Climate change

Where do agricultural greenhouse gases come from, and what can be done to reduce them?

Modern agriculture developed over time to meet a pressing need: feeding the world’s growing population. Agronomists discovered new techniques to increase crop yields. Chemists found ways to harness nitrogen from the air, and agronomists found methods to use this process to make powerful fertilizers. Farmers applied these techniques to make sure they could meet the food demand.

mixture of rye and vetch cover crops

A cover crop mixture of hairy vetch and cereal rye provides nitrogen for the crops to follow, and protects the soil from erosion. Cover crops also reduce greenhouse gas emissions. Photo credit Hanna Poffenbarger.

But, like any science, practices are now changing. Some of the techniques that were applied did, indeed, grow more food. And, yet, there were unintended consequences. Clearing the land for crop production decreased water quality. Some other practices to increase yields increased greenhouse gases.

Agronomists are now researching ways to grow food while at the same time protecting the environment.

Industrial innovations have caused the release of greenhouse gases such as carbon dioxide (CO2), nitrous oxide (N20), and methane (CH4). These gases trap the sun’s radiant energy in our atmosphere. The result is an increasing global temperature.

Worldwide, agriculture contributes about 14% of the global greenhouse gas emissions1. There are several agricultural sources of greenhouse gases, as well as many answers to reducing them. Methane and nitrous oxide are more potent than carbon dioxide in terms of their global warming potential.

Carbon dioxide

Worldwide, to create more farmland to feed the growing population, forested and prairie lands were cleared from the landscape. Subsequently, many soils lost up to 50 – 90% of their original organic carbon as carbon dioxide into the atmosphere. When soil is cultivated – either through plowing, tilling, or even the use of machinery to work the farms – the organic matter, protected for hundreds of years within the soil matrix, is quickly exposed to oxygen and microbes. As a result, carbon dioxide is released into the atmosphere.

But, plants also pull carbon dioxide from the air, and use it as “food”. In this way, carbon is stored (or sequestered) in the soil. During this process, carbon is stored in the soil in various ways such as root biomass and litter. Agronomists have found that in many areas, reducing or eliminating the use of tilling can still maintain good crop yields. Improved methods of weed control and no tillage planting techniques can reduce the loss of soil organic carbon. Research has found that these techniques can increase carbon storage over time. Less plowing also means fewer fossil fuels are used in the farming process.

Another practice farmers are using to reduce their carbon dioxide output is to restore wetlands on the edges of their farms. Wetlands pull carbon dioxide from the atmosphere, and store it in the wetland’s soggy soils. Adding back some trees to the edges of farmland can do the same. As a result, agriculture is making a huge contribution in reducing the global impact of industrial greenhouse gasses in the atmosphere.

Nitrous oxide

Nitrogen is a major element needed for our crops. Nitrogen forms the backbone of all proteins, including plant-based proteins. In the early 1900’s, chemists discovered a way to make ammonia from nitrogen in the air. This discovery led the way to the development of today’s commercial nitrogen fertilizers.

Although using nitrogen fertilizers has increased yields and allowed sufficient food to be produced to feed the population, it takes a lot of fossil fuels to make the fertilizers and distribute them to farms.  When nitrogen fertilizer is added, it becomes part of the natural biological system, including soil microbial transformations and plant uptake to help the plants grow. But, some of these microbial processes can also release nitrous oxide into the atmosphere, especially when the soil is saturated with water.

Agronomists are researching ways for growers to maintain yields with fewer fertilizers. One method involves growing legume cover crops – like clover – that naturally add nitrogen to the soil. Legumes, can pull nitrogen from the air, and don’t need nitrogen fertilizers as do crops like wheat and corn. Another method is called crop rotation. A farmer might grow a crop such as the nitrogen-fixing cowpea or soybean in one season, and another like corn the following. The corn will benefit from the nitrogen left behind by the previous year’s legume harvest and maintain yields with less nitrogen fertilizer.

Another method to reducing greenhouse gas emissions from nitrogen fertilizer application is called precision agronomy. New hi-tech instruments help growers only apply fertilizer where it is needed, resulting in less fertilizer use, which saves them money, and helps the environment. To read a blog about precision ag, click here; or to read about a precision agronomist, visit here.

Methane

Rice is the main crop responsible for producing methane during production. This is because rice is grown in very wet environments. Under these conditions, bacteria in the soil that are “eating” the organic carbon for energy produce methane gas. Agronomists have developed improved rice production methods that reduces the time needed for soil to be flooded and thus reduces the amount of methane produced. Also, crop breeders have developed a low-methane rice breed, and further advances are forthcoming.

cow with trees and fence

Tree legumes are integrated into this grazing system in Northeast Brazil; legumes add nitrogen to the soil, and cattle can feed freely. Photo credit: Jose Dubeux

Another agricultural source of methane is the direct release from animals during production. Changing the type of food fed to cows – or adding supplements – can help the amount of methane they produce. In addition, animal manure is used as an important organic-matter fertilizer on farms. Agronomists are researching the best practices to “recycle” this important source of nutrients, while reducing the greenhouse gases they emit. By using manures as a nitrogen fertilizer, the amount of the industrial nitrogen fertilizer needed for crop production can be reduced.

One promising area of research to reduce methane emissions and improve overall soil quality is called silvopasture. This method grazes animals on land that is also used for growing crops. Research at the University of Missouri’s Center for Agroforestry showed that this reduces greenhouse gas emissions. It also improves the animal and ecosystem health. Silvopastural production systems are more economical and productive compared to other systems of animal farming. Such systems also reduce manure accumulation and methane production.

Some current agricultural production systems are responsible for our overall greenhouse gas emissions, and agronomists and farmers are working hard to improve the sustainability of our agricultural land. In addition to feeding the world, our current agricultural land provides fibers for clothing (cotton, hemp), and biofuels. The answers are not in reducing the amounts of food grown, but by finding the best practices to do so in a way that preserves – or improves – the environment. By implementing better management practices, it is possible to reduce the level of greenhouse gasses in the atmosphere.

Answered by Sougata Bardhan, Center for Agroforestry, University of Missouri

  1. From Agronomy Grow With It – Energy supply results in 26% of greenhouse gases, industry 19%, forestry 17% and transportation 13%. The remainder of greenhouse gases comes from Buildings and Water/Wastewater programs. To learn more about Agronomy Grow With It, visit http://agronomy4me.org/.

About us: This blog is sponsored and written by members of the American Society of Agronomy and Crop Science Society of America. Our members are researchers and trained, certified, professionals in the areas of growing our world’s food supply while protecting our environment. We work at universities, government research facilities, and private businesses across the United States and the world.

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