Bruce Linquist, UCCE specialist, Dept. of Plant Sciences, UC Davis

The goal of this project is to develop fertilizer management guidelines that are economically viable and environmentally sound. Research objectives in 2018 were:

Remote sensing of nitrogen status

This study was initiated in 2015 to evaluate the potential for sensor-based technologies to assess nitrogen status in rice and to determine the need for a top-dressed nitrogen application. A handheld sensor, the Green Seeker NDVI, evaluated multiple farmer fields from 2015 to 2018. (NDVI is an acronym for “normalized difference vegetation index” and is used to analyze remote sensing measurements.) Additionally, in 2018 a drone was incorporated into the study.

In 2018, four nitrogen response trials were established with five preplant nitrogen rates ranging from zero to 210 pounds/acre. Additionally, at panicle initiation, each plot was split into three subplots receiving zero or 30 pounds nitrogen top-dressed. This study confirmed the following.

The Green Seeker NDVI is a good indicator of aboveground nitrogen content at panicle initiation but is a relatively poor indicator of biomass and of nitrogen concentration in the plant at panicle initiation.

At panicle initiation, the crop needs at least 100 pounds/acre in the aboveground biomass to achieve maximum yields. This equates to a Green Seeker NDVI value of 0.69. An NDVI value lower than this indicates the need for a top-dressed nitrogen fertilizer.

A response index (maximum observed NDVI/treatment NDVI) is a more robust way of determining the need for top-dressed nitrogen. A response index of 1.1 or greater indicates a greater chance of a positive yield response.

The Green Seeker NDVI was superior to the drone NDVI for detecting crop nitrogen status. However, the drone showed potential for use in other indices to evaluate crop nitrogen status. This will be evaluated further in 2019.

Ammonia volatilization study

Previous research showed that nitrate leaching losses are low in California rice systems. Nitrous oxide losses through leaching or runoff make up only 6% of all fertilizer nitrogen applied. This is good compared to other rice systems globally, as well as other cropping systems.

A study was initiated in 2017 to test a method for quantifying ammonia volatilization from soil in preplant nitrogen fertilizer applications. In 2018, this approach was used to evaluate ammonia losses at eight on-farm locations. The four treatments examined were zero nitrogen, aqua-ammonia, urea broadcast, and urea drilled. Rates were typical of grower practices. At panicle initiation, a similar experiment was conducted at seven locations to quantify ammonia volatilization from top-dressed applications of aqua-ammonia or ammonium sulfate.

Overall, 0.2% or less of the fertilizer was lost through ammonia volatilization in the aqua- and urea-drilled treatments. With urea broadcast as pellets, 1.7% of the fertilizer was lost on average across on-farm sites. In all cases, losses were low and did not significantly affect yields.

In the top-dressed applications, losses from ammonium sulfate were higher than urea. The reason for this is not clear. Overall, losses represented 2.6% of the amount of ammonium sulfate applied and only 0.1% of the urea applied. Under current management practices, where the bulk of nitrogen is applied as aqua-ammonia, about 1% of the fertilizer is lost (assuming a 180 pounds/acre application).

Alternating wet/dry rice

Current irrigation practices keep most California rice fields continuously flooded through the majority of the rice growing season. This strategy helps provide high yields, good weed control, and efficient nitrogen use. Nonetheless, there is interest in exploring alternative production practices such as alternating flooding with periods of dry soils (AWD).

An ongoing study at the Rice Experiment Station has been evaluating this practice. Research on AWD from 2012 to 2016 focused on two dry-down periods 45 days after seeding between two and 12 days in length. In all AWD treatments, grain yields were the same as in the conventional water-seeded treatment. The optimum nitrogen rate required to achieve maximum yields was similar among treatments. This research also showed that AWD reduces methane emissions between 40% to 90%. Nitrous oxide emissions were negligible. Concentrations of arsenic and methyl-mercury in grains were reduced by more than 50%. However, arsenic concentrations were not reduced in “Safe” AWD, where dry-downs were only two to three days.

In 2017, research focused on a single dry-down beginning around 36 days after planting. A yield decline was noted for the first time. In addition, harvest was delayed by a few days. The early dry-down also resulted in increased nitrous oxide emissions. However, the season was unusually hot, and that probably affected results.

In 2018, a single drain was examined in each treatment. At the Rice Experiment Station, the following treatments were replicated three times: