“Carbon capture” is a phrase on everybody’s lips these days. Engineers dream of building high-tech facilities to grab carbon dioxide from the air, concentrate it and pump it deep into the earth. Others look to low-oxygen burning systems, called pyrolysis, to create biochar that can be used as a fertilizer or buried. Foresters count trees, while ecologists in Europe look to peat bogs for a massive carbon sink.
What many carbon capture champions forget is that the answer to vast amounts of carbon capture is right in front of their noses, on their dinner plates and in the clothes they wear. Agriculture holds the key to capturing vast amounts of atmospheric carbon, and irrigated agriculture pumps that effort into high gear.
What’s more, when farmers capture carbon, they also make the world a better place, building soils and producing food, feed, fiber and fuel.
Irrigation is a pivotal carbon capture tool in many agricultural areas. First, irrigation increases yields in most crops in which it is used, often dramatically. Higher yields mean more carbon dioxide is converted into plant matter, which in turn is either consumed as food or other usable products, or used to build soil organic matter, which is agriculture’s method of sequestering carbon over the long term.
Irrigation also reduces the risk of crop loss to drought, ensuring that plants will capture the carbon we expect them to. More precise irrigation techniques, whether they’re high-efficiency pivot nozzles, microemitters or drip systems, increase the amount of crop per drop.
As irrigation becomes smarter, the potential grows for farmers to sequester even more carbon, as well as to help mitigate other greenhouse gases and precursors to smog. Spoonfeeding nitrogen not only helps maximize the amount of nutrient that is available in the crop’s root zone, it can reduce the amount of unused N left to volatilize as nitric oxide, nitrous oxide or ammonia.
That is a huge issue. For farmers, lost nitrogen is lost money. It’s fertilizer that they paid to buy and apply that drifts away in the wind or leaches out of reach of their crops’ roots. Nitrogen in the environment can also represent an environmental threat. Nitrous oxide is the third most problematic greenhouse gas, after methane and carbon dioxide. Though it is not as plentiful as the top two greenhouse gases, nitrous oxide is much longer-lasting, delivering 300 times more global warming effect over a 100-year period than carbon dioxide does.
According to the U.S. Environmental Protection Agency, 74% of the nitrous oxide emitted in the United States is created in agricultural soils. Similarly, a paper in Science Advances estimated that agricultural soils in California’s Central Valley release 25% to 40% of the state’s nitrogen oxide emissions. European farmers have already been targeted by strict nitrogen regulations that have forced down nitrous oxide emissions. It’s likely that American farmers will, too.
Smart irrigation can help. Applying oxygenated irrigation water through subsurface drip goes even further.
Data from California State University, Fresno; Memorial University of Newfoundland; and University of the West Indies have documented shifts in the soil microbial population toward microbes known to fix nitrogen in crop-available forms where this type of system delivered what they described as an “air/water slurry.” In plots watered with conventional subsurface drip, the soil microbial balance favored nitrate-reducing bacteria that convert nitrite into nitrous oxide and other NOx compounds. Other studies around the world consistently demonstrate significant yield improvements where aerated subsurface drip water improves conditions in the root zone and creates an environment conducive to effective nutrient transfer.
The old-fashioned way of aerating the root zone was to plow, accelerating the degradation of crop biomass into the soil and mixing air into the top several inches of topsoil. Unfortunately, plowing releases a significant amount of stored soil carbon, evidenced by the steady loss of soil organic matter in tilled soils, as well as vast amounts of other, nitrogen-based greenhouse gases. Applying oxygenated irrigation water through subsurface drip has been called “plowing without a plow,” a method to aerate soils while keeping carbon stores and nutrients intact. In a carbon-focused world, that is a huge win.
Irrigated agriculture’s role in the cycle of carbon is a great complement to its role in the cycle of water, capturing carbon from the atmosphere and turning it into plant matter, then sequestering much of it in the soil. The term “circular economy” is becoming almost as ubiquitous as “carbon capture” in discussions of the environment. Like carbon cycling, the concept of a circular water economy also puts farmers in a strong position, as the circular water economy begins and ends with agriculture.
In a circular water economy, a parcel of water is drawn from a source, treated to drinking water standards and used for home or commercial consumption, then reused to maximize efficiency. Gray water can be reused again for irrigation. In Israel, the paragon of a circular water economy, 87% of the nation’s water is reused; here in the United States, the figure is closer to 10%. Runoff following irrigation returns to rivers, lakes or groundwater and becomes source water once again.
The distinctive purple pipes of wastewater reuse systems have not become commonplace here in the U.S. quite yet, but I am optimistic that they will. As they do, they will provide yet another opportunity for the nation’s farmers and our irrigation community to demonstrate their tremendous role in natural cycles, turning potential environmental challenges into the food we eat, the fiber we wear or build with, and the fuel we consume. For capturing carbon and cycling water, irrigated agriculture is the most effective approach to achieving our environmental goals while sustaining life and health and the most productive way to protect our planet.