Introduction

1. Rice culture in the Sacramento-San Joaquin Delta to mitigate past agricultural impacts, improve water quality and sequester carbon William R. Horwath1, Leslie Butler1, Bruce Linquist1, Dennis Baldocchi2, Philip Bachand3, Steve Deverel4, Jacob Fleck5, Stuart Siegel6, and Amy Merrill7 1University of California, Davis, 2Univerersity of California, Berkeley, 3Tetratech, 4Hydrofocus, 5USGS, 6Wetlands & Water Resources, Inc., 7Stillwater Sciences Introduction The Sacramento-San Joaquin Delta (Delta) is a large wetland that serves as the confluence of major California rivers, which supply the majority of the water for California State Water Project. Two thirds (740,000 ac) of the Delta is in agriculture. The drained fertile peat soils have been used for agriculture since the late 1800s. Today, the Delta is dramatically deteriorated as evidenced by subsidence up to 20 feet or more below sea level. With subsidence, extensive greenhouse gas (GHG) emissions have resulted with an estimated 5.5 million CO2eq tons of carbon dioxide and 0.5 to 2 million CO2eq tonnes of N2O lost annually (1 to 2 percent of California’s total emissions). Severe subsidence has required the construction of 1100 mile levees to protect the Delta islands and the Delta’s importance in conveying water to 28M Californians and for irrigating 3M acres in the Central Valley. The levee system prevents inundation of the island network and the ingress of salty oceanwater. Yet the Army Corp of Engineers assesses the condition of this levee network as second in catastrophic failure potential to the New Orleans levee system. Thus, subsidence has resulted in risk to California’s water supply. Finally, peat oxidation has degraded water quality through releasing dissolved organic carbon (DOC) and methyl mercury (MeHg) impacting public health (drinking water) and ecosystem integrity. Results GHG chamber technique Results Subsidence Mg CH4-C m-2 hr-1 • The Chamber GHG method shows higher methane emissions than the Eddie Covariance method. Differences need to be resolved. • Groundwater flow model results showing the effect of subsidence under business as usual (BAU) and rice cultivation. • After 50 years of continuing subsidence under BAU conditions (corn and alfalfa), the model estimates seepage will increase by about 28 %. • For two hydrologic conditions for rice cultivation, water levels at land surface and 30 cm above land surface, seepage rates onto the island were reduced by 18 and 23 percent, respectively. Results Mercury Approach This project will address key research questions associated with hydrology and water quality, soil C dynamics and soil carbon balance. These questions will be addressed at a variety of scales – agronomic plots, experimental 1-acre cells, fields of many acres, crop complex, and island-scale. We propose work that 1) will quantify the effect of rice improvement and cultivation on land-surface elevations, GHG emission and the soil carbon balance; 2) define best management practices for widespread rice cultivation in the Delta; and 3) quantify the logistical, environmental and economic opportunities and constraints to a regional water solution. These impacts are assessed across three scenarios (Business –as-Usual, Agrarian Delta, Water Conveyance) for assessing economics across scales or different potential future pathways. Results Eddy covariance technique • Comparison of MeHg concentrations and percent MeHg by ditch type for samples collected from October 2008 to September 2011. MeHg concentrations increased in ditches receiving all or part of their drainage from rice (P=0.002). The percent MeHg (ratio of MeHg to THg) was highest in ditches receiving exclusively rice fields but the difference was not significant. A direct comparison between corn and rice ditches was significant only to P=0.056. Results Soil Carbon Soil carbon increases with depth, typical of a degraded peat wetland. The 13C content of soils previously in corn shows no corn carbon accumulating corresponding to a general decline in total carbon. Results Economic & Descriptive Storyline δ13C • Eddy covariance techniques to determine the carbon balance of rice. Results show a positive carbon balance, which supports subsidence reversal. Results Agronomics We have estimated current and future costs and benefits of converting about 14 of the most commonly grown crops in the Delta to rice production. Taking into account the risks and uncertainties associated with rice conversion in the Delta, We have found that rice is a feasible replacement for many of the crops grown in the Delta. Further monitoring and economic analysis is needed to determine the impacts on water quality, GHG emissions and on the states’ water supply for agriculture and municipal use. Strategies are being developed for integrating a critical mass of rice into the Delta under three Delta scenarios (Business-As –Usual, Agrarian Delta, Water Conveyance) Forward • Grain yield showed no significant response to N application rates between 0-160 kg N/ha • Weed management is a major challenge in this region due to organic soils binding herbicides • Eddie covariance techniques methane emissions estimates from Delta rice paddies for the last 4 years are lower than from mineral soils.