Tilling the soil releases carbon into the atmosphere, but does the compost that organic growers apply compensate for that

Tilling the soil releases carbon into the atmosphere, but does the compost that organic growers apply compensate for that?

Emissions of nitrous oxide from on-farm fertilizer applications are thought to be a significant contributor to the rising levels of greenhouse gases in the ­atmosphere.

Although the amount of nitrous oxide emitted into the atmosphere is significantly less than the carbon dioxide emitted in the United States, nitrous oxide is far more potent. The effect of one molecule of nitrous oxide is equivalent to that of almost 300 molecules of carbon ­dioxide, according to David Granatstein, sustainable agriculture specialist with Washington State University in Wenatchee.

That’s why WSU researchers are interested in learning more about the greenhouse gas emissions resulting from various types of fertilizers, particularly those used on organic farms.

Key greenhouse gases
Gas Percent of greenhouse effect CO2 equivalents(warming potential per molecule)
Water vapor H20 36–72
Carbon dioxide CO2 9–26 1
Methane CH4 4–9 25
Nitrous oxide N2O 6 298
Ozone O3 3–7 0.25
Source: David Granatstein, WSU, based on information from the Intergovernmental Panel on Climate Change and Wikipedia.

“If you’re going to pick a place where you can have a big impact, the soil is the place,” Granatstein said. “That’s where we have the biggest effect, because of nitrogen management and the carbon cycle. We enrich the ecosystem with nitrogen to grow our crops, which are generally not very efficient in using the nitrogen.  There’s going to be some leakage, there’s some conversion to nitrous oxide as part of that process, and it ends up in the atmosphere.

“That’s why we’re focusing our study on organic nutrients to find out if they operate any differently from a ­conventional fertilizer.”

One of the challenges, however, is that emissions of nitrous oxide are extremely difficult to measure.

Granatstein is working with Dr. Lynne Carpenter-Boggs, WSU’s biologically intensive and organic agriculture research leader; Dr. Dave Huggins, a soil scientist with the U.S. Department of Agriculture; and others in Pullman, Washington, on a new project funded with a $1.5 million federal grant to study how carbon and nutrients are cycled through organic farming systems and their impacts.

It’s a follow-up to a six-year research project on Climate Friendly Farming conducted by WSU’s Center for Sustaining Agriculture and Natural Resources with funding from the Paul G. Allen Family Foundation.

An irrigated orchard is among the five farms that they are studying in the new project.  They’re measuring gas emissions from various fertilizers, both in field trials and laboratory studies. A previous study in Italy indicated that green manure and urea emit far more nitrous oxide than compost.

The scientists plan to develop models in the form of spreadsheets that growers, consultants, and others can use to assess farming operations and compare their practices to other options to see if there are ways to improve what they’re doing, Granatstein said. There’s a possibility that the models might also be accepted by organic certifiers to demonstrate a grower’s compliance with soil quality requirements or as verification for carbon credit marketing.

Apart from applying more organic amendments and not applying synthetic fertilizers, organic fruit growers are more likely to till the soil, primarily to control weeds.

Granatstein said he thinks the project will help show organic growers the implications of tilling from a carbon standpoint. “You’re burning off more carbon with more tillage, but you can compensate for it. They figure, ‘We have to till for weed control, but we’re putting the compost back on, and, hopefully, it breaks even.’ Our goal is to come up with a tool to answer that question—simply model out what growers are doing and get a reasonable estimate of whether they’re breaking even on their carbon.”

This is important, he said, because part of the organic standard requires growers to maintain or improve the soil quality, and carbon is the most central indicator of soil quality.

Life cycle assessment

Granatstein said the scientists plan to interface the crop model data with a life cycle assessment tool, so growers can easily assess the overall environmental impacts of their operations.

Life cycle assessments were developed for industrial processes and are more difficult to apply to biological systems in agriculture, where the weather and the inputs change from year to year. To overcome these variations, a cropping systems simulation model (CropSyst) is used to study a farming system under typical conditions. As part of the Climate Friendly Farming project, the CropSyst model was upgraded to handle greenhouse gas emissions, along with many other aspects of crop production.

Funding for the project comes from the Organic Agriculture Research and Extension Initiative of the National ­Institute of Food and Agriculture.

Editor’s note: This story originally included a link to a webinar that is no longer online.