Carlisle, Eli (Univ. of California, Davis, University of California, Dept. of Viticulture, Davis, CA, 95616; Phone: 530-754-7144; Email: ecarlisle@ucdavis.edu)

Conversion of Oak Woodlands to Vineyards Alters Physical Constraints on Soil CO₂ Respiration in a Mediterranean Climate Ecosystem

E.A. Carlisle *, K.L. Steenwerth, D.R. Smart

We examined constraints on annual soil CO₂ respiration at unmanaged oak woodlands and adjacent vineyards that were converted from oak woodlands approximately 30 years ago in the Oakville Region of Napa Valley, California. All sites were located on the same soil type, a Bale (variant) gravelly loam (Fine-loamy, mixed, superactive, thermic Cumulic Ultic Haploxeroll) and dominated by C₃ vegetation. Seasonal soil CO₂ efflux was greatest at the oak woodland sites, although during the dry summer periods the rates measured from oak sites were at times similar to those measured from vineyards. In spite of the fact that CO₂ efflux rates were higher, soil profile CO₂ concentrations were lower at the oak-woodland sites, except at the shallowest depth (15cm). Soil gas diffusion coefficients for the oak sites were larger than for the vineyard sites. Long-term annual cultivation of the vineyard soils apparently increased bulk density, diminished porosity, and thus altered the effective gas diffusivities. In addition to lower CO₂ production, lower diffusion coefficients may help account for the lower rates of CO₂ efflux and higher CO₂ concentrations in vineyard soil profiles. Vineyard soil CO₂ was more depleted in ¹³C throughout the soil profile below 30 cm as indicated by more negative d¹³C ratios, and points to different respiratory C sources in vineyard soils. Annual C losses were less from the vineyard soils (7.02 ± 0.58 Mg C ha-1 yr -1) as compared to the oak soils (15.67 ± 1.44 Mg C ha-1 yr-1), and both were comparable to losses reported in previous investigations.