Biomass. Corn stover. Residue. Fodder. There are lots of names for matter that's critical to soil, and it's now becoming critical to biofuels. The majority of biomass — stalks, cobs and leaves left in the field after grain harvest — in the Corn Belt comes from corn. According to Jane Johnson, soil research scientist with the USDA Agricultural Research Service, about half of the aboveground plant mass in corn is grain and the other half is stover. And that stover can be used for ethanol.
As our nation strives to become more independent from overseas oil and ethanol becomes a bigger player, new sources of ethanol are being researched. One of those is above-ground biomass.
“Through emerging technology, (biomass) can be converted to ethanol and used as a transportation fuel,” says Johnson. However, “the risks of removing too much biomass include increasing soil erosion, removing valuable topsoil; increasing run-off of nutrients and pesticides and losing soil organic matter.”
Biomass for energy may be a great idea, but not if it causes more damage to the field than benefits realized from using biomass as a renewable fuel.
“Biomass left on the field serves a critical role in supplying carbon and nutrient cycles, and through decomposition, builds humus,” she says. “Removing any residue on some soils could reduce soil quality, promote erosion and lead to loss of soil carbon, which in turn lowers crop productivity and profitability.”
Tillage practices can also have a negative impact on necessary field residue. “In a long-term study established in 1888 (and still continuing today), different crop rotations, manure, tillage treatments and continuous corn production, coupled with intensive tillage, decreased soil organic matter by almost 64%,” says Mahdi Al-Kaisi, Iowa State University Extension agronomist. “The loss of original soil organic matter exceeds any potential additional carbon from crop residue because the majority (70-80%) of residue carbon after decomposition will be lost as CO2 to the atmosphere.”
RESIDUE REMOVAL FOR any reason — ethanol, tillage — can increase losses in organic matter, carbon and nutrients. Using corn stover for cellulosic ethanol or any other purpose needs to be weighed against the potential impact on water quality and soil productivity, Al-Kaisi cautions.
“The reduced nutrient supply associated with corn stover removal represents an economic loss in the short term, as well,” says Al-Kaisi. “Possible short-term impacts can include an increase in nitrogen, phosphorus, potassium, calcium and magnesium applications. One study estimates the replacement cost for these macro-nutrients at $10/ton of harvested residue.”
How you manage your crop impacts the rate of organic matter decomposition and soil erosion, greatly varying harvestable stover rates.
“The amount of stover that could be harvested annually depends on minimum residue requirements, yield, tillage management, cropping system, soil and climate,” says Johnson. “Initial estimates of minimum residue required to maintain soil carbon are much larger under moldboard plow systems compared to no-till or conservation tillage.
“For example, using the 2005 national yield average of 147 bu./acre, no biomass harvest would be recommended from either a continuous corn or corn-soybean rotation that was moldboard plowed, and less than 30% from continuous corn managed with reduced or no tillage,” Johnson says.
NOT ONLY DOES stover removal affect what's in the soil, it affects soil erosion and water quality, too. It will have a long-term negative effect on soil quality, water quality and agriculture sustainability.
“If unsustainable amounts of stover are removed from the field, wind and water soil erosion will intensify while accelerating the lost of soil organic carbon and other nutrient levels, potentially reducing future yields,” says Al-Kaisi. “In normal rainfall, raindrops 6 mm in diameter hit the ground at 20 mph. The cumulative impact of raindrops can be incredible, dislodging soil particles and splashing them up to 3-5 ft. away.
“The splashed particles clog soil pores, effectively sealing off the soil surface and leading to soil crust and poor water infiltration,” he says. “Instead of soaking into the soil, water collects and moves down-slope in sheet or rill erosion, forming gullies and carrying soil particles into rivers and streams.”
The tough part in managing biomass is that there aren't any clear guidelines for removal. Clear guidelines that protect soil from erosion and loss of nutrients, including organic carbon, ensure future productivity.
There is an “immediate need for more field trials to understand the impact of removing biomass, especially in systems with conservation or no tillage and modern high-yielding production practices and hybrids,” says Johnson. “We cannot afford to overlook the potential costs associated with wide-scale removal of crop residues from the land. We suggest a cautious approach to harvesting crop biomass.”
What needs to happen is implementation of conservation systems that will help sustain soil and improve environmental quality, says Al-Kaisi.
“Sustainable soil and improved environmental quality have to be considered in the current trend toward the increase of continuous-corn acreage and future thinking of corn residue removal for cellulosic ethanol production,” he says. “Sustainable stover removal depends on several factors, including soil erodibility, surface slope, cultural practices and climate conditions. Residue removal will have a devastating impact on soil sustainability and environmental quality in the long term.”