Soil Carbon Dioxide Production and Climatic Sensitivity in Contrasting California Ecosystems.

Despite the wealth of information available on respiratory fluxes of CO2 from the soil surface, relatively few studies have examined the depth-related flux and production of CO2 within the soil. We developed a depth-integrated CO2 flux model based on soil profile CO2 measurements to explore both the temporal and spatial patterns of, and controls on, CO2 production in two contrasting California coastal ecosystems: a redwood [Sequoia sempervirens (D. Don) EndI.] forest and a seasonal grassland ecosystem. In the forest, CO2 production increased exponentially with temperature but the dominant control on CO2 efflux was due to changes in soil diffusivity with water content. Here, ∼67% of the total efflux was produced in the top 20 cm, with little seasonal variation. In the grassland, soil temperature and water content both exerted strong positive controls on CO2 production, with CO2 efflux highest in early spring (8 g Cm-2 d-1) and lowest in late summer (0.4 g Cm-2 d-1). Here, the relative proportion of total respiration in the top 20 cm varied from 5% at the end of the dry season to 70% during the wet winter months. Soil respiration in the redwood soil appeared to be well buffered from climate extremes due to the stable microclimate created by the forest canopy. The grassland soil, however, lacking such buffering, was subject to large and rapid changes in CO2 production throughout the year. These results indicate that despite some uncertainty in the calculation of diffusion rates, significantly more information on soil biological processes is gained from the CO2 production-diffusion approach than from surface flux measurements alone. [ABSTRACT FROM AUTHOR]