By planting switchgrass and using certain agronomic practices, farmers can significantly reduce the amount of nitrogen and nitrates that leach from the soil, according to Iowa State University research.
Matt Helmers, professor of ag and biosystems engineering, and Antonio Mallarino, professor of agronomy, have been studying the amount of nitrates that pass through soil into tile drainage systems from several different types of crops and fertilizer treatments for the past three summers. The research is funded by the Iowa Department of Agriculture and Land Stewardship and ISU's Leopold Center for Sustainable Agriculture. Research has found that certain practices can minimize the amount of nitrogen and nitrates that leach from the field into the drainage tiles.
"One of the biggest things we found is that when alternative biomass sources like switchgrass are grown, even when fertilizer is applied, we see dramatically lower nitrate concentrations in the drainage water," says Helmers.
Biomass crops dramatically lower nitrate in drainage water
The research has compared fields that were planted with continuous corn while harvesting just the grain; continuous corn taking the grain and stover; and planting continuous corn taking all possible biomass from the fields. Half of those fields were treated with fertilizer and the other half with manure. Other fields tested systems that rotate corn and soybeans, and others looked at switchgrass plots that received nitrogen fertilizer.
The results show that fields planted in continuous corn and treated with fertilizer had the most amount of nitrates leach below the crop root zone into the tile system. The fields with the least amount of nitrates that leached through the soil were planted in switchgrass and treated with fertilizer or manure.
Helmers says that while switchgrass allows less nitrogen to leach into the soil, farmers need a reason to plant it. "Right now, there is not necessarily an economic market for switchgrass," says Helmers.
What are environmental impacts of planting land to biomass crops?
"What we're trying to do is evaluate what might be the environmental benefits of that type of land use," he explains. "I think finding that out may be able to help inform future policy. If we pursue a strategy for producing additional biofuels from various biomass feedstocks, we need to know what the environmental impacts of those different feedstocks are, because that may play into federal policy.
"If there is enough societal benefit and water quality benefit from growing switchgrass on these soils, there may be potential incentives for farmers to grow switchgrass," he notes.
Helmers estimates that at least a third and possibly as much as half of all farmland in Iowa now uses tile systems to drain excess water from the fields.
Nitrates that leach into the soil can affect Iowa communities that depend on the rivers for clean drinking water, Helmers says. Nitrates that leach into the soil and are carried downstream are believed to contribute to a dead zone in the Gulf of Mexico where few plants or animal life survives.
Overall, there is need for additional information on how biomass feedstock production systems impact nitrate leaching, says Helmers. "We do frequently get questions about what is the nitrate level leaching from grassland systems compared to corn and soybeans."