Team evaluates agricultural management practices in the new nitrous oxide accounting method

As for greenhouse gases, nitrous oxide (N₂O) is a doozy. With a greenhouse effect 273 times greater than that of carbon dioxide, N₂O could make a big difference. But before mitigation can take place, it is important to understand where the substance comes from.

Most analyzes point to agriculture as the main source of N₂Oh worldwide. But there are many variables within agriculture – crop type and fertilizer, soil texture, conservation practices and more – that can affect N.₂O emissions. A recent Urbana-Champaign study from the University of Illinois provides a comprehensive explanation for these factors, including finding that no-till management can effectively reduce N production in the long term.₂O emissions.

The study, “Estimating soil N₂O emissions caused by organic and inorganic fertilizer inputs using a Tier-2, regression-based meta-analytic approach for U.S. agricultural soils,” is published in Science of the total environment

“Our analysis allows us to identify practices that work well in specific regions and encourage programs, including emerging markets for ecosystem services, to reward effective management,” said study co-author Michelle Wander, professor in the Department of Natural Resources and Environmental Sciences, part of the Illinois College of Agricultural, Consumer and Environmental Sciences (ACES).

Wander says previous N₂O the accounting has either been too crude and unable to identify specific agricultural factors influencing emissions; or too complicated, requiring time-consuming calculations and complex algorithms. That is why Yushu Xia, who obtained her PhD at Wander, strove for a middle ground in her analysis.

“We were motivated to bridge the gap between overly simplistic (Tier-1) and overly complex (Tier-3) approaches, so we developed Tier-2 accounting. We have collected a large metadata bank, containing almost 2,000 observations of U.S. agricultural lands, to get relatively accurate estimates without complicated algorithms or the use of supercomputers,” said Xia, now an assistant research professor at Lamont at Columbia University.

Xia created her metadata bank based on published research and public databases, collecting predictors including soil properties, topography, cropping systems, fertilizer types, climate factors and management. She looked at N₂O emissions on a monthly basis instead of on an annual basis to accommodate seasonal differences in flux rates. The team also took into account differences within U.S. regions to see whether groups like the Ecosystem Services Market Consortium should tailor programs to specific areas.

Of the management practices included in the analysis, no-till was most significantly and consistently associated with reduced N₂O emissions in time and space. But the authors are quick to point out that no-till in this context refers to something very specific.

Wander explains that the “no-till” label can be misleading because rotational or alternating tillage does not have the same effect as true, long-term no-till. The latter leads to a more complex soil structure, including stable macropores that can help reduce greenhouse gas production.

Wander said: “In our analysis, reduced tillage practices varied widely in terms of N₂O emissions, which shows that they are not a panacea. Only true no-till management could consistently reduce emissions.”

The type of fertilizer and soil texture were also important factors.

“The type of fertilizer made a big difference,” Xia said. “For example, liquid manure caused many more emissions compared to solid manure, a product that is released more slowly. Anhydrous ammonia had the highest emissions of the manures we evaluated, but emissions from that source were highly variable.”

There are some things that management cannot change. For example, the analysis showed that soils with a finer texture emit more N₂Oh then coarse textured bottoms. It also identified important regional differences in the interaction between soil texture and water.

“Soil microbes process nitrogen in complex ways, and soil moisture and texture can make a big difference when it comes to whether the end product of microbial processing is harmless dinitrogen or the greenhouse gas nitrous oxide,” said Xia. “We need to think about how best to manage emissions for irrigated and non-irrigated systems, but we’re not there yet.”

Although the analysis revealed several key factors contributing to the development of N₂Omissions and identified gaps that need to be filled with further research, the real value of the research lies in improving the older Tier-1 method without requiring the enormous computing power of Tier-3 accounting. However, the metadatabase can be used to calibrate and validate Tier-3 studies; To do this, the authors have shared it with others in the research community.

“To fairly reward farmers for stewardship, we need to know where and when practices can reduce greenhouse gas emissions,” Wander said. “We show that general linear modeling is a practical Tier-2 approach that policymakers can rely on to make recommendations.”

Provided by University of Illinois College of Agricultural, Consumer and Environmental Sciences (ACES)