Carbon Farming Tested in California Delta

The research team has won a three-year, $12.3 million grant from the California Department of Water Resources to test the concept on 400 acres in the Delta beginning next spring.

Called carbon farming, the project involves building wetlands, which is what nature originally grew in the Delta. Following the Gold Rush, developers "reclaimed" the land for agriculture by constructing levees to drain swamplands and contain the rivers that form the estuary.

Over the past 150 years, conventional farming practices have exposed fragile peat soils to wind, rain and oxygen, liberating carbon from the soil and causing subsidence, or sinking, of Delta lands. According to the USGS, most of the islands farmed in the Delta are more than 20 feet below the surface of the water. They are kept dry and intact only because of the levees.

Farmers' fields adjoin the channels of the Sacramento-San Joaquin River Delta. (Photo by Phillip Capper)

The carbon farming project aims to rebuild the rich peat soils by re-establishing wetlands. A pilot project by the USGS and state Department of Water Resources has already shown that it can work.

On an island called Twitchell in the western Delta, researchers planted two seven acre test plots with cattails, tule grass and other wetlands vegetation. As the plants grew, died and decomposed, they left roots and other parts that gradually compacted into a material similar to the original peat. From 1997 to 2005, the USGS measured 10 inches of new soil.

The pilot also showed that the process could sequester up to 25 metric tons of carbon dioxide per acre per year and eliminate the CO2 emissions produced by current farming practices, which cause peat to oxidize, virtually evaporating and blowing away, the USGS reported in a briefing paper.

If California converted into carbon farms an area the size of all subsided lands in the Delta, the amount of greenhouse gas emissions avoided each year would be equivalent to trading all SUVs in the state for small hybrids, the agency estimated.

The state is under a self-imposed deadline to scale back its greenhouse gas output by 2020 to the level emitted in 1990.

As more governments tackle greenhouse gases, the concept of carbon farming is catching on. Usually, the term refers to paying farmers to plant trees and other vegetation that stores carbon for longer periods than crops; or to less frequently till the land, a practice that delivers carbon in the soil into the atmosphere.

The Delta brand of carbon farming specifically involves rebuilding wetlands. The project is not without potential risks. As the USGS briefing paper notes, "Large scale efforts to manage the environment have a decidedly mixed record of success."

One possibility is that the wetlands will emit methane and nitrous oxide, two greenhouse gases more potent than carbon dioxide, potentially canceling the benefit of sequestering the carbon. The USGS said measurements of methane varied widely in the pilot. The scientists did not attempt to measure nitrous oxide.

Another possible drawback is that certain conditions under which carbon is captured may produce methylmercury, a neurotoxin that accumulates in the food chain, concentrating in fish.

Methylmercury is highly toxic to mammals, including people, says the U.S. Environmental Protection Agency. Eating fish high in methylmercury can permanently damage the brain, kidneys, and developing fetus. Effects on brain functioning may result in irritability, shyness, tremors, changes in vision or hearing, and memory problems. If the benefits of wetlands restoration outweigh problems, the project could accomplish three big goals: it would sequester carbon, reverse subsidence and provide a means of making a living from land in a sustainable manner, said Roger Fujii, Bay-Delta program chief for the USGS California Water Science Center.

In a statement, Fujii said, "This project is an investment in California's future that could reap multiple benefits over several decades - for California, the nation and the world."

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