Land-use change influences soil carbon (C) transformations and characteristics as well as soil physical properties. We examined the effects of the land-use change in oak woodlands and adjacent vineyards converted approximately 35-40 years ago in the Oakville region of Napa Valley, California. All experimental sites were located on the same soil type and on similar slopes.
Field experiments found that annual 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 than those observed from vineyard sites. Soil gas diffusion coefficients for oak soils were larger than for vineyard soils. Soil profile CO2 concentration ([CO2]) and δ13C values showed substantial temporal changes over the course of a year. Vineyard soil CO 2 was more depleted in 13CO2 below 25 cm in the soil profile during the active growing season as indicated by more negative δ 13C ratios. It is likely that different C sources were being oxidized in vineyard soils as compared to oak soils. Annual C losses were less from vineyard soils (7.02 ± 0.58 Mg C ha-1 yr-1) than oak soils (15.67 ± 1.44 Mg C ha-1 yr-1).
Long-term 810 day laboratory incubations were used to further address soil organic C (SOC) decomposition and microbial substrate utilization. Greater NO3- and microbial biomass C accumulations in the oak soils, combined with lower DOC, total soil C, and lower respiration rates in the vineyard soils, indicated that vineyard soils were C limited relative to the oak soils. Normalized respiration rates (respiration g-1 C) demonstrated that C quantity rather than other factors such as quality had a stronger impact on microbial activity. Respiration δ 13CO2 values decreased over time with the oak soil respiration having the most depleted δ13C values. Vineyard berms, which were 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 total soil organic matter composition than in oak soils. The δ13C data indicated that microbes utilized different substrates in the vineyard and oak soils. Increasing microbial biomass and decreasing metabolic quotient (qCO2 ) measurements suggested that a shift in microbial community to one better suited to utilize a more highly degraded substrate occurred during the incubation for the oak and vineyard soils.
In the Mediterranean climate of California, water has been found to be the primary limiting resource to soil microbial activity during the hot summer months. Furthermore, season (i.e. wet or dry season) has been found to have differing impacts on soil C transformations in oak woodlands and vineyards that have been converted from oak woodland systems. Conversion of oak woodlands has resulted in changes in profile C characteristics including particulate organic matter (POM) C, bulk soil C, DOC, microbial activity, and soil atmosphere [CO2]. POM C fractions, particularly those associated with the mineral soil (<53 μm) was greater in oak than vineyard soils. This fraction is thought to be the most stable of the POM fractions, and demonstrates that the loss of C from the soil profile during the conversion process was primarily of the most stable POM C pool. Water additions were found to have different impacts on soil C in the oak and vineyard sites, primarily with regard to microbial responses to the increased soil moisture.
|Advisor:||Smart, David R.|
|Commitee:||Dahlgren, Randy, Tate, Kenneth|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||DAI-B 70/06, Dissertation Abstracts International|
|Subjects:||Ecology, Biogeochemistry, Soil sciences|
|Keywords:||Incubation, Land use change, Oak woodlands, Soil carbon, Vineyards|
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