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

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.