There’s been a lot made in the past year about paying farmers to store carbon and install “carbon smart” practices on the farm.
But nitrous oxide, of which manure is a source, is 300 times more powerful than carbon dioxide and remains in the atmosphere for more than 100 years, says Heather Karsten, associate professor of crop production and ecology at Penn State.
There isn’t a simple answer to addressing the amount of manure and, in turn, nitrous oxide that comes from farms, but research by Karsten and others is shedding light on a possible solution: applying manure when the crop really needs it.
The study that Karsten led measured nitrous oxide emissions from the corn phases of two crop rotations — a corn-soybean rotation and a dairy forage rotation — under three different management systems. The results offer clues about how dairy farmers might be able to better time the amount of nitrogen fertilizer they apply to corn, saving money and contributing less to climate change.
“This research suggests that all nitrogen inputs — manure, legumes and fertilizer — contribute to nitrous oxide emissions,” Karsten says. “But farmers could reduce nitrous oxide emissions if they could apply manure after the crop is planted, closer to when the corn begins to take up nitrogen.”
In the corn-soybean rotation, Karsten’s team compared nitrous oxide emissions from broadcast dairy manure, shallow-disk manure injection and the application of liquid urea ammonium nitrate.
Manure was applied before corn was planted, while the liquid urea was applied according to recommended practices — when the corn was growing and taking up nitrogen.
“We sampled N20 [nitrous oxide] emissions by taking gas samples from vented chambers that remained on the soil surface during the study,” Karsten writes in an email. “We sampled twice a week and shortly after rain events to measure N20 emissions when they tend to increase due to microbial denitrification because the soil becomes anaerobic due to water saturation. Three gas samples were collected over 30 minutes to measure the rate of N20 emissions at each time point. Then, we analyzed the samples with a gas chromatograph.”
The better timing of nitrogen application led to less total nitrogen applied. Injecting manure increased nitrous oxide emissions compared to the broadcast treatment in one year of the study, showing that the environmental and nitrogen conservation benefits of injection should be weighed against the additional emissions when selecting it, Karsten says.
“This is primarily because injecting manure conserves more manure nitrogen, especially ammonium nitrogen, that's otherwise lost to the atmosphere as ammonia if manure is applied to the soil surface and not incorporated as in a no-till system," she writes. "Also, the manure was applied prior to corn planting, and a large quantity of it was available to be lost as nitrous oxide before corn was growing and taking up N."
The team also compared nitrous oxide emissions from corn silage or grain in a no-till, six-year dairy forage rotation where corn followed a two-year, mixed alfalfa and orchardgrass forage crop and a crimson clover cover crop.
Manure also was broadcast before corn planting, and nitrous oxide emissions were compared to the rotation, in which corn was planted after soybeans with broadcast manure. The nitrous oxide emissions didn’t differ among the three prior legume treatments.
Karsten says that she and the other researchers only tested sidedressing using the inorganic fertilizer.
"We only sidedressed inorganic fertilizer because we don’t have the equipment yet to sidedress manure," she writes. "But when we sidedressed significant quantities of inorganic N fertilizer in the inorganic fertilizer treatment and the broadcast manure treatment [that the in-season soil PSNT indicated was needed in the broadcast manure treatment], the N20 emissions from those treatments that applied fertilizer N when the corn was growing and taking up N were still lower.
"Since we don’t have the equipment yet to sidedress manure, in a follow-up study we used a crop and soil computer simulation model to simulate sidedressing manure, which allowed us to reduce the total N rate. Those simulations over 25 years of central Pennsylvania weather indicated that better synchronizing manure application with corn uptake would significantly reduce N20 emissions."
In both experiments, nitrous oxide emissions peaked a few weeks after manure was applied and for a short period after fertilizer was applied. Since nitrous oxide emissions are influenced by factors that influence microbial processes, the researchers examined what environmental and nitrogen-availability factors were most predictive of nitrous oxide emissions.
Increasing temperatures spurring corn growth and factors that influence soil nitrogen availability were important factors in both comparisons, according to the study.
Nitrogen availability from organic inputs such as manure and legume cover crops can contribute to nitrous oxide emissions from corn, says lead researcher Maria Ponce de Leon, former graduate student in Karsten’s research group and now a doctoral candidate at the University of California, Davis. Identifying how to time organic nitrogen with corn uptake represents an opportunity, she says, to reduce nitrous oxide emissions from dairy systems.
But Karsten adds that the total amount of nitrogen lost as nitrous oxide in manure is still a relatively small amount and, in most cases, is relative to how much is lost from the soil surface as ammonia or leaching.
“For instance, in our study, we measured N20 losses from mid-April to early September, and although some N20 was also likely lost during the colder periods of the year, the percentage of total N applied lost as N20 relative to the total N applied we found was less than 2%. But because N20 is such a potent greenhouse gas, its impact is significant,” she writes in an email. “Further, best management practices such as the 4Rs that help farmers capture more fertilizer and manure N can also reduce N20 emissions, providing an opportunity for farmers to reduce N costs and protect the environment.”
The study was recently published in Nutrient Cycling in Agroecosystems.
Ohio State University has done research on replacing commercial sidedress nitrogen with liquid manure in emerged corn.
Here’s a video that describes the work: