Temperate peatlands, such as those in the Sacramento -San Joaquin Delta (Ca) study site which is the focus of this project are hotspots of soil-carbon storage and biological diversity. They are additionally known to be vulnerable to climate change, land use, and altered water management practices. A comprehensive set of eddy covariance flux estimates for the carbon gases CH4 (methane), CO2 (carbon dioxide) and water vapor on daily, seasonal and interannual time scales will be measured and interpreted in the context of a suite of biotic (e.g. leaf-area index, plant type isotope discrimination, other edaphic and microbiological indicators) and abiotic (water table height, temperature, soil moisture, oxygen, hydrological, climatic, geohazards such as storms and earthquakes) processes. Soil biogeochemistry studies will be directed towards understanding how microbiological consortia of methanogens and methanotrophs affect the net flux of methane from these vulnerable wetland systems.
Using these results, processes level models of peatland and wetland storage and emissions of carbon gases will be scaled up with remote sensing and Geographic Information System (GIS) techniques used to indicate vegetation type and net primary production, with the intention that these models be then extended and compared to peatlands in related ecosystems. Outreach activities of the project will include environmental education in this increasingly populated area as well as contributions to community decision making and land management issues. Connections to ongoing international activities such as the FLUXNET project as part of the ILEAPS (Integrated Land Ecosystem Atmosphere Processes Study) of the International Geosphere-Biosphere Program (IGBP) are an important means of evaluating processes and feedbacks of carbon and water cycle components on a truly global scale.
This work is supported under the NSF Carbon and Water in the Earth System solicitation, an interdisciplinary funding opportunity from the Directorate of Geosciences.
We have completed a multi-year study of the greenhouse gas exchange of contrasting landscapes in the delta of the Sacramento-San Joaquin Rivers. The objectives of the project were to understand biophysical and biogeochemical controls on carbon dioxide, methane and water vapor fluxes, to construct their annual trace budgets for a number of representative ecosystems (peatland pasture, rice and wetlands) and quantify the temporal and spatial dynamics of greenhouse gas exchange. From a societal point this research addresses the biogeochemical consequences of restoring drained peatlands from agriculture back to native wetlands; with business-as-usual the soils will continue to decompose and subside, while flooding and restoring these lands will stop and revise soil subsidence, but produce methane, an undesirable greenhouse gas. Methane flux measurements were made with two novel and new generation tunable diode laser spectrometers; one was an open path system and the other was an open path system. We conducted a series of comparative flux and concentration measurements between our closed path Los Gatos sensor and a new open path sensor methane sensor from LICOR Biosciences. The open path sensor performed very well in terms of accuracy, stability and detection limit of measuring methane fluxes. Because it does not need a high power pump and its one power draw is low, it has great potential for being used off the power grid over remote wetlands where methane fluxes are greatest and the scientific questions most challenging. The drained peatland pasture, which represents business-as-usual, was a net carbon source and this management choice contributed to continued soil subsidence of the peatland; this ecosystem lost between 175 and 299 gC-CO2 m-2yr-1. This pasture was also a small source of methane, losing less than 10 gC-CH4 m-2 y-1. Spatial sampling with chambers enabled us to detect hotspots of methane and nitrous oxide emission across the landscape; these were associated with the drainage ditches that cross the fields in regular patterns that comprise < 5% of the landscape and account for 84% of the areally-upscales emissions. The complex spatial patterns of methane hot and cold spots affected the interpretation of methane concentrations and fluxes measured with our eddy covariance system. A pronounced feature of our methane flux measurements was a strong diurnal pattern with maximal values at night at both the peatland pasture field site; conventional wisdom expects either a daily maximum or a flat daily pattern. A series of case-study field campaigns were conducted to explore the cause of this unexpected diurnal pattern in methane emission; we measured flux divergences at the anchor site, the drained and grazed peatland pasture, advective fluxes on the levee upwind from the anchor site and downwind from an extensive tule wetland, and methane fluxes at a site without cows, a rice paddy. Elevated methane concentrations and fluxes, at night, were found to be due to a combination of the extension of the flux footprint under stable conditions to wet, methane hot spots, upwind, and the presence of cows, close to our tower. In comparison to the drained peatland pasture, planting flooded rice enables the landscape to be a net carbon sink, exclusive of the carbon harvested and removed from the landscape. There is a water-based cost to cultivating rice in this Mediterranean climate as the rice paddy and wetland evaporated 45-95% more water than the drained grazed peatland. The newly restored wetland was a small carbon source, its first year of operation. But, it has as great potential to be a large carbon sink as the tules infill the wetland. On the other hand, the newly restored wetland is a significant source of methane, producing over 15 gC-CH4 m-2 y-1, which is two to three times the amount of methane emitted from the rice paddy and peatland pasture, respectively. In conclusion, the eddy covariance method demonstrated great promise for measuring long-term budgets of methane and carbon dioxide in remote wetlands at the field scale.. We also found a strong trade-off between the roles of draining and flooding soils on the potential to sequester carbon and produce methane. In summary, the future practice of agriculture in the drained peatlands of the Delta has limits due to the fact it is mining the soil and is weakening the infrastructure of levees that keep the fields drained. On the other hand, restoring wetlands will build soils back, reduce the vulnerability of the levee system, but produce prodigious amounts of methane.
