Effects of Land Use on Soil Respiration

Abstract

We examined constraints on soil CO₂ respiration in natural oak woodlands, and adjacent vineyards that were converted approximately 30 yr ago from oak woodlands, 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 summer drought the rates of soil CO₂ efflux measured from oak sites were generally similar to those measured from the vineyards. Soil profile CO₂ concentrations at the oak woodland sites were lower below 15 cm despite higher CO₂ efflux rates. Soil gas diffusion coefficients for oak sites were larger than for vineyard sites, and this indicated that the apparent discrepancy in soil profile carbon dioxide concentration ([CO₂]) may be caused by a diffusion limitation. Soil profile [CO₂] and δ¹³C values showed substantial temporal changes over the course of a year. Vineyard soil CO₂ was more depleted in ¹³CO₂ below 25 cm in the soil profile during the active growing season as indicated by more negative δ¹³C ratios. This result indicated that different C sources were being oxidized in vineyard soils. Annual C losses were less from vineyard soils (7.02 ± 0.58 Mg C ha⁻¹ yr⁻¹) as compared to oak soils (15.67 ± 1.44 Mg C ha⁻¹ yr⁻¹), and both were comparable to losses reported in previous investigations.