Improving Fertilizer Guidelines for California's Changing Rice Climate, 2011

 

Project Leader

Bruce Linquist, professional researcher Department of Plant Sciences, UC Davis

Robert Hijmans, assistant professor, Dept. of Environmental Science and Policy, UC Davis

Richard Snyder, UCCE specialist, Dept. of Land, Air and Water Resources, UC Davis

Richard Plant, professor, Dept. of Plant Sciences, UC Davis

Chris van Kessel, professor and chair Department of Plant Sciences, UC Davis

Jim Hill, UC Cooperative Extension specialist, Department of Plant Sciences, UC Davis

 

 

This project continues its focus on improving fertilizer management guidelines that are economically viable and environmentally sound. Research in 2011 examined nitrous oxide and methane emissions, quantifying nitrate leaching, and further development of a Web-based decision tool to predict weed growth.

Greenhouse gas potential

Flooded rice fields contain conditions favorable for the production of greenhouse gases (GHG) such as methane and nitrous oxide.

Methane is about 25 times more potent than carbon dioxide. Incorporation of organic matter in flooded fields stimulates methane emissions.

Nitrous oxide has about 296 times the warming potential of carbon dioxide. The main source of nitrous oxide in rice systems is nitrogen fertilizers.

Thus, improved quantitative estimates of the amounts of methane and nitrous oxide coming from rice fields are critical to prioritize effective mitigation practices.

Two on-farm experiments were conducted in 2011 with a conventionally farmed rice field growing M-206 and a drill-seeded field growing Koshihikari. Water management varied between the two fields, with the conventional field receiving a permanent flood maintained throughout the season and the drill-seeded site flooded and drained several times before permanent flood. Both sites were drained one month before harvest. Several different nitrogen treatments were included to determine possible effects on emission levels.

Principal findings from this research include:

• Practices that affect nitrification (conversion of ammonia to nitrate) appear to regulate GHG emissions more than nitrogen fertilizer rate. Nitrification appears to be the major process involved in nitrous oxide emissions, although denitrification (anaerobic conversion to nitrogen gas) during dry-down periods may also contribute to overall emissions.

• Methane emissions were not directly affected by the addition of nitrogen fertilizer, but high fertilizer applications may lead to higher crop residue and eventually higher methane emissions.

• Frequent flood-drain cycles resulted in higher nitrous oxide emission events. Continuous flooding practices and avoiding flood-drain cycles during the growing season may reduce nitrogen losses from rice fields and consequently lower global warming potential.

• Applying nitrogen deep into soil as aqua ammonia may reduce nitrous oxide and methane emissions (compared to surface nitrogen applications).

• Application of high-nitrogen fertilizer does not necessarily increase global warming potential, provided that rice is grown with best management practices resulting in high resource-use efficiency.

Nitrate leaching study

Water quality restrictions may eventually affect agricultural practices that allow nitrates to enter surface and ground water. In California, rice soils are relatively impermeable. The potential for nitrate leaching is believed to be minimal but needs verification through research.

A study was conducted in 2010 to document the extent of nitrate leaching losses. Soil samples were taken to a depth of seven feet from seven fields.

If leaching were a potential problem in these fields, elevated nitrate concentrations would be found below the root zone. Nitrate concentrations in excess of 10 ppm are considered a health hazard by the U.S. Environmental Protection Agency. In that study the highest nitrate level found was 4.2 ppm in surface soil. In general, surface soils had more nitrate, ranging from 0.4 ppm to 4.2 ppm. Below the rooting zone, nitrate levels were all 3 ppm or less; in most cases less than 0.5 ppm.

In a laboratory study in 2011, the top soil from each of these sites was used to determine the rate at which nitrate breaks down and is lost to the atmosphere as nitrogen gas. Results indicate that upon flooding a rice field 98% of the nitrate is lost to the atmosphere in a day and a half and thus would not have time to leach. Furthermore, the hydraulic conductivity of the rice soils was extremely low. These results suggest that there is not adequate time for nitrate leaching to occur in California rice systems.

Web-based tools

The goal of this work is to develop a Web-based tool for rice weed management in alternative stand-establishment systems in the Sacramento Valley. Central to this approach is the ability to predict early-season weed emergence. In 2011, work focused on watergrass species and on smallflower umbrellasedge.

Researchers quantified the spatial variability of species-specific physiological temperatures for these weeds during rice establishment. Physiological temperatures cover the range of optimal growth for a particular plant species. Each species and biotype has a distinct range of optimum temperature.

The field-scale variability of weed emergence predictions in stale-seedbed and drill-seeded fields was quantified for 2010 and 2011 in Glenn County and Sutter County. This work is being done in cooperation with weed scientists and serves as an initial step toward applying more elaborate germination, emergence, and early-growth models being developed at the laboratory and greenhouse scales.

Scientists also initiated construction of an online interface that will deliver information from emergence models to rice growers. This Web-based tool would enable growers to choose a location, weed of interest, and date of first post-tillage flush of water and then provide real-time emergence estimates for a given date. It could also serve as a platform for the delivery of information from future rice models—weed related or not.