LINDELL BEACH, B.C. — Researchers may have determined that scientists should include emissions from the crops themselves when measuring greenhouse gas emissions from cropland.
Expressing emissions per unit of crop yield rather than on a more conventional per area basis produced starkly different results, says Rod Venterea, a research soil scientist with the U.S. Department of Agriculture and adjunct assistant professor with the University of Minnesota’s soil, water and climate department.
His team found that total nitrous oxide emissions were not greatly affected by tillage practices when calculated on an area basis.
However, emissions were significantly greater under no tillage compared to conventional tillage when they were calculated on a per unit yield of grain.
Nitrous oxide (N2O), a byproduct of many agricultural practices, is a potent greenhouse gas with an ability to trap heat in the atmosphere 300 times greater than that of carbon dioxide.
The findings, which were published in theJournal of Environmental Quality,are important because they have implications for how greenhouse gases generated by agriculture are reported, evaluated and potentially mitigated.
“On an area basis, the average N2O emissions with no-till were slightly greater than conventional tillage, by about 19 percent, but statistically speaking, the values were not different from each other,” said Venterea.
“On a yield basis, we actually calculated it three different ways: per unit of grain yield, per unit of grain nitrogen yield and per unit of total plant nitrogen yield. In all three cases, no-till did have greater emissions, by 52 percent, 66 percent, and 69 percent, respectively, and the differences were highly statistically significant.”
He said this is because researchers divide by the yields when calculating emissions on a yield basis.
Because the yields were smaller with no-till, after dividing by a smaller number you end up with a larger (final) number.
Venterea said his team also figured out why there are greater percentage emissions on no-till grain than grain grown on conventionally tilled land:
• Fertilizer was placed on the surface of the soil, which in no-till management is where most of the microorganisms that convert fertilizer into N2O are located.
• The crop yields in the no-till treatment were lower.
“These two factors combined to produce the net result,” he said.
Venterea said nitrous oxide emissions and yield need to be looked at together. Other studies have shown that practices such as fertilizer management and tillage management affect nitrous oxide emissions, but few studies have reported the effects of these practices on crop performance at the same time.
Greenhouse gas emissions are often expressed with respect to the area of field, such as the amount of nitrous oxide emitted per acre. However, the study has shown that expressing greenhouse gas emissions per unit of yield is more meaningful.
Different sites studied
Venterea’s studies were conducted in collaboration with the University of Minnesota.
“This has been a very productive collaboration because of the expertise and facilities that each of our organizations can offer,” he said.
“We’ve been able to utilize three different university agricultural research stations, each of which has different soil types and site-specific factors.”
The team measured the effects of tillage and nitrogen fertilizer management on nitrous oxide emissions, grain yields and crop nitrogen absorption over three consecutive growing seasons in Minnesota.
The experiment focused on three research plots used for corn and soy-b ean production that had been maintained under either no-till or conventional tillage for 18 years.
When they calculated nitrous oxide emissions per unit yield of grain or grain nitrogen, they found that emissions under no tillage were 52 percent and 66 percent higher, respectively, than with conventional tillage.
This meant that, for this cropping system and this specific climate, no-tillage practices resulted in substantially more nitrous oxide than conventional tillage for the same amount of grain. The effect was due to lower yields under the no till system.
Reduced yields under the no-till method in parts of the upper Midwest have been attributed to lower soil temperature in spring, which may slow plant development.
However, no-till can increase yields in other geographic regions.
Venterea said calculating greenhouse gas emissions on a yield basis in those regions could show benefits to no-till management that might not otherwise show up if emissions were calculated the conventional way.
“Yield-scaled emissions provide additional information for evaluating GHG impacts,” he said.
“For example, if we looked only at area-scaled nitrous oxide emissions in this study, then we might conclude that the no-till and conventional systems performed equally well.
“But based on the yield-scaled results, we can conclude that if the same amount of grain were produced using no till and conventional tillage, the no till system would emit about 50 percent more N2O compared with conventional tillage under the same fertilizer regime. The no-till system would also require additional land area in order to achieve the same production.
“In other cases, it is conceivable that a particular practice, tillage or some other practice, could at the same time reduce N2O emissions and increase crop yields. In these cases, yield-scaled emissions data would be valuable because they would express the synergistic benefit of these two effects.”
The agricultural industry is a significant contributor of greenhouse gas emissions, a fact not to be taken lightly by producers.
“The U.S. Environmental Production Agency publishes a greenhouse gas inventory every year that includes the agricultural sector, and within that sector, the various GHG sources,” Venterea said.
“Nitrous oxide emissions are a very large component of the total agricultural sector. Soil management accounts for approximately half of all GHG emissions that come from agriculture.”