Land-use change and soil carbon dynamics in vineyards and oak woodlands

There has been extensive conversion of oak woodlands to vineyards in California. to examine the effects of land use change on soil C dynamics, we conducted an 810 day incubation of soils collected from natural oak woodlands and adjacent vineyards that were converted approximately 30 years ago from oak woodlands.  Changes in availability of soil C were assessed by monitoring rates of CO₂ evolution and changes in microbial biomass and dissolved organic C (DOC). Changes in inorganic N pools also were monitored. Microbial substrate utilization and changes to total soil C were measured using ¹³C analyses.  Greater NO₃ accumulations in the oak soils, combined with lower DOC, total soil C, and respiration rates, indicated that vineyard soils were C limited relative to the oak soils. Headspace respiration δ¹³C values decreased over time with the oak soil respiration having the most depleted values. Total soil C δ¹³C increased during the incubation for all soils as a result of the loss of relatively more depleted C sources. Vineyard berms, which are managed to remove weed biomass under the vines, showed the most enriched δ¹³C values, suggesting that root and rhizosphere deposition make a greater relative contribution to soil organic matter composition than in oak soils. The δ¹³C data indicate that microbes utilize different substrates in the vineyard and oak soils. Increasing microbial biomass and decreasing qCO₂ measurements suggest that a shift in microbial community to one better suited to utilize the more highly degraded substrate occurred during the incubation.