Improving Fertilizer Guidelines for California's Changing Rice Climate, 2019


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 2019 were:

  • Assess top-dressed nitrogen need through remote sensing.
  • Study potential nitrogen losses early in the season due to flooding and draining events.
  • Continue research on rice grown under alternate flooded/dry (AWD) soil conditions.
  • 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 hand-held sensor, the Green Seeker NDVI, has evaluated multiple farmer fields. (NDVI is an acronym for “normalized difference vegetation index” and is used to analyze remote sensing measurements.)

    UC Cooperative Extension Rice Specialist Bruce Linquist breaks down his research on using remote sensing of nitrogen status during Rice Field Day 2019
    More recently, a drone was incorporated into the study. It demonstrated that another remote sensing metric called NDRE (Normalized Difference Red Edge) also is a strong predictor of crop nitrogen uptake. In 2019, researchers developed response indexes to predict grain yield at different values. This research should provide farmers with a robust decision-support tool to guide them in midseason nitrogen fertilizer management.

    Nitrogen rate trials with M-206 were conducted at three on-farm locations and one site at the Rice Experiment Station. The main plot treatment used preplant nitrogen injected into the soil as aqua ammonia at rates ranging from zero to 210 pounds/acre.

    At panicle initiation, NDRE was measured using the drone and NDVI was measured with both the drone and the GreenSeeker. After index measurements were taken, plant samples were collected from each plot. Plots were then split into subplots that received top-dressed fertilizer at either zero or 30 pounds/acre. Soil samples were collected from the zero and 150 pounds/acre plots, and again at heading. At maturity, rice plants were harvested from each subplot to measure yield.

    Data from 2018 and 2019 research show that NDVI from both the drone and the GreenSeeker plateau at higher nitrogen rates. The NDVI from the drone has higher values but plateaus at even lower crop nitrogen uptake values, making it a less reliable estimate of crop nitrogen status than the GreenSeeker.

    The relationship between average yield response to top-dressed nitogen fertilizer applied at panicle initiation and GreenSeeker NDVI Response Index
    The relationship between average yield response to top-dressed nitogen fertilizer applied at panicle initiation and drone NDRE Response Index
    However, comparing NDVI and NDRE from the drone shows that the NDRE response to crop nitrogen uptake is linear across a wide range of crop nitrogen values. This confirms observations from 2018, as well as what others have seen with other crops. This suggests that for assessing crop nitrogen status and the need for a midseason nitrogen application, the NDRE may be a better option than NDVI taken from either a drone or a GreenSeeker.

    Results from 2016 through 2019 were combined to develop the yield response to top-dressed nitrogen in each plot. With an NDVI response index below 1.1, the average yield response was close to zero or negative. With a response index above 1.1, there was a positive yield response.

    Combined results from 2018 and 2019 for the NDRE response index showed similar results. Values below 1.1 did not respond to top-dressed applications.

    This research suggests that the GreenSeeker NDVI and NDRE are both useful tools in determining the need for a top-dressed nitrogen application. When needed, a top-dressed nitrogen application increased yields by 200 to 1,000 pounds/acre.

    Potential early season nitrogen loss

    In conventionally managed rice, fields are flooded shortly after aqua nitrogen is applied. Flooding protects the aqua nitrogen fertilizer from converting to nitrate because the field is an anaerobic state. This results in good nitrogen use efficiency.

    However, when a field is drained that still has aqua nitrogen in the soil, oxygen is introduced and microbes can then convert the ammonium to nitrate through a process called nitrification. Nitrate, in turn, can then denitrify and is lost as nitrogen gas (N2). The process is driven by how aerobic soils get during a dry down period, soil temperature, and soil properties such as texture and carbon content.

    To understand this process better, two field studies were conducted—one at the Rice Experiment Station and one near Knights Landing. With knowledge of how fast the conversion of ammonia to nitrate takes place under different soils and drying conditions, growers will have a better idea of how much nitrogen is still available in the soil.

    At the Knights Landing location, nitrogen was applied at a rate of 80 pounds/acre as anhydrous-ammonium. At RES, 150 pounds/acre was applied as aqua ammonia. As expected, at both locations there was no nitrate in the continuously flooded treatment. The soil remained anaerobic and denitrification did not occur. In contrast, in the drained treatment at both locations nitrate increased when the soil was drained and dropped to zero after reflooding. The total amount of nitrate lost during the two drain events was 24 pounds/acre at Knights Landing and 19 pounds/acre at RES.

    While denitrification represents the primary nitrogen loss pathway when rice fields are drained, nitrogen can also be lost as nitrous oxide, a greenhouse gas. Greenhouse gases were measured at the RES site. No nitrous oxide was detected in the continuously flooded field, but about 1.5 pounds/acre was lost as nitrous oxide gas. This loss occurred primarily when the fields were drained, not when they were being reflooded.

    Adding the denitrification losses and the nitrous oxide emission losses together, suggests that about 20 to 25 pounds/acre of fertilizer nitrogen is lost during these drainage events when there is a lot of fertilizer nitrogen in the soil (early in the season). These losses may be partially offset by increased mineralization of soil organic matter when the soil is dried down. Further research will quantify these separately and help develop nitrogen recommendations for these systems.

    Alternating wet/dry rice

    While the conventional system of growing rice under continuously flooded conditions produces good grain yields and maintains high nitrogen use efficiency, California rice growers should be prepared for situations where they might face limited water availability or legislative pressure to implement alternative water management strategies.

    The alternating of wet (flooded) and dry (drained) conditions, known as AWD, has the potential to mitigate concerns about greenhouse gases, arsenic uptake, and methyl-mercury formation in flooded soils. Thus, it is important to evaluate the agronomic viability of AWD as a potential option.

    Research on AWD from 2012 to 2016 with two dry-down periods varying in intensity from two to 12 days between 45 days after seeding to heading indicate:

  • No evidence of yield reductions relative to the continuous flood control.
  • Nitrogen and weed management are the same for AWD as for the control.
  • Methane emissions are reduced by 40% to 90%, while nitrous oxide emissions are negligible.
  • Arsenic and methyl-mercury concentrations in grains are reduced by more than 50%.
  • In 2017, research began on a single dry-down. This fits better into a California cropping cycle. Two years of research have shown:

  • Yields, nitrogen rate, and weed management are the same with a single dry-down as with a continuously flooded field.
  • With a five-day drain period, methane can be reduced by up to 50% and no nitrous oxide emissions. Arsenic uptake also is reduced in grain.
  • In 2019, research on three farms examined AWD where a full check was drained at 40 to 45 days for a two to six day period. Results confirm findings from research at the Rice Experiment Station that draining a field during the midseason for a short period of time does not reduce yields. However, adoption of this practice is unlikely because there are no direct economic incentives (i.e. increased yields or reduced water use) to manage a field in this manner.

    Other experiments

    A very wet May in in 2019 forced some farmers to flood their fields before they had applied their aqua-ammonia. This meant having to supply the nitrogen needs of the crop with other forms of nitrogen such as urea. Little research has been done in this area, so a field trial at the RES was run to evaluate different nitrogen sources.

    In all cases, 150 pounds of nitrogen per acre was applied. Yields were low overall because of late planting. Yields were highest in the aqua and ammonium sulfate treatments and significantly lower in the urea treatment.

    Also in 2019, a study was conducted to determine whether applications of the herbicide Regiment® reduce rice yields and, if so, whether a top-dressed nitrogen application alleviates this effect. Examining results with or without Regiment® indicates that yields were similar regardless of Regiment® treatment. Because of the inconclusive nature of the results, however, recommendations have not been made from this research.

    It is not uncommon when rice fields are fallowed for a season that farmers level the field during the summer. Would it be advantageous to plant rice onto the leveled field the following spring without any additional tillage? An experiment compared rice yields in no-till versus tilled fields on a half-acre plot. Yields were low and also similar between the two treatments.