Decisions farmers make over the spring and summer can dramatically increase greenhouse gas (GHG) emissions later in the winter, a new study from the University of Vermont concluded.

According to an announcement from the university, the study shows, for the first time, that the impacts of farmers' manure use decisions extend beyond the growing season to influence emissions on warm winter days.

"This could have big impacts as winters become warmer and soils thaw more frequently," said lead author Carol Adair of the University of Vermont's Rubenstein School of Environment & Natural Resources and the Gund Institute for Environment. "If croplands move farther north with warming climates, this could increase the contributions of agriculture to global GHG emissions."

The study, published in Soil Science Society of America Journal, provides some of the first measures of GHG emissions from agricultural soils in Vermont and highlights important trade-offs with current agriculture practices, such as injecting manure into soils, the announcement said.

While it's known that farmers' decisions to add nutrients to their fields affect GHG emissions during the growing season, Adair and her colleagues found that these choices have long-lasting effects: They can increase emissions of powerful GHGs, especially nitrous oxide, during wintertime thaws.

Emissions of GHGs — carbon dioxide and nitrous oxide — from agricultural soils have been well-studied during the growing season, but much less so during winter, the university said. Understanding the lasting consequences of management decisions is becoming more critical because agriculture is expected to expand and intensify in northern regions as the climate warms, the researchers said.

"By injecting manure, farmers are trying to do the right thing and keep manure on the farm, in their soils and crops and out of waterways," Adair said. "These results, in conjunction with our previous research that found injection to also increase emissions during the growing season, suggest that there may be important trade-offs to consider when deciding on a method of manure application."

GHG emissions result from the activity of soil microbes such as bacteria, which break down manure into nutrients useable by crops. "During typical winters, when soils are very cold, microbes basically hibernate, but they are just waiting for the right conditions to be active again," Adair said. "When soils warm up just a bit — or thaw — microbes wake up and quickly start producing GHGs."

Researchers conducted a laboratory study on frozen soils collected from field trials in Vermont. During the growing season, the agricultural lands received different methods of manure application: broadcast or broadcast plus incorporation by plow or by injection. In the lab, researchers subjected soil cores to either a frozen, freeze/thaw or thaw treatment for eight days.

The research team found that the method of manure application strongly influenced emission rates of carbon dioxide and nitrous oxide — a GHG roughly 300 times more powerful at trapping heat than carbon dioxide — from soils. During winter thaws, nitrous oxide emissions from manure-injected soils were up to 20 times greater than emissions from soils with surface broadcast or broadcast plus plow manure application, the researchers reported.

Release of carbon dioxide and nitrous oxide was up to eight times greater from soils that thawed than from soils left frozen, but this varied depending on manure application method. Nitrous oxide emissions from injected soils were two to three times greater than from soils broadcast with manure and 4-19 times greater than from soils broadcast and plowed. The type of manure application also affected carbon dioxide, but not nearly as much as it affected nitrous oxide.

The researchers have some theories as to why GHG emissions are greater with manure injection and plan further study, the university said.

"Microbes that produce carbon dioxide and nitrous oxide need carbon and nitrate, and injections of manure 6-8 in. below the soil surface may increase availability of those nutrients," Adair said. "Another potential reason is that the manure application treatments change microbial communities; there may be more of the type of microbe that produces nitrous oxide in injected soils."

"This study gave us an incredibly useful launching point for expanding this research and enabled us to work with more Vermont farmers," said Lindsay Barbieri, a doctoral student in the Rubenstein School and Gund Institute. "Together, we're monitoring GHG emissions, alongside water quality, crop yield and other measurements, from agricultural soils and practices. This is happening directly in the field, for longer periods of time, as we work to better understand the complexities of agricultural practices and the role of GHG emissions in Vermont."

In addition to Adair and Barbieri, study co-authors included Heather Darby of the University of Vermont College of Agriculture & Life Sciences and Gund Institute and University of Vermont undergraduate Kevin Schiavone.

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