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Study examines biomass harvesting, water quality

By Larry Dreiling

(Journal photo by Larry Dreiling.)

It's been said that one man's trash is another's treasure. The "trash," or residue, left in no-till fields is considered valuable for soil preservation. Grain sorghum, a crop becoming more popular on the High Plains because of its low moisture needs and heat tolerance, is rich in residue.

With the construction of the Abengoa Bioenergy refinery near Hugoton, Kan.--the nation's first commercial-scale cellulosic ethanol refinery--expected to be fully completed by 2011, the dream of producers selling the refuse of their crops to make ethanol from cellulosic biomass to reduce both dependence on imported fossil fuels and emissions of greenhouse gases is within reach.

The Hugoton facility will process about 700 tons per day of crop residues--corn stover, wheat straw and, yes, sorghum stubble--as well as other feedstocks, like switchgrass, into ethanol.

It's seen as a big change in ethanol production, away from the use of grains for feedstocks and toward cellulosic biomass. It is seen to likely reduce the controversy over the use of grains causing an increase in food prices and accelerating food insecurity.

A different controversy

One controversy not being examined by many experts is how the large-scale removal of crop residues would affect water quality. This is because crop residues are the early targets as cellulosic ethanol feedstocks, according to a Kansas State University researcher.

"Identifying and growing lignocellulosic-rich plant biomass and developing technologies for processing cellulosic biomass into ethanol are receiving more attention than ever before," said Dr. Humberto Blanco, assistant professor of agronomy and soil scientist at the Kansas State University Agricultural Research Center-Hays.

"While producing ethanol from renewable biomass feedstocks is a plausible goal to reduce our dependence on fossil fuels, excessive removal of crop residues may adversely impact water quality by increasing transport of non-point source pollutants in runoff, such as sediment, nitrogen and phosphorus (N and P), to downstream water bodies."

Although termed by some people as wastes, crop residues are essential to conserve soil and water and improve crop production, Blanco said. Crop residue mulch increases water infiltration and thus reduces the amount of rainfall available for runoff. It filters pollutants from runoff and also increases soil organic matter content, which is essential for adsorbing and retaining pollutants.

A good idea?

One crop rich in residue is grain sorghum. Blanco, along with students in an agronomy class at Fort Hays State University, led by Dr. Bob Stephenson, is working on a project designed to advance the understanding of the potential impacts that harvesting crop residues for ethanol production may have on water quality.

The project was funded by grants from the Kansas Department of Health and Environment and the U.S. Environmental Protection Agency Section 319 grant program through the Kansas WaterLINK initiative, Blanco said.

"Crop residue is in high demand in some areas of Kansas and other states, either as feedstocks for cellulosic ethanol production, industrial uses, livestock feed, or other uses," Blanco said. "Producers can get paid for selling their crop residue for these uses. But, we had to ask, is it really a good idea to remove and sell crop residue? What is the cost of crop residue removal in terms of loss of soil quality and productivity, and potential impairment of surface water quality?"

Leaving crop residue on the soil surface is the best and simplest way of reducing water and wind erosion, Blanco added. Widespread residue removal for other uses may accelerate soil erosion and increase the loss of sediments, nutrients, and pesticides in runoff water. Sediment and nutrients leaching into runoff water are the main culprits of non-point source pollutants into downstream water bodies.

"But crop residue removal may not be all or nothing. The producer may want to remove some residue and leave some. We have to ask: How much residue can be removed from crop fields without creating erosion and runoff problems?"

Crop quality question

The answer is a hard one to grasp, Blanco admits. He said it partially depends on the level of crop productivity. In some cases, particularly in semiarid regions such the High Plains, not enough residue is produced, most years, to protect soil from water and wind erosion and maintain adequate levels of soil organic matter.

"In those cases, any removal of residues may further degrade soil quality, increase water pollution, and reduce crop production," Blanco said.

The Blanco project is out to demonstrate to watershed partners the beneficial role of crop residue mulch in plowed and no-till soils in reducing runoff transport of non-point source pollutants. In some watersheds, a fraction of crop residue may be available for removal without negatively affecting water quality.

"We also have to look at excessive mulch cover, because in some soils, it reduces crop production," Blanco said. "So we need to find a balance of how much residue we can take off the field."

In some regions, crop residue production is often limited and not sufficient to protect soil from erosion and maintain adequate levels of soil organic matter. Thus, a study of the impact of a partial removal of crop residues was developed, in turn, to bring about guidelines to manage residues in a way that their removal does not adversely impact water quality.

"The objectives of the project are to quantify the impacts of wheat and sorghum residue removal on runoff, soil loss, and nutrient loss from two gently sloping soils in western Kansas and suggest the threshold of removal of residues for biofuels production," Blanco said.

Study explained

The challenge was in finding a long-term no-till field, then asking the producer if, in the interest of science, he'd mind having a small plot of land torn up to help conduct the study.

Leave it to local producer Harold Kraus, a member of the National Biodiesel Board, who, along with stepson, Lance Russell, have been no-tilling at their farm a few miles south of KSU-ARCH for more than 20 years to come to the scientists' assistance. The combination of a study of biodiesel feedstocks and crop residue was very intriguing to the cooperating producers and they readily offered a small plot for the experiment.

The project was keyed to variable rates of residue removal from wheat and sorghum fields on water erosion in two soils late last fall. A 20-year no-till field under continuous winter wheat and a three-year conventional-till grain sorghum field (plowed in the spring before planting) were selected. The soil in both fields is silt loam with a slope of six 6 percent for wheat and three 3 percent for sorghum.

The stubble remaining after harvest was removed at 0, 25, 50, 75, and 100 percent. The average height of the standing stubble was 12 inches for wheat and 23 inches for sorghum. The plots were parallel to the dominant slope. Simulated rainfall was applied to the plots to give the effect of a rainstorm with a return period of 25 years for western Kansas.

In addition, a second set of plots within each field was established by tilling the soil a few days before rainfall simulation. Concentrations of sediment, soil organic carbon, and nutrients in runoff were measured.

Runoff samples were collected from each plot under simulated rainfall to quantify the amount of runoff and sediment as well as N and P loss.

Study results

Results of this study showed that wheat and sorghum residue removal after harvest exponentially increased loss of sediment, soil organic carbon, and nutrients in runoff regardless of tillage system, Blanco reported.

"The single rainstorm of high intensity caused large and immediate runoff loss of sediment and nutrient pollutants when residues were removed. Where most or all of the residue was left intact after harvest, the runoff water after the high-intensity rainstorm was clearer. Where half the residue was removed, sediment loss increased after the rainstorm," Blanco said.

Meanwhile, the freshly tilled wheat plots (tilled immediately after the residue removal) lost more sediment, soil organic carbon, and nutrients than no-till wheat plots for the same level of residue removal.

"This suggests that residue removal in combination with intensive tillage can accelerate soil erosion to unsustainable levels," Blanco said. "Differences in soil slope affected the amount of pollutants lost in runoff. More runoff, sediment, soil organic carbon, and nutrients were lost from the freshly tilled wheat plots with six 6 percent soil slope.

"Another finding, which may surprise many, was that removing 75 percent or more of the residue after harvest can negate many of the benefits of no-till in reducing runoff. We found that the loss of non-point source pollutants from no-till soils was equal to those from plowed soils when residues were removed at or above 75 percent.

"This indicates that no-till may be no better than plow tillage if residues are removed at high rates. Excessive residue removal from no-till soils can negate the erosion control benefits attributed to no-till. No-till benefits for controlling soil erosion are quickly lost when residue is removed at rates above 25 percent. Residue cover is needed to keep the soil in place."

Making a choice

The study also showed removing residue after harvest also has a negative effect on soil carbon, even in no-till wheat systems. The large losses of soil organic carbon with residue removal at rates as low as 50 percent indicate that a single rainstorm of high intensity could eliminate all the gains in the soil organic carbon pool attributed to no-till.

"The large losses of soil organic carbon with sediment may have large soil, agronomic, and environmental implications," Blanco said. "Soil organic carbon is essential to increase the soil's ability to absorb and filter non-point source pollutants and improve the productivity of the soil.

"Residue removal also increased losses of essential nutrients-particularly total N and P. Loss of nutrients in runoff increased with residue removal above 50 percent in no-till wheat and reduced nutrient pools through nutrient removal with residues and via increased runoff."

Results of this study show that crop residues are indeed essential to reduce sediment, soil organic carbon, and nutrient loss in runoff, regardless of tillage system, Blanco added.

"Crop residue removal is not recommended if soil and water conservation, non-point source pollution control, and soil carbon buildup are high priorities. Residue left on the soil surface protects the soil against impacting raindrops, helps maintain the integrity of soil aggregates, and improves rainwater infiltration."

The bottom line: Only a small fraction--about 25 percent--of residue may be available for removal for biomass energy production from no-till soils.

"Further studies that will look at pollutants losses under different scenarios of rainfall intensities as well as soil and topographic characteristics are needed to more fully determine the amount of harvestable residue, Blanco said.

Larry Dreiling can be reached by phone at 785-628-1117, or by e-mail at ldreiling@aol.com.

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