Linking changes in early season water management to changes in nitrogen dynamics in California rice systems
Kaden Koffler, Bruce Linquist, Luis Felipe Tiene Da Silva, Jim Hill, Cass Mutters, Chris Greer, and Chris van Kessel
Department of Plant Sciences, University of California, Davis
Many rice growers are beginning to change early season water management to accommodate the use of newer herbicides, such as Clincher. Growers drain their fields within a couple weeks following seeding, and once dry enough, apply herbicides by ground. Shortly following the herbicide application, the fields are reflooded. This wetting and drying of soil changes the soil redox potential. Fluctuations in redox potential can cause nitrogen (N) pools to undergo transformations from one mineral form to another, resulting in N losses. During the dry-down the soil is reoxygenated, causing ammonium to be nitrified to nitrate. When the field is reflooded, and the soil once again begins to become depleted in oxygen, the nitrate becomes susceptible to loss via denitrification.
In an on-farm experiment near Live Oak, we are measuring changes in soil mineral N and N uptake by rice resulting from the draining and reflooding of rice fields in the early season. These changes in soil mineral N are being linked to changes in soil redox potential, soil moisture, and soil temperature, at different soil depths. Two on-farm field experiments are being conducted: one on a field where rice straw was burned and the other where rice straw was incorporated. Each field has two treatments, each replicated 3 times in a randomized complete block design. The treatments were imposed in the fields by forcing 32" diameter metal rings into the soil, creating a seal so that water could either be kept in or out of the rings, depending on the treatment. The first treatment is the farmer practice of draining the field within 2 weeks after seeding, a period of drying, followed by a Clincher application, and finally reflooding. In the second treatment, the flood water is maintained throughout the early season.
Plant and soil samples were taken from the rings just before the drain began, directly after the Clincher application, and at 7 and 20 days after the fields were reflooded. A final plant and soil sample will be taken at harvest. By comparing a continuously flooded environment with an environment subjected to wetting and drying in the early season, we hope to identify key points in the dry-down / reflood cycle where N losses might occur and to quantify these N losses.