Written by Jonathan Eisenthal

During the growing season, nighttime low temperatures are rising. In the fall, winter, and spring, daytime highs are higher than they used to be. All these effects of the changing climate appear to intensify a microbial reaction called denitrification: In saturated soils and warmer temperatures, the plant food that crops depend on, nitrate nitrogen, is converted into nitrous oxide — a greenhouse gas that is emitted to the atmosphere.

With less food in the soil, corn and other crops will yield less.

“We need to come up with strategies that reduce nitrous oxide emissions,” said Tim Griffis, a professor of biometeorology at the University of Minnesota.

The first step to formulating these strategies is to take a farm field and put it in a bottle. A very big bottle, called a mesocosm. Griffis and his team have created a range of these controlled settings, each filled with 9,000 lbs. of agricultural soil, where they can simulate the temperature fluctuations experienced by a growing crop, and by the fields after the crop has come off, to see what controls denitrification.

The University of Minnesota mesocosm facility was founded with university support, and it also received initial funding from the National Science Foundation and USDA. Through the Minnesota corn check-off, Minnesota corn farmers have covered operating costs and underwritten the work of students and researchers involved in the mesocosm projects over the last several years.

“A significant amount of nitrous oxide is lost during early spring when we are going through that freeze-thaw cycle,” Griffis said. “We are seeing as much as 35 percent of the annual nitrous oxide production in this period. Our colleagues in Canada have field observations that estimate as much as 70 percent of nitrous oxide emissions occur in early spring. When it comes to strategies to reduce denitrification, taking care of the freeze-thaw losses is the low-hanging fruit.”

Nitrous oxide is a greenhouse gas, whose warming effect is estimated to be 265 times more potent than carbon dioxide, adding to the cycle that appears to be increasing emissions. So, any reductions in nitrous oxide emissions will not only help maintain soil fertility, but it may dampen the warming effect that is working against fertilizers. Of course, agriculture is only one source of nitrous oxide, with the transportation sector and wastewater treatment facilities also producing the gas.

Griffis describes the research in the mesocosms: “We apply fertilizer and we grow corn. (After harvesting the corn),we are trying to freeze the soil column using this top-down radiative freezing approach. We have cooling units and radiators that hang over the soil. We freeze the upper few centimeters of the soil and then we take it through a thaw cycle and then we repeat this. This has been very challenging. It has been harder to do than we thought, but we have been successful in recreating these cycles, and quantified these large increases in the nitrous oxide emissions from the soil. What we are trying to do now is to prove to ourselves that we can repeat these freeze-thaw cycles, and then what we want to do, and this ties in directly with the Minnesota Corn funding, is to test some ways in which we can reduce those emissions.”

Griffis outlines two main strategies, already applied by some farmers, which may gain wider use as the research demonstrates climate-induced denitrification. Now that he feels confident in how the mesocosms mimic the climate, he can test nitrous oxide reduction methods.

“A couple hypotheses here … if you have residual nitrogen left over from the end of the season that is in the soil, that’s available to undergo denitrification,” Griffis said. “Can we use cover crops to mop up some of the excess nitrate in the soil? To go along with that strategy, we can select cover crops that can also act as a thermal mulch, like when we mulch our gardens, and that can dampen the temperature response in the soil. … This might reduce the nitrous oxide production. When you have large cycles in the soil temperature near the surface, that physically degrades some of the soil aggregates and releases nitrogen substrates.”

This process makes nitrates more readily available to become nitrous oxide.

“We are also testing whether enhanced efficiency fertilizers are a better choice with respect to lowering these nitrous oxide emissions that happen during the freeze-thaw cycle,” Griffis said. “(These products) have inhibitors that act to reduce nitrification and reactive nitrogen losses. We are wondering if those inhibitors can help.”

Read more about this project throughout the year at mncorn.org.