Thursday, August 9, 2007 - 10:10 AM

COS 99-7: The effects of early season soil wetting-drying cycles on N dynamics in California rice agroecosystems

Kaden B. Koffler, Bruce Linquist, James Hill, Randall Mutters, Christopher Greer, and Chris van Kessel. University of California, Davis

Many rice growers are changing early season water management to improve weed management. Some growers drain their fields within 2 weeks following seeding, and once dry enough, apply an herbicide by ground. Shortly following the herbicide application, the fields are reflooded. In two on-farm experiments near Live Oak, California, we measured changes in N dynamics resulting from the flooding-drying-reflooding of rice fields in the early season. Each field had two treatments, each replicated 3 times in a randomized complete block design. The treatments were imposed by forcing 76cm diameter iron rings into the soil, creating a seal so that water could either be kept in or out of the rings. The first treatment (Drained) was the farmer practice of draining the field within 2 weeks after seeding, a period of drying, followed by an herbicide application, and finally reflooding. In the second treatment (Undrained), the flood water was maintained throughout the early season. Draining the fields for a period of 7 to 10 days led to an accumulation of 22-34 kg soil nitrate-N ha-1, which disappeared within 20 days following reflooding. At harvest, N uptake and yield were higher in the Undrained than in the Drained treatment, by 17 and 381 kg ha-1, respectively. These data suggest the drain led to nitrate loss through denitrification, which adversely impacted N uptake and yield. Rice growers using early season wetting-drying practices need to reevaluate and adjust their N management strategies to maintain competitive yields and improve N cycling in their systems.