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Twitchell Island USGS project

In the California Delta, a freshwater tidal marsh thick with tules and other marsh vegetation formed carbon-rich peat soils 60 feet deep in places. In the 1870s, farmers began to build dikes, drain the marshes, burn the tules, and farm the peat soils, which oxidized rapidly on exposure to air. The soil surface descended below sea level, sometimes by inches per year. The levees were reinforced, and the water table lowered some more by pumping, so that farming could continue. By the 1990s the soil surface on some of these peat lands was down to 15, 20, and even 25 feet below sea level. The biological crater left by the oxidation of peat soils in the Delta is about the size of the debris flow from the eruption of Mount St. Helens in 1980. The carbon loss was probably on the order of 10 tons per hectare per year.

About ten years ago the US Geological Survey began a small experiment to see if this loss due to oxidation could be reversed. On Twitchell Island, about 15 feet below sea level, they flooded about 6 hectares shallowly, and put in some clumps of tules and cattails. As the plants grew, they raised the water level. After ten years, this experiment has built two feet of peat soils that you can stand on. The plants drew carbon dioxide from the air in photosynthesis, and built complex carbon compounds. The partially decayed dead stalks and roots, protected from further oxidation by careful management of the near-stagnant water, became organic matter or peat. More carbon went in than out.

San Francisco Estuary and Watershed Science, 6(3) (2008)

USGS summary