and Pinus coulteri. Archived samples of soil originally used for filling the lysimeters and litter samples from the surface of each lysimeter were analyzed to determine initial and boundary conditions. Although detectable changes in carbon content were limited to the topmost 20 cm of the profiles, variations in δ 13C were found to depths of 80 cm, indicating that processing of carbon occurs much deeper than indicated by carbon concentrations alone, underscoring the utility of carbon isotopes in the study of soil carbon dynamics. A one-dimensional model that considers surface carbon input, downward transport, and loss through decomposition is developed to describe the evolution of carbon concentration and stable carbon isotope ratios in the four soil profiles. Comparison of measured and calculated profiles yields estimates of carbon fluxes, turnover rates, and accumulation of soil carbon. A set of physical parameters, including the rate constant of decomposition, downward transport rate of organic carbon, and rate of carbon input from the surface are derived from the model and can be related to the species and environmental conditions. The calculations indicate the importance of species on soil formation and carbon cycles, which is important for understanding the effects of changes in land use on ecosystem processes. Our results also suggest that fire may increase the rate of soil carbon accumulation in a chaparral ecosystem.