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Moving biofuel cropping systems toward long-term sustainability

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The switchgrass nitrogen fertility trial at Kellogg Biological Research Center in Hickory Corners, MI (left), and GLBRC researcher Laura Smith collecting switchgrass tissue samples at Chiwaukee Prairie in Kenosha City, WI (right). Photos by Laura and Matt Smith.

By now most of us are accustomed to filling our cars with fuels that are part ethanol, and we know that corn is not only in our tortillas but also in our gas tanks. Great Lakes Bioenergy Research Center (GLBRC) researchers, however, are moving beyond corn and other first-generation biofuel feedstocks in an attempt to fill our tanks with environmentally sustainable biofuels.

Agronomy professor Randy Jackson, GLBRC’s sustainability research group co-leader, says “the focus of agricultural biofuel research has changed recently from ‘agronomic intensification’ to ‘ecological intensification.’ In other words, it’s not just about how much money you can make growing a crop anymore…it’s about how we can grow what we need and nurture the land at the same time.”

“One way to move towards a system of ecological intensification,” Jackson continues, “is to move from fields of corn, which need to be planted annually and require lots of fertilizer, to mixed varieties of perennial grasses such as switchgrass.”

Perennial grasses dramatically reduce soil erosion, provide protective cover and food for wildlife, encourage bird populations and insect pollination, foster methane-consuming microbes, and suppress the invasion of agricultural pests. Perennial grasses also use nitrogen more efficiently than annual monocultures such as corn, which could mean less nitrogen fertilizer in our fields.

Reducing the use of nitrogen fertilizer, in addition to saving farmers time and money, is a big plus for the environment. Most crops are unable to retain the majority of applied nitrogen. Some is lost in run-off where it contributes to nitrate contamination of ground water, streams, and rivers. At the same time, a significant amount of excess nitrogen ends up in the soil, where microbes convert it to the greenhouse gas (GHG) nitrous oxide (N2O).

N2O is a highly potent greenhouse gas — the atmospheric-warming impact of a single pound of N2O is more than 300 times that of a pound of carbon dioxide. In addition, N2O is the most significant ozone-depleting chemical resulting from human activity. And currently, N2O accounts for about 6% of GHG emissions resulting from human activities, with 75% of those emissions coming from the agricultural use of synthetic fertilizers.

Although the ideal field of biofuel feedstock, from an ecological standpoint, would contain a variety of perennial grasses, studying switchgrass, a native prairie grass, is an important step toward realizing that ideal.

smith_switchgrass2“Switchgrass is a promising biofuel feedstock and represents a kind of halfway point between agronomic intensification and ecological intensification,” says GLBRC doctoral researcher David Duncan, “It has the downside of being a single species, but it’s perennial so you don’t have to replant it every year. And it doesn’t require as many inputs, such as nitrogen, as a crop like corn.”

Switchgrass also has the advantages of being fast-growing, productive, and able to grow on marginal land unsuited for food crops.

But it’s a major transition for farmers accustomed to tending monoculture crops such as corn to add perennial cropping systems to their crop selections. In a corn field, every plant is genetically identical and the varieties have become so predictable that farmers know, almost to the day, how long they will take to mature.

In a switchgrass field, on the other hand, you find many genetic differences within a single variety, and the time required for crop maturation varies significantly depending on weather, light conditions, moisture, and nutrient availability.

Moving biofuel cropping systems toward long-term sustainability and profitability will thus require a better understanding of how perennial grasses develop under differing conditions. GLBRC doctoral researcher Laura Smith, for example, is trying to determine the relationship between biomass productivity and nitrogen uptake in switchgrass fields.

“We are trying to understand the ways that perennial grasses such as switchgrass use nitrogen so we can reduce the use of nitrogen fertilizer and still maximize biomass harvest,” Smith explains. “A significant challenge to the use of these grasses as biofuel feedstocks is the inefficiency of harvest — currently, if we wait until fall to harvest, we are only able to collect about 60% of harvestable biomass from switchgrass plants and we need to figure out how to improve on that.”

One important factor limiting maximum biomass harvest is harvest timing. Plants typically reach peak biomass in August. Yet the long-term health of perennial plant systems depends on their having the chance to senesce, or let the plant system’s nutrients return to the roots, before harvest.

“The time of peak biomass yield is also the time when plants have the highest nutrient content.” Smith says.  “If that’s when you harvest, you’re pulling huge amounts of nutrients off the field. You are losing a whole lot of nitrogen and that means you’ll have to use more fertilizer for next year’s crop.”

But waiting for senescence significantly compromises biomass yield, as plants recycle carbohydrates and leaf litter falls to the ground.

“Finding the balance point where we can maximize yield and still ensure nutrient resorption moves us toward ecological intensification,” Smith says. “Some of our work indicates that fertilized switchgrass plants take longer to resorb nitrogen than unfertilized plants. If we can reduce added fertilizer and get the same amount of biomass off a field, it’s good for the farmer and very good for the environment.”

In focusing on ecological intensification, GLBRC researchers are tackling a central challenge of “second generation” biofuel feedstocks: promoting highly productive harvests that also nurture the environment.

This story was originally published on the GLBRC website here

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