Agricultural management practices evaluated

As greenhouse gases go, nitrous oxide is a doozy. With a global warming potential 273 times that of carbon dioxide, mitigating nitrous oxide could make a big difference. But before mitigation can happen, it’s important to understand where the compound is coming from.

Most analyses point to agriculture as the major source of nitrous oxide globally. But there are a lot of variables within agriculture — crop and fertilizer type, soil texture, conservation practices and more — that can affect nitrous oxide emissions. A recent University of Illinois Urbana-Champaign study provides a comprehensive accounting for those factors, finding, among other things, that long-term no-till management can effectively cut nitrous oxide emissions.

“Our analysis enables us to identify practices that work well in specific regions and encourage programs including emerging ecosystem service markets to reward effective management,” said study co-author Michelle Wander, a professor in the department of natural resources and environmental sciences, part of the College of Agricultural, Consumer and Environmental Sciences at the University of Illinois Urbana-Champaign.

Wander said previous nitrous oxide accounting has either been too crude, unable to pinpoint specific agricultural factors influencing emissions or too complicated, requiring time-consuming calculations and complex algorithms. That’s why Yushu Xia, who completed her doctorate with Wander, aimed for a middle way in her analysis.

Xia said, “We were motivated to fill the gap between overly simplistic (Tier-1) and overly complicated (Tier-3) approaches, so we developed Tier-2 accounting. We collected a large metadatabase, which has almost 2,000 observations from U.S. agricultural lands, to get relatively accurate estimates without complicated algorithms or the use of supercomputers.”

Xia, now a Lamont assistant research professor at Columbia University, created her metadatabase from published studies and public databases, pulling in predictors including soil properties, topography, cropping systems, fertilizer types, climate factors and management. She looked at nitrous oxide emissions on a monthly rather than annual basis to capture seasonal differences in flux rates. The team also considered 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 the most significantly and consistently associated with reduced nitrous oxide emissions across time and space. But the authors are quick to point out no-till in that context refers to something specific.

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

While the analysis pinpointed several key factors contributing to agricultural nitrous oxide emissions and identified gaps to be filled with further research, the study’s real value is in improving the older Tier-1 method without requiring the massive computational resources of Tier-3 accounting. However, the metadatabase can be used to calibrate and validate Tier-3 studies; the authors have shared it with others in the research community to do so.

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