Working together to manage nitrogen oxide emissions from farmland

Feb 08, 2018 Diane Nelson
Researchers seek solutions that benefit agriculture and the environment

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UC Davis researchers collaborate with California farmers, environmentalists, industry and public agencies to support crop productivity and environmental health.
Researchers from the UC Davis College of Agricultural and Environmental Sciences (CA&ES) are working with farmers and ranchers, environmentalists, industry, and public agencies to find practical, science-based solutions to the world’s most pressing problems, including managing nitrogen oxide (NOx) emissions from farmland. 

NOx has been in the news lately as UC Davis researchers grapple with the difficult task of measuring NOx and greenhouse gas emissions in California, tracking the source and developing the tools agriculture needs to sustainably produce enough food to feed the world.

What is nitrogen and why does it matter?

All plants need nitrogen to grow. Nitrogen is important for producing high-yield crops, and productive agriculture is vital to feeding billions of people worldwide. But excess nutrients can pollute air and drinking water, and cause serious health and environmental risks. That’s why dozens of UC Davis researchers collaborate with California farmers to find the right rate, timing, amount and application of nitrogen to support crop productivity and environmental health.

Crops suffer if they don’t get enough nitrogen, but too much is bad, too. Excess nutrients can leach into water resources or be emitted into the air in the form of nitrogen oxides, such as NOx, which are a family of air-polluting chemical compounds. It’s tricky to find that perfect balance because every crop has different nutrient needs, and even those requirements vary based on a long list of factors, such as soil type, weather, irrigation, and how much nitrogen from soil organic matter and other organic sources crops can access. 

Is NOx natural?

The production of NOx is a natural microbial process occurring in all soils but can increase if farmers apply too much nitrogen. It’s not the only form of volatilized nitrogen from soil. One form—N2 or nitrogen gas—is nontoxic and makes up 78 percent of the air we breathe. Another form—N20 or nitrous oxide—is a potent greenhouse gas that is 300 times more efficient at trapping heat than carbon dioxide. It is not a risk to local health or the environment. 

Do excess NOx emissions benefit anyone?

Nobody wants excess NOx emissions. NOx emissions are central to forming ground-level ozone that can contribute to health concerns such as heart disease and asthma and harm crops and other plants, as well. Excess NOx emissions are also a waste of nutrients needed to nourish a growing population. Farmers don’t want expensive inputs such as fertilizers to escape into the air. Like everyone else, farmers benefit when they apply only the amount of fertilizer needed to increase crop quality and yield.

How do you measure NOx emissions?

Measuring NOx emissions is not easy. It requires specialized instruments, and emission rates vary widely depending on temperature, irrigation, rainfall, fertilizer timing and application amounts, and other events. Emissions will rise after the first rainfall after applying fertilizer, for example.

Have NOx emissions changed in California?

NOx emissions from California’s farmland have dropped significantly over the last three decades for two primary reasons: conversion to perennial cropping systems and efficient micro-fertigation-irrigation.

About 60 percent of California’s cropland has been converted from less profitable, high-input row crops watered with flood irrigation to perennial crops like almonds, which utilize nutrients efficiently and remove more carbon dioxide from the atmosphere. 

Over the past decade, more farmers have switched from furrow irrigation to subsurface drip and other efficient irrigation, which reduces water use, nutrient runoff and greenhouse gas emissions. Professor Will Horwath with the Department of Land, Air and Water Resources and the J. G. Boswell Endowed Chair in Soil Science has shown that drip irrigation can produce on average 30 percent greater tomato yields with similar amounts of water and nitrogen fertilizer, and also reduce nitrous oxide emissions dramatically compared to flood irrigation practices. Today, more than 90 percent of tomato growers use drip irrigation and, as a consequence, have significantly reduced nitrous oxide emissions. Researchers are finding that drip irrigation can provide similar yield and environmental benefits with corn, lettuce, alfalfa and other crops.

Micro-irrigation technology allows farmers to precisely feed plant nutrients in the same way it can customize water application, which increases fertilizer efficiency. And, because drip systems are buried beneath 8 to 10 inches of soil, microbiota in the soil consume nitrogen gases before they can escape into the atmosphere. Researchers are finding that precision fertilization can reduce nitrogen oxide gas emissions to “below baseline,” or below emissions found from fields that apply no fertilizer under old irrigation techniques.

Would less farmland mean fewer NOx emissions?

If farmland in California was converted to other uses, such as housing, scientists estimate that greenhouse gas emissions from that land would be 70 times greater. Therefore, conserving farmland is an excellent approach to mitigating NOx and harmful greenhouse gas emissions.

The healthy soil connection

The more fertile the soil, the fewer nutrients you need to add. Professor Horwath and Professor Kate Scow, along with agronomy specialist Tim Hartz, are engaging growers to enhance and measure soil fertility. They are showing that practices that increase soil aeration, reduce compactions, and enhance soil structure with organic matter could decrease nitrous oxide emissions from agriculture land by increasing nitrogen use efficiency. 

Removing the guesswork

Applying the right amount of nitrogen is a challenge. To remove some of the guesswork, Daniel Geisseler, Cooperative Extension specialist with land, air and water resources, has created online nutrient-management guidelines for many California crops, from alfalfa to walnuts. Professor Patrick Brown in the Department of Plant Sciences has developed a nutrient calculator for almonds  based on yield history, current conditions, and previous nitrogen applications.

On the UC Davis Fruit and Nut Research and Information Center site, farmers can find nitrogen prediction models for almond and pistachio.

Michael Cahn, Cooperative Extension farm advisor in Monterey County, developed an online tool called CropManage that helps farmers calculate water and nutrient needs for cool-weather crops like lettuce. Geisseler is working to expand the tool to include data for hot-weather crops like tomatoes. 

A wealth of nitrogen-management resources is also available at the Agricultural Sustainability Institute Solution Center for Nutrient Management.

Protecting rural farming communities

Professor Ben Houlton and Professor Ian Faloona in the Department of Land, Air and Water Resources are working to understand how the gains in crop nitrogen efficiency are helping to reduce soil NOx emissions in rural farming communities statewide. Using new modeling and air-sampling techniques, their work is helping us to understand the co-benefits of agricultural efficiency and the health and livelihood of Californians and the environment. It is also essential for providing large-scale information on the fossil fuel and fire-related air quality risks facing the Central Valley.

Seeking balance

The Central Valley is one of the world’s most highly productive agricultural areas. Roughly half of the fruits and nuts produced in the United States are grown there, including nearly all the nation’s almonds, walnuts, raisins, avocados and tomatoes. California farmers produce one in every four food items on dinner plates in America. 

The College of Agricultural and Environmental Sciences continues to seek balance between agriculture and the environment and develop solutions that meet the needs of all our communities. Air quality, clean water, healthy soil and sustainable agriculture are issues that matter to everyone. Our partnerships with scientists, farmers, policymakers, and conservationists will bring us closer to the scientific answers and solutions to these shared challenges.

Additional resources

For more information about efforts to measure and manage NOx emissions, contact Will Horwath, [email protected], 530-754-6029 or Ben Houlton, [email protected], 530-752-2210.

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