Assessing the Impacts of Land-Use Change and Ecological Restoration on CH4 and CO2 Fluxes in the Sacramento-San Joaquin Delta, California: Findings from a Regional Network of Eddy Covariance Towers
- Author(s): Knox, Sara Helen
- Advisor(s): Baldocchi, Dennis
- et al.
The Sacramento–San Joaquin Delta in California was drained for agriculture and human settlement circa 1850, resulting in extreme rates of soil subsidence and CO2 emissions due to peat oxidation. As a result of this prolonged ecosystem carbon imbalance where ecosystem respiration exceeded primary productivity, much of the land surface in the Delta now lies 5 to 8 m below sea level. To help reverse subsidence and convert Delta ecosystems from net carbon sources to carbon sinks, land managers have begun converting drained agricultural lands back to flooded ecosystems including wetlands and irrigated rice paddies. However, this comes at the cost of increased CH4 emissions, a much more potent greenhouse gas than CO2.
To evaluate the impacts of drained to flooded land-use change on the biosphere-atmosphere exchange of CO2 and CH4 in the Delta, I conducted a full year of simultaneous eddy covariance measurements at two conventional drained agricultural peatlands (a pasture and a corn field) and three flooded land-use types (a rice paddy and two restored wetlands). This research showed that the drained sites were large CO2 and greenhouse gas (GHG) sources. However, this study also found that converting drained agricultural peat soils to flooded rice paddies or wetlands can help reduce or reverse soil subsidence and reduce GHG emissions, despite the potential for considerably higher CH4 emissions. In particular, wetlands offer the greatest potential for reversing subsidence since both restored wetlands were large net carbon sinks.
Since natural and managed ecosystems can exhibit large year-to-year variation in CO2 and CH4 exchange, I analyzed 6.5 years of measurements from the irrigated rice paddy to investigate the factors affecting CH4 fluxes across diel to interannual timescales and quantify interannual variability in CO2 and CH4 budgets. Using wavelet analysis, I found that photosynthesis induced the diel pattern in CH4 flux, but soil temperature influenced its amplitude. At the seasonal scale, linear and neural network models indicated that photosynthesis and water levels were the dominant factors regulating daily average CH4 fluxes. However, across years, much of the variability in annual and growing season CH4 sums was driven by soil temperature. Soil temperature also strongly influenced ecosystem respiration, resulting in large interannual variability in the net carbon budget at the paddy. This study emphasizes the need for long-term, continuous measurements particularly under changing climatic conditions.
With a growing interest in including wetlands in carbon markets worldwide due to their ability to accumulate large amounts of carbon, there is a need for models that can accurately and cheaply predict wetland CO2 and CH4 fluxes. In the final chapter of this dissertation, I combined eddy covariance CO2 fluxes measurements, flux footprint analysis, and near-surface (i.e. digital cameras) or satellite remote sensing data to investigate the potential of using the light use efficiency approach to accurately and cost-effectively model photosynthesis in wetland systems. Through this analysis, I showed that digital camera and Landsat imagery can be used to model carbon uptake in wetlands, providing inexpensive means of monitoring carbon cycling in these environments that can be used in carbon markets.
By measuring trace gas exchange across multiple sites for multiple years, this dissertation provides new and important insights on the impacts of land use change in the Delta, improves our understanding of factors influencing CO2 and CH4 fluxes from agricultural and restored wetlands across diel to interannual timescales, and presents cost-effective and accurate ways of estimating photosynthesis in restored wetlands by combining flux measurements with near-surface and satellite remote sensing. This work helps bridge understanding between biometeorology, biogeochemistry and climate policy, and provides valuable information to help inform management decisions regarding carbon and water management of the Delta.