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Returning Carbon To The Farm

Replacing synthetic fertilizers with organic ones could help reduce climate impacts and improve agricultural systems

by Deirdre Lockwood
July 15, 2013 | A version of this story appeared in Volume 91, Issue 28

Credit: Sylvia Kantor
An eddy flux tower monitors CO2 at a Washington State University test farm.
This is a photo of an eddy flux tower that monitors carbon dioxide gas exchange at the Washington State University Cook Farm.
Credit: Sylvia Kantor
An eddy flux tower monitors CO2 at a Washington State University test farm.

In efforts to mitigate climate change, we’re more likely to think about changing how we power our cars than how we grow our food. But as University of Washington geologist David R. Montgomery likes to remind people, farming has a huge impact on the carbon dioxide content of the atmosphere.

One-third of the CO2 humans added to the atmosphere by the end of the 20th century was released by cutting, burning, and plowing forests and grasslands to convert them to cropland, according to Montgomery.

Montgomery, a recipient of a MacArthur Fellowship, known as a “genius grant,” visited the heart of the Dust Bowl region in southwest Kansas several years ago. He found soils “like beach sand that they essentially drop a bunch of inorganic nitrogen on to grow things.” Plowing for so many years had stirred up the once organic-rich topsoil, releasing CO2, causing erosion, and depleting the soil’s fertility.

The Dust Bowl is an extreme example, but the problem exists to different degrees in major agricultural regions across the globe. Policy­makers, scientists, and environmental groups are eager to reverse the trend. They are encouraging farming and land-management approaches that both replenish carbon in the soil and offset anthropogenic CO2 emissions. By some estimates, the efforts could sock away 10% or more of global emissions from using fossil fuels.

Last month in Seattle, policymakers and scientists, including Montgomery, gathered to discuss strategies to sequester carbon in the soil at the Northwest Biocarbon Summit. The summit was organized by a nonprofit group called Climate Solutions.

Many of these strategies are as old as human agriculture. They include applying compost and manure, planting cover crops such as rye and alfalfa, and limiting tilling. Commercial farmers aren’t strangers to these practices, but they are becoming more interested in implementing them when they see the benefits: better crop yield and resiliency and improved water retention in the soil.

The climate mitigation impact of these changing agricultural practices is hard to predict, however, according to Chad Kruger, director of Washington State University’s Center for Sustaining Agriculture & Natural Resources. Kruger said at the biocarbon summit that results vary widely depending on climate, type of crop, farming practice, and site history.

What’s needed is a full cost-benefit, or life-cycle, analysis on the practices, Kruger said. But the process is challenging. For example, although researchers have found that more carbon is stored in irrigated soils than in dry soils, the benefits of irrigation could be offset by the fossil fuels required to pump irrigation water.

Through WSU’s Climate Friendly Farming initiative, Kruger and his colleagues have integrated the results of decades-long agricultural studies and developed a predictive model called CropSyst. They are consistently finding that the most effective practice for increasing carbon storage is switching from synthetic inorganic to organic sources of fertilizer—material derived from manure, food and yard scraps, and treated sewage sludge known as biosolids.

University of Washington soil scientist Sally Brown studies the carbon sequestration potential of this switch. For each ton of dry organic fertilizer applied, she said, carbon equivalent to up to a ton of CO2 is stored in the soil, whereas little or none is stored with synthetic fertilizer. “It’s a better return than putting money in the bank,” Brown said.

But moving to organic fertilizer has its own costs. Its purchase price can be three to six times as much as synthetic fertilizer to get the equivalent amount of nitrogen, Kruger said. On top of that comes the greater expense of applying the bulky organic material.

Picking an organic fertilizer is another challenge. Biosolids turn out to be the most cost-effective source of organic fertilizer in Washington, but availability of the material is limited. Municipalities are required under the Clean Water Act to dispose of residual solids from wastewater treatment by burying it in a landfill, incinerating it, or recycling it as fertilizer.

Only about 50% of available biosolids in the U.S. are recycled on land. Washington is one of few states where nearly all of the material is used for agriculture. Because residents pay most of the cost of biosolids processing and transport in their water bills, the compost is available to farmers at a cost that is competitive with synthetic fertilizer. Farmers have embraced this subsidized resource, Kruger and Brown noted.

Kruger added that farmers in Washington state are increasingly finding that the switch is worth the money because of improved crop yield, soil fertility, and water retention. In a study of wheat in eastern Washington, where yield strongly depends on water, Brown and colleagues found that, compared with synthetic fertilizer, biosolids application increased water availability by 10%, crop yield by 25%, and soil carbon storage by 50% (Environ. Sci. Technol. 2011, DOI: 10.1021/es2010418). Recycling biosolids can also help reduce greenhouse gas emissions by sinking the carbon in the soil rather than releasing it to the atmosphere during incineration or landfill disposal, which can also release the greenhouse gases methane and nitrous oxide.

The use of biosolids in agriculture is controversial because of “the ick factor,” as Brown puts it, and the threat of introducing pathogens, pharmaceuticals, and industrial chemicals into the soil or water supply. The treatment process for class A biosolids, which can be used without restrictions, including on community gardens, kills or chemically digests all pathogens. Treatment for class B biosolids, restricted to crops like wheat that usually grow far from running water, removes about 98% of pathogens. Application of class B biosolids has been banned in Washington’s Wahkiakum County and California’s Kern County in recent years because of concerns about water contamination, although the California ban was reversed in a court ruling in February. The Wahkiakum ban has been challenged by the Washington State Department of Ecology.

Brown said decades of studies show that biosolids are safe if used as regulations dictate. Many chemicals of concern, such as estrogens derived from birth control pills, break down in the soil within hours to days, she said. Still, she added, studies continue on the impact of industrial chemicals such as bisphenol A, an endocrine disruptor with uncertain human health effects.

Even if all biosolids in the U.S. were applied on farmland, the material would cover only 1% of arable land, Brown said. Animal manure would cover 10 to 30%, but because of antibiotic use in livestock, it poses risks of increasing antibiotic resistance in addition to introducing pathogens. With commercial partners, WSU researchers are developing anaerobic digestion technologies to convert animal waste on dairy farms to decontaminated fertilizer. A bonus by-product is biogas, a renewable, methane-rich fuel that can be used for heating and transportation.

Although a goal of new soil conservation policies is to develop certification for carbon offsets that would give farmers a monetary incentive to adopt these practices, Kruger doesn’t think this is realistic. He estimates the offsets would provide at the most 1% of a commercial farmer’s production budget.

Carbon conservation has to be balanced with other priorities, Kruger said. But it could help farmers weather risks posed by climate change, such as water scarcity. “We need to increase the resiliency of the agricultural system,” he said, “and to do that we need to remain conscious about putting carbon back in the soil.”


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