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Study offers insights on reducing nitrate contamination from groundwater recharge
Light irrigation before flooding stimulates microbes to remove nitrates from soil
With California enduring record-breaking rain and snow and Gov. Gavin Newsom recently easing restrictions on groundwater recharge, interest in “managed aquifer recharge” has never been higher. This process – by which floodwater is routed to sites such as farm fields so that it percolates into the aquifer – holds great promise as a tool to replenish depleted groundwater stores across the state.
But one concern, in the agricultural context, is how recharge might push nitrates from fertilizer into the groundwater supply. Consumption of well water contaminated with nitrates has been linked to increased risk of cancers, birth defects and other health impacts.
“Many growers want to provide farmland to help recharge groundwater, but they don't want to contribute to nitrate contamination of the groundwater, and they need to know how on-farm recharge practices might affect their crops,” said Matthew Fidelibus, a University of California Cooperative Extension specialist in the UC Davis Department of Viticulture and Enology.
A recently published study by UC scientists sheds new light on how nitrates move through an agricultural recharge site and how growers might reduce potential leaching. Researchers analyzed data from two grapevine vineyards at Kearney Agricultural Research and Extension Center in Fresno County – one flooded for two weeks, and other for four.
Understanding initial nitrate levels crucial
A key factor in mitigating contamination is understanding how much nitrate is in the soil at the outset, said study author Helen Dahlke, a UC Davis hydrologist and leader of UC Agriculture and Natural Resources' strategic initiative on water. In areas with little precipitation and cropping systems that require greater amounts of synthetic fertilizer, the accumulation of residual nitrate – resulting from nitrogen in the fertilizer not taken up by the plants – can be quite high.
“The percentage of nitrates in some soils can really increase over the years, particularly if you have many dry years in a row where you don't have access to irrigation water or natural precipitation flushing some of those nitrates out of the soil,” Dahlke said.
While intense rains in recent weeks have helped dilute nitrate concentrations naturally, farmers looking to participate in recharge during the dry years ahead should consider flooding their fields with greater volumes of water.
“If you're doing this for the first time – on-farm recharge in the winter – check your residual soil nitrate levels because if they're very high, you should apply a lot of water in order to make sure that the residual nitrate is diluted down,” said Dahlke, who also added that growers should check their soil properties for suitability of recharge projects.
She recommended using, as a “good first approximation,” the online Soil Agricultural Groundwater Banking Index map, a project led by Toby O'Geen, a UC Cooperative Extension soil resource specialist.
Researchers looking at other ways to reduce nitrates
Even before flooding the fields for recharge, there are several practices that can lower initial nitrate levels and risk of leaching. Cover crops such as alfalfa and triticale, for example, can help take up residual nitrates that accumulate from fertilizing a main crop over time.
Dahlke and Fidelibus – a co-author of the San Joaquin Valley vineyard study – both pointed to pre-flooding irrigation that encourages denitrification, a process in which soil microbes transform nitrates into gaseous forms of nitrogen.
“Those denitrifying microbes need to be stimulated to do the work,” said Dahlke. “What we have found is that if you do a little bit of irrigation before you start the flooding, increasing the soil moisture can get those microbes started and they can take out more nitrate from the soil.”
The timing and quantity of fertilizer applications are also major factors in reducing leaching. Although more growers are following high-frequency, low-concentration practices to maximize uptake by crops, Dahlke said there needs to be more emphasis on incorporating nitrogen transformation processes – such as denitrification – in the nutrient management guidelines that farmers follow.
“Implementing thoughtful nutrient management plans will play a particularly important role in participating farms,” Fidelibus added.
A more holistic view of groundwater recharge
In short, choices made during the growing season can affect those in the winter recharge season – and vice versa. For example, applying compost or other organic amendments to soil can give microbes the “fuel” they need for sustained denitrification.
“What we have found is that our denitrifying bacteria often run out of steam because they don't have enough carbon to do the work,” Dahlke said. “Like us, microbes need energy to do the work, and for microbes this energy comes from soil carbon.”
Then, adding moisture via recharge to that field with high organic content can stimulate mineralization and nitrification, processes in which microbes transform the organic nitrogen into ammonium – and subsequently nitrates – that the plants can then take up. Those naturally occurring nitrates would thus reduce the need for the grower to apply synthetic fertilizer.
“The winter on-farm recharge experiments have shown that altering the moisture regime in the winter has consequences for the nitrogen budget in the summer growing season,” Dahlke explained. “Theoretically, what we need to be doing is better integrating both seasons by keeping an eye on the soil-nitrogen balance across the whole year so that we can ensure, at the end of the growing season, the residual nitrate in the soil is minimized.”
The study, published in the journal Science of The Total Environment, was part of the post-doctoral work of former UC Davis researcher Elad Levintal. In addition to Fidelibus and Dahlke, other authors are Laibin Huang, Cristina Prieto García, Adolfo Coyotl, William Horwath and Jorge Rodrigues, all in the Department of Land, Air and Water Resources at UC Davis.
Growing cereal crops with less fertilizer
Discovery could reduce nitrogen pollution, save farmers billions
Researchers at the University of California, Davis, have found a way to reduce the amount of nitrogen fertilizers needed to grow cereal crops. The discovery could save farmers in the United States billions of dollars annually in fertilizer costs while also benefiting the environment.
The research comes out of the lab of Eduardo Blumwald, a distinguished professor of plant sciences, who has found a new pathway for cereals to capture the nitrogen they need to grow.
The discovery could also help the environment by reducing nitrogen pollution, which can lead to contaminated water resources, increased greenhouse gas emissions and human health issues. The study was published in the journal Plant Biotechnology.
Nitrogen is key to plant growth, and agricultural operations depend on chemical fertilizers to increase productivity. But much of what is applied is lost, leaching into soils and groundwater. Blumwald's research could create a sustainable alternative.
“Nitrogen fertilizers are very, very expensive,” Blumwald said. “Anything you can do to eliminate that cost is important. The problem is money on one side, but there are also the harmful effects of nitrogen on the environment.”
A new pathway to natural fertilizer
Blumwald's research centers on increasing the conversion of nitrogen gas in the air into ammonium by soil bacteria — a process known as nitrogen fixation.
Legumes such as peanuts and soybeans have root nodules that can use nitrogen-fixing bacteria to provide ammonium to the plants. Cereal plants like rice and wheat don't have that capability and must rely on taking in inorganic nitrogen, such as ammonia and nitrate, from fertilizers in the soil.
“If a plant can produce chemicals that make soil bacteria fix atmospheric nitrogen gas, we could modify the plants to produce more of these chemicals,” Blumwald said. “These chemicals will induce soil bacterial nitrogen fixation and the plants will use the ammonium formed, reducing the amount of fertilizer used.”
Blumwald's team used chemical screening and genomics to identify compounds in rice plants that enhanced the nitrogen-fixing activity of the bacteria.
Then they identified the pathways generating the chemicals and used gene editing technology to increase the production of compounds that stimulated the formation of biofilms. Those biofilms contain bacteria that enhanced nitrogen conversion. As a result, nitrogen-fixing activity of the bacteria increased, as did the amount of ammonium in the soil for the plants.
“Plants are incredible chemical factories,” he said. “What this could do is provide a sustainable alternative agricultural practice that reduces the use of excessive nitrogen fertilizers.”
The pathway could also be used by other plants. A patent application on the technique has been filed by the University of California and is pending.
Dawei Yan, Hiromi Tajima, Howard-Yana Shapiro, Reedmond Fong and Javier Ottaviani from UC Davis contributed to the research paper, as did Lauren Cline from Bayer Crop Science. Ottaviani is also a research associate at Mars Edge.
The research was funded by the Will W. Lester Endowment. Bayer Crop Science is supporting further research on the topic.
Editor's note: Blumwald is affiliated with UC Agriculture and Natural Resources through the Agricultural Experiment Station at UC Davis.
New ‘big data’ tools help California wheat farmers reduce fertilizer guesswork
Growers in California grapple with plenty of climate uncertainty – but a new set of tools can help wheat farmers make crucial fertilizer decisions with more precision and confidence.
An interactive website integrates these tools – developed or adapted by researchers at the University of California, Davis and University of California Cooperative Extension – that provide farmers with recommendations for applying nitrogen fertilizers, specific to their own sites and conditions.
While factoring in those weather variables, the management tool also draws data from two indicators of nitrogen sufficiency or deficiency: the results of a soil nitrate quick test (a simple test previously used in vegetable crop systems along the coast), and comparisons of plant health in the broader field to that in a “nitrogen-rich reference zone” (a practice originally developed in the Midwest).
Using them in tandem, in the context of California wheat growing, is a novel approach. In a Nov. 4 webinar, Lundy will introduce the use of the nitrogen-rich reference zone, a small area in a field where extra fertilizer is added at the beginning of the season.
“This project is a unique example of digital agriculture at work in an applied setting,” he explained. “We are integrating ‘big data' sources like site-specific soil and weather data, as well as satellite, drone and other sensor measurements into an interactive web interface. This allows users to receive straightforward yet highly customized recommendations from somewhat complex agronomic models.”
The team conducted 11 on-farm demonstrations in fields representing a wide range of agroecosystems, including the Sacramento Valley, San Joaquin Valley, Delta region, and Tulelake Basin.
Fritz Durst, a western Yolo County-based grower who participated in one of the case studies, said that the process of gathering the data was “actually pretty simple” and the tool “eliminates much of the guesswork” for managing nitrogen fertilizers.
“This tool is extremely helpful for me to make decisions about the most efficient and cost-effective method for applying nitrogen to my wheat,” Durst said.
In addition to potentially increasing crop productivity and farmer net-income, the tool can benefit the environment by reducing the amount of nitrate leaching from fertilizer applications, according to Lundy.
“It's not only trying to say how much fertilizer to put down, sometimes it's trying to confirm you don't really need any fertilizer,” he said.
More resources and events related to the Nitrogen Fertilizer Management Tool for California Wheat – including demonstration activities – will appear on the UC Small Grains blog.
UC is offering a nitrogen management course for certified crop advisors and the general public
The University of California has opened registration for a seven-part video course on nitrogen management. The series, taught by UC researchers and extension specialists, begins May 10.
The course is open to anyone interested nitrogen management or seeking certified crop advisor continuing education units related to nutrient or soil and water management. The curriculum addresses all the learning objectives set forth by the American Society of Agronomy for the new California Nitrogen Management Specialty Exam. Taking the course helps certified crop advisors prepare for the exam, but doesn't substitute for taking and passing the exam.
The course topics, release dates and continuing education units for the series are:
- Environmental impacts of nitrogen losses, May 10, 0.5 unit
- Nitrogen cycling soil transformations, May 17, 1.0 unit
- Nitrogen cycling plant utilization, May 24, 1.0 unit
- Nitrogen sources, May 31, 1.0 unit
- Nitrogen budgeting, June 7, 1.0 unit
- Irrigation and nitrogen management, June 14, 1.0 unit
- California cropping systems, June 21, 2.0 units
Registration for the course is $120. Students with a valid educational email address (.edu) receive a 50% discount. July 31 is the last day to register and Sept. 30 is the last day to access the course content.
To register or for more information visit http://ucanr.edu/NitrogenCourse
If additional information is needed, contact course organizer Sat Darshan Khalsa at sdskhalsa@ucdavis.edu, or visit the FAQ page.
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UC ANR publishes nitrogen management advice for fruit, nuts and other crops
California growers can download a new series of publications summarizing efficient nitrogen management practices from UC Agriculture and Natural Resources. The publications are designed to assist growers in complying with state regulations for tracking and reporting nitrogen fertilizer applied to crops, in an effort to prevent nitrogen from leaching into groundwater.
The science-based publications are associated with a series of trainings for growers and Certified Crop Advisers to develop efficient nitrogen management practices, an effort coordinated by UC ANR's California Institute for Water Resources.
“Our role is to provide farmers, agricultural consultants and policymakers the best science possible for making decisions on managing and protecting California groundwater,” said Doug Parker, director of the water institute.
The free publications, created from training materials, lessons learned from the training sessions and from additional UC research, can be downloaded at http://ucanr.edu/nmgmtpublications.
The following publications are now available for download:
· Principles of Nitrogen Cycling and Management
· Irrigation and Nitrogen Management
· Nitrogen Management for Nut Crops
· Nitrogen Management for Deciduous Fruit and Grapes
· Nitrogen Management for Citrus and Avocado
· Nitrogen Management for Cool-Season Vegetables
· Nitrogen Management for Strawberry Production
· Nitrogen Management for Processing Tomato
· Nitrogen Management for Corn on California Dairies
The publications were authored by Parker of California Institute for Water Resources; Patrick Brown, professor in the UC Davis Department of Plant Sciences; Allan Fulton, UC Cooperative Extension advisor, Tehama County; Tim Hartz, UC Cooperative Extension specialist emeritus, UC Davis Department of Plant Sciences; Dan Munk, UC Cooperative Extension advisor, Fresno County; Daniel Geisseler, UC Cooperative Extension specialist, UC Davis Department of Land, Air & Water Resources; Michael Cahn, UC Cooperative Extension advisor, Monterey, Santa Cruz and San Benito counties; Richard Smith, UC Cooperative Extension advisor, Monterey, Santa Cruz and San Benito counties; Marsha Campbell, UC Cooperative Extension advisor emeritus, Stanislaus County; Sat Darshan Khalsa, UC Davis project scientist; and Saiful Muhammad, UC Davis graduate student.
Developed in 2014, the training program has been offered at 11 different locations around the state, most recently in Fresno. More than 1,000 Certified Crop Advisers have taken the training.
The nitrogen management training curriculum was developed by a group of UC ANR faculty, specialists and advisors. The first day focuses on the nitrogen cycle in crop production systems, nitrogen sources, irrigation and nitrogen management, and nitrogen budgeting. The second morning covers annual and permanent crops and nitrogen planning practices.
For more information on the nitrogen management training materials, visit http://ciwr.ucanr.edu/NitrogenManagement.
The Nitrogen Management Training and Certification Program is a joint effort between the California Department of Food and Agriculture, UC Agriculture and Natural Resources, California Association of Pest Control Advisers' Certified Crop Adviser Program and the Regional Water Boards.
