Improving Fertilizer Guidelines for California's Changing Rice Climate, 2012

 

Project Leader

Bruce Linquist, professional researcher Department 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

The overall objective of this project is to develop economically viable and environmentally sound fertilizer management guidelines for California rice growers. Research in 2012 focused on four areas: quantifying emissions of nitrous oxide and methane, quantifying nitrogen losses from nitrate leaching, preplant nitrogen fertilizer availability and loss following application, and assessing potassium status in rice soils.

Greenhouse gas potential

Flooded rice fields contain conditions favorable for the production of greenhouse gases (GHG) such as methane and nitrous oxide. Improved quantitative estimates of methane and nitrous oxide coming from rice fields are critical to prioritize effective mitigation practices.

Research in 2012 investigated nitrogen management practices and whether choice of variety affected emissions. Studies evaluated the effect of aqua ammonia and urea nitrogen sources and the effect of subsurface nitrogen applications on methane and nitrous oxide emissions.

Results from this research were inconclusive, however, because of problems establishing rice. Bird damage, poor weed control, and problems with application equipment were at issue. While differences in emissions were noted, factors other than nitrogen management or variety must be taken into consideration.

Nitrate leaching study

Research in 2012 directly quantified nitrate leaching in rice systems, a follow-up to work initiated in 2010. Four fields that were sampled in 2010 were selected for this study. Soil samples to a depth of 40 inches were taken and analyzed.

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 to be a health hazard.

Results showed that before planting, levels of nitrate and nitrogen below the root zone were below10 ppm, confirming previous results. After fertilizing and flooding, nitrate levels in water below the root zone were at their highest but never above 10 ppm. As the season progressed, nitrate levels became undetectable.

An experiment with labeled nitrogen showed that fertilizer nitrogen is not the primary source of nitrate below the root zone. Fertilizer nitrogen contributed, at most, 15% of the nitrogen below the root zone.

Findings from both years suggest that nitrate leaching in rice fields is not a major concern. This can be explained by the impermeability of many rice soils and by the loss of nitrate to the atmosphere as nitrogen gas.

Preplant nitrogen

Following nitrogen application, planting is sometimes delayed because of spring rains. This delay can lead to nitrogen losses through denitrification or ammonia volatilization.

A study was begun in 2012 to quantify nitrogen losses so growers would have a better idea of how to manage nitrogen following planting delays. Nitrogen as aqua ammonia and urea were applied 15, 10, five, and zero days before flooding and planting. Weather conditions during this period were dry with no rainfall. As a result, less than one pound of nitrogen per acre was lost as either nitrate or through denitrification.

Under dry conditions there was no perceived cost to delaying flooding. However, delaying flooding after nitrogen application is not recommended.

Potassium status

Scientists assessed the potassium status of 31 rice fields in the Sacramento Valley in 2012. This involved grower surveys to determine the history of each field—fertilizer and straw management, yields, and water source. Soil samples were taken from three checks within each field. Water samples were taken twice during the season. Flag leaf samples were taken at heading.

Fourteen of 31 fields were usually fertilized with potassium. Average rate was 27 pounds/acre. Soil potassium levels ranged from 35 ppm to 350 ppm. Potassium levels, however, were not related to fertilizer history. The three fields below the critical threshold of 60 ppm were located east of Highway 99. Other fields with potassium levels below 100 ppm were also located on the east side of the valley. On the west side of the valley, all fields had potassium levels above 100 ppm—regardless of fertilizer management.

The relationship between soil potassium and flag leaf potassium values in fields where potassium fertilizer was not applied.

Fields on the west side are irrigated with Sacramento River water, which has the highest potassium concentration (1.2 ppm) of all irrigation water tested. On the east side of the valley, fields are irrigated with water from the Feather, Yuba, or Bear rivers, with an average potassium concentration below 0.8 ppm. This may be one factor causing lower soil potassium values on the east side of the valley.

Fields with potassium levels below 100 ppm had lower flag leaf potassium values unless they were fertilized. Thus, potassium fertilization is recommended in fields below 100 ppm. This study may be expanded to include fields where potassium is not used and where straw is routinely removed.