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.