Using a Network of Flux Towers to Investigate the Effects of Wetland Restoration on Greenhouse Gas Fluxes

The Sacramento-San Joaquin Delta, a peatland and former wetland, was drained a 100 years ago for intensive agriculture. In the interim, over 10 m of peat has been lost, mostly through oxidation. Current land use is not sustainable, if this region is to maintain its integrity and serve as a conduit for freshwater pumped from northern to southern California. There is great interest in restoring this disturbed landscape with tule wetlands or rice; one is an effective carbon sink, the other an economically viable alternative. Questions arise to how effective are these new landscapes in sequestering carbon and what are the unintended consequences, such as the production of methane and high rates of evaporation from flooded lands, in a semi-arid climate? We are currently operating 6 eddy covariance flux systems that measure short and long term fluxes of carbon dioxide, water vapor and methane. We are making flux measurements over landscapes that represent business as usual (irrigated pasture and corn) and new alternatives (rice, a newly restored wetland and a 14 year old wetland). The pasture and corn operate as carbon sources and are weak emitters of methane on annual time scales. The rice is a modest sink of carbon dioxide, but becomes a carbon source when harvesting is considered. It is a smaller source of methane compared to rice growing in the Sacramento Valley on clay. It seems that there are sufficient alternative electron acceptors (iron, nitrate) that moderate methane production for rice growing on peat soils. The newly restored wetland is a huge methane source, with fluxes exceeding 300 nmol m-2 s-1. In the first two years of functioning, following disturbance, it is switching from being a carbon source to a sink, as tules fill the landscape. The older wetland remains a strong methane source, and its carbon dioxide sink potential is diminishing at it becomes derelict with much undecomposed vegetation.