Wetlands and suburbs co-exist in the shadow of Mt. Diablo. Photo: Janet Byron

ESTUARY NEWS: Sniffing the Delta for Greenhouse Gases

by Janet Byron, Estuary News Group

Dutch Slough in Oakley, on the southern edge of the Sacramento-San Joaquin Delta, is less than a mile from where biometeorologist Dennis D. Baldocchi grew up on his parents’ orchard and fished with pals in nearby Marsh Creek. In October 2021, the California Department of Water Resources breached the levees here, restoring nutrient-rich tidal flows to degraded ranchland.

Early in the 20th century, Baldocchi’s father grew dry beans and sugar beets on the peat soils of the Delta’s Liberty Island, and his aunt’s family raised asparagus, sugar beets, and corn on Sherman Island. In 1952, his dad started growing almonds and walnuts at the junction of Sellers Avenue and Cypress Road in Oakley.

“My family mined the Delta for agriculture, which led to subsidence and contributed to climate change,” says Baldocchi, a University of California, Berkeley, professor whose current research focus is the ability of restored tidal wetlands to sequester carbon. “Now my generation is trying to use science to stop it.”

Beginning in the mid-19th century, virtually all of the Delta’s wetlands were drained and transformed into ranches and fertile farms via the construction of a vast network of levees. Without natural tidal flows, the land between those levees — with rich peat soil up to 50 feet deep — has been sinking rapidly, as much as 30 feet in some places. “Soil microbes eat carbon compounds and convert them into energy and carbon dioxide gas, which causes the land to collapse and subside,” Baldocchi says.

Of the Delta’s 740,000 acres, about two-thirds are in agriculture; top crops include corn for forage, alfalfa, wheat, wine grapes, and processing tomatoes. Global food production contributes an estimated 21% to 37% of the greenhouse gases that are warming the planet, including huge releases of carbon when vast aquatic ecosystems like the Delta are transformed for agriculture. Baldocchi and colleagues are now trying to figure out if bringing back tidal wetlands like the one at Dutch Slough can help reverse these processes.

“These are some of the most productive ecosystems in the world,” Baldocchi says. “We want this to be a greenhouse gas sink to stop global warming.”

Lab for Restoration Strategies

When Baldocchi was growing up in the 1970s, Oakley was a hamlet of about 1,300 people. Now it’s a growing city of nearly 44,000, east of the Antioch Bridge. Before the city of Oakley incorporated in 2002, Contra Costa County had slated 4,500 homes for the Dutch Slough site, precariously below sea level; now a newish subdivision skirts the upland area above the flood and tide lines on its southern border.

Wetlands and suburbs co-exist in the shadow of Mt. Diablo. Photo: Janet Byron

The $71 million Dutch Slough Tidal Restoration Project is a laboratory for ecosystem restoration strategies. When complete it will include tidal wetlands, marsh managed for the elusive black rail, nest-tree plantings for raptors, preservation of Indigenous lands and an historic vineyard, and a 55-acre community park with regional access trails.

Katherine Bandy, who manages the project for the Department of Water Resources, says that Baldocchi’s research is important because Dutch Slough uniquely functions as both a freshwater and tidal marsh. “Most freshwater marshes are impounded, and most tidal wetlands are saline,” she says. “This type of tidal marsh hasn’t been widely studied before.”

In the years before tidal action was restored, portions of the land were graded and the Department of Water Resources and River Partners planted 25,000 native tule plugs and 50,000 shrubs and trees. Just seven months after the levee was breached, the marsh is thickly vegetated with reeds and sedges swaying in the breeze; the trees and shrubs will need more time to become established. With restored tidal action, new soil is accreting at a rate of two to three centimeters per year, Baldocchi says.

The marsh is humming with wildlife. Red-winged blackbirds cackle and trill in the tule, egrets and a great blue heron fish in the channels, a pintail alights from the marsh, turkey vultures cruise overhead, and killdeer clown around in the upland area. This reporter was delighted to see an otter waddle across the levee.

Arias-Ortiz on teabag patrol. Photo: Janet Byron

On a clear spring morning, Baldocchi shows where the Department of Water Resources breached the levee, with a DEAD END sign marking the end of the road. Nearby, UC Berkeley Biometeorology Lab technician Daphne Szutu is up on the weather station downloading data, while fellow technician Joe Verfaillie makes some adjustments to a narrow walkway out into the marsh.

Crouching among the tules, Ariane Arias-Ortiz, a UC Berkeley postdoctoral researcher and recent NOAA Climate and Global Change fellow, checks the teabag index samplers. A few weeks ago, she and Baldocchi enlisted 5th and 6 graders at nearby Knightsen Elementary School in a global citizen-science project that is burying household teabags in wetlands all over the world to measure how well they decompose carbon.

“By calculating the weight loss of the teabag after three months, students can estimate the rate of decomposition of plant material by microbes in wetland soils and compare it to that of other soils, such as their school garden,” Arias-Ortiz says.

Carbon Uptake by Marsh Plants

Baldocchi’s research program uses the tools of biometeorology — the study of relationships between living things and atmospheric phenomena — to gauge how well plants in restored marshes take carbon out of the air and return it to the soil, and how the reintroduction of aquatic plants influences overall greenhouse gas levels.

Here at Dutch Slough and other restored tidal wetlands in the Delta, his team is continuously monitoring vertical fluxes of carbon dioxide and methane between water, soil, plants, and the atmosphere, as well as lateral fluxes of carbon as tides flow in and out. “We’re sniffing the delta,” Baldocchi says. “We can monitor pockets of air moving up and down and across the marsh. Our methods allow us to measure the breathing of the biosphere.” A three-year, $700,000 grant from the Delta Science Program is helping to fund the work.

Joe Verfaillie works on walkway. Photo: Janet Byron

Baldocchi’s current research at Dutch Slough and Hill Slough — a new wetland at Suisun Marsh in Solano County that was intentionally breached about the same time — builds on more than a decade of studies measuring the exchange of greenhouse gases on agricultural lands that have been restored as wetlands.

In a 2021 article in the journal PLOS ONE, Baldocchi and colleagues reported on 21 site-years of carbon flux measurements on five restored freshwater-to-brackish wetlands at the Delta’s Twitchell Island and Sherman Island, which had been restored between three and 23 years ago (see figure).

Researchers found that a minimum of 55% of vegetation cover was needed for wetlands to become carbon sinks, and site-specific conditions including water levels, soil nutrients, and planting methods mattered in terms of how well they captured carbon from the atmosphere.

Measuring Greenhouse Gas Balances

Another Baldocchi study, published in 2019, compared the amounts of carbon and methane that were absorbed and emitted by restored wetlands and drained agricultural lands, also at Sherman and Twitchell islands, for 36 site-years.

While sequestering carbon, natural and restored wetlands also release methane, which is created when soil microbes decompose plant matter under the anaerobic, or low-oxygen, conditions of land that is under water. A potent greenhouse gas, methane effectively traps 25 times more heat in the Earth’s atmosphere than carbon dioxide.

Szutu collects data on the Dutch Slough weather station. Photo: Janet Byron

After the initial year of restoration, Delta wetlands extracted an average of 339 grams per square meter (± 55 g/m2) of carbon each year, while the agricultural sites released between 200 and 1,541 g/m2 of carbon into the atmosphere annually.

But the Delta wetlands also released methane at an average rate of 44 g/m2 per year, with significant variability among sites due to water management, soil nutrient levels, and other conditions. The agricultural sites, which included rice, pasture, corn, and alfalfa, released up to 12 g/m2 of methane per year, and none in some locations.

In a subsequent analysis, Baldocchi and colleagues found that the restored wetlands quickly became carbon sinks once vegetation was established, and, if well-maintained, had the potential to be net greenhouse gas sinks within a century, and likely decades sooner. “The reality is these wetlands are very effective carbon sinks, and as time goes on the methane production and its release into the atmosphere will go down,” Baldocchi says.

In addition to understanding the potential of restored wetlands to mitigate climate change, the biometeorology researchers are learning which restoration, water management, and planting strategies work best to provide long-term benefits for marsh ecosystems. “The Dutch Slough project,” Bandy says, “is helping climate scientists to figure out what type of wetlands are the best bang for the buck for restoration when it comes to carbon sequestration.”

Dutch Slough. Photo: Janet Byron

Mitigating Climate Change

Baldocchi’s studies are finding that Delta wetlands match upland forests in their ability to pull carbon from the atmosphere, providing important evidence to support their inclusion in the state’s carbon-trading markets, a key component of California’s strategy to lower greenhouse gas emissions.

“The research that is being conducted at Dutch Slough will help validate the efficacy of tidal wetlands in providing greenhouse gas benefits in the Delta,” says Michelle Jesperson of the Department of Water Resource’s California EcoRestore program, which seeks to bring back 30,000 acres of Delta wetlands.

Cal EcoRestore is on track to meet this goal, adds Department of Water Resources project manager Charlotte Biggs, with 10,000 acres of wetlands completed, 10,000 acres under construction, and the remaining acreage in planning and permitting.

Back at the Dutch Slough research station, Arias-Ortiz says the UC Biometeorology Lab’s work clearly demonstrates that carbon extracted by marsh plants has immediate benefits for climate mitigation. “As wetlands age, the carbon capture these ecosystems provide exceeds the negative effects of methane emissions, while additionally providing many other valuable services such as the reversal of subsidence and habitat for fish and wildlife,” she explains. “The atmospheric carbon taken up by plants is being buried in the ground, and most of it stays locked up there.”

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