Growing rice typically requires a lot of water, and researchers in Arkansas are looking for ways to conserve water and minimize greenhouse gas emissions with sustainable practices that farmers can use here in the Natural State.
Arkansas rice accounted for 47.5% of total U.S. rice production in 2021, and rice ranks as one of the top three crop commodities in cash receipts for Arkansas farmers, according to the University of Arkansas System's research and extension arm.
With a team of researchers, and using a pit-less tailwater recovery irrigation system developed by Chris Henry, Kristofor Brye has been learning how greenhouse gas emissions could be reduced by changing the amount of water used to cultivate rice.
Brye is professor of applied soil physics and soil science at the university's Arkansas Agricultural Experiment Station. Henry is an associate professor and water management engineer who conducts research at the Rice Research and Extension Center near Stuttgart.
The way farmers choose to grow rice and how much water they use comes down to whether they use furrow or flood irrigation. Furrow irrigated rice, also called row rice or upland rice, is planted in beds with furrows between them and water is sent down the furrows routinely every couple of days, as opposed to flood irrigation.
"About 10 years ago, I realized there's a lot of opportunity in furrow irrigated rice we hadn't really explored," Henry said. "So I started working on being able to pick up the water from furrow irrigated rice and returning it in this recirculatory system that allows us to keep and maintain a bed system through the year."
Henry's system maintains consistent moisture levels just below saturation on rice fields. Furrow irrigation generates nitrous oxide, but with Henry's system, the rows are kept wet enough to minimize that effect.
"It allows us to keep nitrous oxide at bay and I think we can cut about the same yields with this system," Henry said. "Normally in a row rice field, we documented a 16-bushel yield penalty and farmers are OK with that generally, because there's enough extra costs using a flooded rice system in tillage, in building levees and spills and everything we have to do with that, that it almost pays for the 16-bushel yield penalty."
"One of the challenges with a tailwater system is you're losing ground to production because you're digging a hole," Henry continued. "In our system, you only lose an area the size of the top of your kitchen table. So when you're looking at it from an irrigation efficiency perspective, it's about twice as cost-effective as the conventional system, which is what a farmer would normally install in their fields."
It's hard to say yet whether Henry's new system will be less costly to farmers because until recently, there was only one system.
Five of Henry's irrigation systems have been placed on Arkansas farms, three of which were installed last year.
Anecdotal evidence from farmers suggests they are closing the gap in yield, Henry said.
Farmers can get funding for improving water conservation, Henry said. These incentive payments come from the Environmental Quality Incentives Program via the Natural Resources Conservation Service.
Henry's goal is to make his system more accessible to farmers. His team is also looking for a commercial partner and funding to develop the system on a larger scale and demonstrate how it works to farmers throughout Arkansas.
FURROW AND FLOOD
Furrow irrigation has become popular in recent years not just because it saves water, but also because farmers won't face costs for putting in levees around conventional flooded rice fields.
"There's some time savings, there's some labor savings, there's less costs to implement a furrow-irrigated production system," Brye said. "I think this is an attractive alternative that many producers have already been using for other crops and now they're just implementing it and applying it to rice."
It also allows farmers to plant cover crops, which improves the soil.
Jeff Rutledge has a 3,500-acre farm in northeast Arkansas where he grows corn, soy and rice in rotation. Rutledge has used both furrow and flood irrigation to grow rice for the past five or six years, though he increases his furrow-irrigated acres each year. Rutledge also plants cover crops.
"You have a growing, living ecosystem there that's covering the soil, that's reducing erosion and runoff," Rutledge said. "You've got these roots going down in the soil that allows more water infiltration, so you've got more internal drainage, you've got more biological activity. Farmers can then terminate the cover crop and get the cash crop in the ground in time for spring planting."
Furrow irrigation in Arkansas has increased from less than 1% to more than 10% as of 2019, according to the University of Arkansas research and extension center. The researchers concluded that in addition to water savings, furrow irrigation can also minimize greenhouse gas emissions.
Carbon dioxide, nitrous oxide and methane emissions from human activity are major contributors to climate change.
Carbon dioxide accounted for about 79% of all U.S. greenhouse gas emissions from human activity in 2020, but nitrous oxide is more potent, warming the atmosphere 300 times more than carbon dioxide over a 100-year period, according to the Environmental Protection Agency.
"With furrow irrigation, if we can keep the soil moisture uniformly wet -- and by uniformly wet, I mean just below saturation, so there's a little bit of air space in the soil but the soil is really wet -- we've got evidence from the greenhouse that's pretty convincing that that is sort of the happy medium to keep methane emissions low, nitrous oxide emissions low and [carbon dioxide] emissions low," Brye said.
Because flood irrigation is the most popular method of growing rice, Brye and his team investigated factors that affect methane emissions during the growing process. The agriculture sector is the second-largest source of methane emissions in the U.S., according to the EPA.
"Rice production is a perfect scenario for methane emissions," Brye said. "No one had been doing any work in Arkansas and there hadn't been much work in flood-irrigated rice and methane emissions even in the country, so very few people have been working on this."
In 2018 and 2019, Brye and the researchers simultaneously measured methane, nitrous oxide and carbon dioxide at three slope positions in a furrow-irrigated field.
One of Brye's graduate students finished last summer studying nitrous oxide emissions data from the project field, and in her master's thesis, she summarized fluxes and emissions from the different positions in the field and tillage treatments over those two years.
A current doctoral student is adding to that nitrous oxide emissions data, adding carbon dioxide and methane data to calculate a single parameter -- global warming potential -- integrating all three of those gases, Brye said.
"[Over] two consecutive years, we'll be able to evaluate these spatial positions in the field, the tillage treatments for global warming potential; and sort of simultaneously considering all three of the gases integrated together," Brye said.
A FERTILIZER SEARCH
Farmers are also looking for more sustainable fertilizers because of rising prices.
Global reserves of minerals like phosphorus, which is mined and manufactured into plant fertilizer, are dwindling, becoming more expensive, and now largely concentrated in only a few countries. Russia accounts for 11% of global phosphate trade, according to the University of Arkansas's enterprise crop budget report for 2022.
Farmers and researchers are looking for more sustainable fertilizers, but researchers like Brye are also interested in products with lesser environmental impact.
Brye is also part of a collaborative research team studying how struvite -- a mineral containing nitrogen, phosphorus and magnesium -- can minimize greenhouse gases, particularly nitrous oxide.
A chemical engineer, Brye uses a technique to recover nitrogen and phosphorus from wastewater sources like municipal wastewater treatment plants, dairy farms and livestock operations.
"They are loaded with fertilizer nutrients," Brye said, referring to the wastewater. "If we can get them out, that's going to clean up the water that gets returned back to the environment and then we've got this material."
The technique precipitates struvite into a solid, crystallized material.
"It's got the potential to serve as a blended fertilizer," Brye said.
Nitrogen, phosphorus and magnesium are already common in modern fertilizers, according to The Fertilizer Institute.
The team has used struvite to fertilize rice in a greenhouse, and preliminary evidence shows there is potential for struvite to minimize greenhouse gas emissions, particularly nitrous oxide, Brye said.
The team is returning to Henry's field this year to run more tests, Brye said.
Alternative fertilizers are worth investigating right now, Brye said, particularly because the war in Ukraine has led to increased costs for fertilizer, fuel and other farming inputs. This is already affecting Arkansas farmers and will continue to do so next year.
The university's crop budget report, which was amended in March to reflect changing prices after Russia invaded Ukraine, shows total variable costs for growing rice in Arkansas are up 47% over 2021, with fertilizer at 26% as the biggest contributor followed by fuel costs at 16%.
Fertilizer prices were rising before the war in Ukraine began, Rutledge, the farmer, said.
There were times farmers weren't sure fertilizer would be available, or whether they would even get a chance to bid to purchase some by the end of last year, he said.
"That made it very difficult to plan for costs and pre-pay some of those costs for the upcoming year or do any kind of financial planning, so it's been a challenge really since the end of harvest last year with the increase in these costs, on fertilizer alone," he said.
In response to rising fertilizer costs, Rutledge is supplementing his traditional fertilizer with larger amounts of chicken litter, though it would be difficult to rely solely on chicken litter for fertilizer unless farmers are based near poultry producers because of the effect rising fuel prices might have on transporting it, he said.