Solving the Puzzle

Fueled by severe droughts, dozens of wildfires scorched more than a million acres of land in the western United States last summer. Among them, the Bootleg Fire in Oregon was so intense that it generated its own weather. Even areas of the country that weren't affected by fires suffered as a massive heat dome bringing record-breaking high temperatures stretched across the western and central regions of the country. Meanwhile, monsoon rains drenched the Southwest and caused flash floods. And powerful Hurricane Ida caused flooding from Louisiana through the Appalachian spine and the Northeast.

"As the climate continues to warm, extreme weather events like these are expected to occur more frequently, and the agricultural sector will be particularly affected," says Armen Kemanian, professor of production systems and modeling. Specifically, he says, climate change can adversely impact agricultural productivity through alterations to average and extreme rainfall and temperature events that will overrun the buffer capacity of the system and reach damaging thresholds more frequently. Not to mention that these changes may increase pressure from pests. These impacts are already affecting markets, including the prices of food, fiber, and energy, as well as farmers' incomes.

To help the agricultural sector face both the gradual changes and the extreme events that are already happening and prepare for those that are yet to come, Kemanian and others in the College of Agricultural Sciences are researching tactical and strategic "climate-smart" practices, such as cover cropping, polycultures, and precision nutrient management. They are also studying climate trends over time and associated impacts on agriculture and creating models to evaluate the potential outcomes of various climate-smart strategies.

"Over the last approximately 70 years, grain crop yields have been increasing," says Kemanian. "This gives producers and the public a disjointed message: on the one hand, catastrophe is around the corner, but on the other hand, average crop yields keep increasing. Which one is it?"

It is both, he says. "The spectacular advances in agricultural technology have made it more difficult to internalize the reality of the ongoing changes. The longer we maintain the status quo, the more we'll see problems develop."

Even if technology and yield keep increasing, he adds, temperatures will become too hot more often, drought spells too long, and precipitation too intense, and at some point, the disruptions will become too great or widespread.

But Kemanian notes, agriculture is in a position to make a significant positive impact on climate change. "With the development and implementation of climate-friendly farming practices, we can adapt; we can reduce greenhouse gas emissions and not only protect the livelihoods of farmers but also improve food security across the world. And we can play offense, too."

Planting Cover Crops

It turns out that one of the most important tools for tackling climate change is right under our feet--soil. This mixture of minerals, dead plant and animal matter, microorganisms, water, and gas is a vast carbon sink, containing more carbon than the Earth's atmosphere and all its plants combined. When plants photosynthesize, they remove carbon from the atmosphere, and when they die, that carbon is returned to the soil as the plants are decomposed by microorganisms.

"The amount of carbon that soils can absorb and the length of time that they can store it varies by how the land is managed," says Jason Kaye, Distinguished Professor of Soil Biogeochemistry. "Disturbances to the soil can cause carbon to be lost to the atmosphere. But good land management--such as avoiding tillage and planting cover crops, like clover and legumes--can compensate for carbon losses."

Good land management can also reduce dangerous emissions of nitrous oxide--a greenhouse gas that is 300 times more potent than carbon dioxide. According to the U.S. Environmental Protection Agency, nearly three-quarters of the world's nitrous oxide emissions come from agricultural soils. When nitrogen fertilizer is added to the soil only about half is taken up by plants; the rest is either converted directly into nitrous oxide by soil microorganisms and released to the atmosphere, or it is leached from the soil, where it can pollute nearby waterways.

Cover crops--crops that are planted after a cash crop is harvested--have long been touted for their ability to reduce soil erosion, balance water by helping soil soak up heavy rainfall and retain water during drought conditions, and control weeds. But recently they have been recognized for their role in mitigating and adapting to climate change by influencing the amounts of carbon and nitrogen that remain in the soil versus the amounts that are lost to the atmosphere.

"Protecting soil by planting cover crops is among the most important things that many farmers are already doing," says Kaye. "Cover crops are a good carbon sequestration strategy because, compared to a fallow field with nothing growing on it, they draw down carbon from the atmosphere, and nitrogen in the cover crop can then be reused by the cash crop. And, depending on the type of cover crop that is planted, they can also help to manage nitrous oxide emissions. Our research is showing that the type of cover crop that is planted is important and that mixtures of species may provide the greatest benefits."

Illustration: Michele Lauriha

To study these benefits, Kaye and his colleagues are conducting a series of long-term field experiments at Penn State's Russell E. Larsen Agricultural Research Center at Rock Springs. The team first grew a rotation of corn-soybean-wheat and then planted cover crop monocultures of triticale, canola, and crimson clover, as well as a five-species mixture dominated by those three species.

In one study, published in 2020 in Renewable Agriculture and Food Systems, the researchers found that the mixtures increased total carbon inputs to soil to a greater extent than the monocultures because they had high carbon-containing root and shoot inputs and they promoted higher carbon inputs from the following corn crop residues.

"The corn crop was more productive following the mixtures than following grasses, and while we harvest a lot of that productivity, some gets left behind in residues," says Kaye. "I think this is really interesting because it shows that the effects of cover crops on soil carbon are not just related to their own roots and shoots, but also how they affect growth of the cash crops."

The study is important, Kaye explains, because the increased knowledge of cover crop root traits it yielded improves the understanding of the linkages between root traits and the services cover crops provide. Going forward, he expects to design cover crop mixtures that deliver unexpected ecosystem benefits, like climate change mitigation, and added boosts to cash crops that follow.

"We'll now be able to think about what we want to occur in the soil and then design mixtures that have the root traits that are best able to provide those benefits," he said. "I envision that we will exploit lots of different cover crop plants for different traits, both above and below ground."

Farm-Tuning Cover Crop Mixtures

In another 2020 study, published in PLOS ONE, Kaye and other colleagues, including Charles White, assistant professor of soil fertility and nutrient management and extension specialist, and Mary Barbercheck, professor of entomology, found that when it comes to cover crop mixtures, what you grow is not always what you get. They demonstrated that they could take the exact same number of seeds from the same plants, put them in agricultural fields across the Mid-Atlantic region, and get profoundly different stands of cover crops a few months later.

"We call it 'farm-tuning' cover crop mixtures," says Kaye, noting that the study demonstrates the need to customize cover crop mixes to achieve desired ecosystem services, depending on soil and climatic conditions.

The team tracked a five-species cover crop mix planted over two growing seasons on eight organic farms in Pennsylvania and New York and on research plots at the Russell E. Larson Agricultural Research Center. In the University's experimental plots, the researchers manipulated cover crop expression with nitrogen inputs and planting dates to learn the response of the various species to soil conditions and growing days.

"There have been very few studies like this--especially looking at cover crop mixtures that comprise more than two species--that analyze how species interact with each other, so I think it's important research," Kaye says. "There has been a misguided assumption that you plant a cover crop mixture, and you get the same result wherever you put it. Commercial seed companies sell many preformulated seed mixtures, but they can also make customized mixes. Our results show that with fixed, preformulated mixtures, what you grow is not always what you expect."

In the study, all eight of the participating farmers seeded the standard mixture and a farm-tuned mixture of the same five species--canola, Austrian winter pea, triticale, red clover, and crimson clover--with seeding rates adjusted to achieve farmer-desired services. At each location, the researchers parsed out the effects of soil inorganic nitrogen and growing days on cover crop mixture expression.

From the same seed mixture, cover crop mixture expression varied greatly across farms, and researchers hypothesized that this variation was correlated with soil inorganic nitrogen concentrations and growing days. For example, they found that canola dominated the mixture when soil inorganic nitrogen and growing days were high, especially in the fall. They also found that low soil inorganic nitrogen favored legume species, while a shorter growing season favored triticale.

The results show that when soil inorganic nitrogen availability is high at the time of cover crop planting, highly competitive species can dominate mixtures, which could potentially decrease services provided by other species, especially legumes. And early planting dates can exacerbate the dominance of aggressive species.

"Based on this study, farm managers could choose cover crop species and seeding rates according to their soil inorganic nitrogen and planting dates to ensure the provision of desired services, including, for example, climate change mitigation," says Barbercheck, suggesting that the real value of this research is that it provides usable information to farmers who want to take advantage of it.

Managing Nitrogen

As with any agricultural practice, however, there are trade-offs with cover cropping. For example, certain cover crops, like legumes, help the soil retain nitrogen needed for the following cash crop, but the residue left behind after these legumes are plowed into the soil slowly releases nitrous oxide into the atmosphere. The situation becomes more complex when farmers then apply manure to further boost soil nutrition.

To investigate this effect, Kemanian and Kaye recently measured soil nitrous oxide emissions for two growing seasons in four corn-soybean-winter grain rotations under various management conditions, including tillage, cover crops, and manure applications. In a study published in August in Ecological Applications, the researchers report that the application of manure after the growth and demise of legume cover crops in the rotations dramatically increased nitrous oxide releases--by roughly 10 pounds per acre--during ensuing corn growth.

"That is a very large rate," said Kemanian. "It represents 80 percent of the three-year rotations' total emissions. We knew that cover crops needed to be managed, but we weren't aware that they needed to be managed for nitrous oxide emissions. We just found a big leak."

The researchers suggest that innovative management strategies are needed to reduce these emissions. Among the strategies they recommend are removing a fraction of the legume aboveground biomass before planting corn and applying manure. In the study, preventing the "co-location" of fresh biomass and manure decreased nitrous oxide emissions by 60 percent during the corn phase.

Another important strategy is knowing precisely how much fertilizer to add to get maximum crop growth while minimizing nitrous oxide emissions. For example, White explains, to avoid applying fertilizer during the really wet times of the year, when it will just wash away, farmers can apply the majority of their fertilizer mid-season in June or July rather than at the beginning of the season in April.

To help farmers sort through all the factors that affect nutrient management and determine the optimal amounts of fertilizer to apply, White and his colleagues have developed an online tool in which farmers can input information--including yield goal, soil texture, soil organic matter concentration, biomass nitrogen content of cover crops, and cover crop carbon-to-nitrogen ratio--from their own farms to see how those activities will affect their fertilizer requirements.

"The more we can provide the best recommendations on the right rate, timing, and form of fertilizers to apply," says White, "the better we can reduce the greenhouse gas footprint of agriculture and improve farmers' bottom line."

For example, the online tool can help farmers determine the least amount of fertilizer they can add while still getting a high crop yield. "The tool predicts the corn yield that can be produced without any fertilizer added," he said. "If it predicts 164 bushels without applying any fertilizer, but the farmer knows from past experience of growing fertilized crops that they can get 200 bushels, we call that difference the nitrogen yield gap, and that's what we base our recommendation on. In this example, the tool will predict that you need 107 pounds of nitrogen to close that nitrogen yield gap and get your 200 bushels of corn. That's about half as much as would have been recommended in the traditional system, where the recommendations are set to be high enough to protect against yield losses in 95 percent of cases. What this system does is takes out some of that overprotection and gives a more fine-tuned recommendation for a specific site."

In 2020, the team received a $500,000 grant from the U.S. Department of Agriculture's National Institute of Food and Agriculture to perfect the tool. Specifically, they will use laboratory experiments to expand their understanding of how soil microbes slowly release nitrogen, making it available to plants. These laboratory studies will help them predict the conditions under which the model will work well. They will also work with a small group of farmers to test the tool on their fields to make sure it is easy to use and helpful in making fertilizer decisions.

The development is notable because while the wide use of cover crops in rotations with corn in the last decade has resulted in reductions in nutrient pollution and sedimentation, the introduction of cover crops has complicated growers' decision-making regarding how much nitrogen fertilizer to apply to meet their cash crop demands.

"Around the world, a lot of nitrogen is lost when too much fertilizer and manure is applied," says White. "A lot of it has to do with a lack of tools available to farmers to accurately predict the correct amount of nitrogen that is needed for crops in any given year."

Harnessing Genetics

Another way to avoid applying unnecessary amounts of nitrous-oxide-emitting fertilizers is to create plants that are better able to capture the nitrogen that is already available in the soil. Jonathan Lynch, Distinguished Professor of Plant Science, and his colleagues recently discovered a gene that regulates the angle of root growth in corn, a finding that may enable the breeding of deeper-rooting crops with enhanced ability to take up nitrogen. "Corn is the most important crop in the world," says Lynch. "In rich countries like the U.S., the biggest energy, economic, and environmental cost of growing corn is nitrogen fertilizer, and more than half of the nitrogen fertilizer applied to corn is never even taken up. It's just wasted--washed deeper into the soil, where it pollutes groundwater, and some of it goes into the atmosphere as the greenhouse gas nitrous oxide."

In findings recently published in Plant, Cell, and Environent, the researchers found that a gene called ZmCIPK15 was missing in a naturally occurring mutant corn line that grows roots at steeper angles to make them go deeper into the soil. The team found that plants without this gene had markedly improved nitrogen capture.

The results of the research are eye opening, Lynch points out, admitting that he was surprised by the outcome. It's quite unusual when you knock something out that the plant gets better, he explains, because plants are like finely tuned machines.

"You take a gene out of that finely tuned machine, you don't expect it to work better. But this shows that if you knock out the single gene, you'll get deeper roots and better nitrogen capture," he says. "For places like America, here's a way to reduce a major cost and environmental impact from corn production. For lower-income countries like Africa, this discovery could result in higher corn yields that will reduce food insecurity. If we can move corn from being 50 percent efficient with nitrogen to 60 percent efficient, we will save money and there will be an environmental gain as well."

Looking to the Future

According to Lynch, climate change is already threatening food security for millions in the world, and it's only a matter of time until the United States also feels the effects. "We can no longer assume that citizens of more affluent nations like the United States will be immune to the worst effects of our destabilizing climate," he says. "We're all in this together, facing changes that will affect every one of us within our lifetimes."

A case in point is a recent study by Kemanian and others in which the team found that if warming continues unabated, the best areas for corn and soybean production may shift from Iowa and Illinois to Minnesota and the Dakotas in the next 50 years.

Using machine learning--a form of artificial intelligence that enables a computer system to learn from data--the team considered more than three decades of county-level crop-yield data from the U.S. Department of Agriculture's National Agricultural Statistics Service for 18 states in the central region of the United States.

"This kind of research was impossible before the era of big data we are living in now, and of course, it can be done only by using the powerful computing capacity that we can access at Penn State," says Kemanian. "This study is important because in a climate that is changing relatively quickly, these techniques allow us to foresee what may happen and prepare."

The findings, published in Environmental Research Letters, do not necessarily mean that the shift north and west in corn and soybean production will occur, he says. But based on the data, the team concludes that such a shift is in progress, and there is a strong probability that it will continue.

"We are not suggesting that such a shift would be a catastrophe," Kemanian says. "It doesn't mean that Iowa will stop producing crops, but it might mean that Iowa farmers adapt to a warmer climate producing two crops in a year or a different mix of crops instead of the dominant corn-soybean rotation. The changes are likely to be gradual, and farmers and the supply chain should be able to adapt. But things will change."

Kemanian warns that while farmers can adapt their crops to respond to slow changes over time, dealing with extreme changes will be much more challenging. By building virtual landscapes that represent with fidelity agricultural and natural landscapes, Kemanian and his colleagues are able to explore the implications of various land management strategies under climate warming scenarios. "These models allow us to tinker at a scale we cannot handle experimentally," he says. "For example, if we were to change a farm that is being tilled to no-till, we can ask, 'What is the estimated gain of carbon in the soil? How about nitrous oxide emissions?'"

Ultimately, adds Kaye, the goal much of the college's climate-change-related research is to help farmers minimize their greenhouse gas inputs while simultaneously maximizing their yields, as well as to adapt to the changes that are occurring as a result of nonagricultural greenhouse gas emissions.

"I think of agricultural system design as a puzzle," he says, "putting the pieces together in a way that maximizes the benefits for farmers and the environment, while minimizing negative outcomes."

-- Sara LaJeunesse and Jeff Mulhollem
Illustrations by Dan Page