Study: soil maintenance needed to ensure sustainability of cellulosic biofuels

Producing energy and bioproducts from crops may well hold the key to a sustainable future, but the transition towards such a biobased economy should be approached with care. A complex set of agro-ecological resources interacts and needs to be kept in balance in order to make the bioeconomy last. Ensuring the health of soils is crucial because soils are the nutrient factories that feed crops. Moreover, soils store vast amounts of organic carbon and thus play an important role in the global carbon cycle. Bioenergy production can only be green if it doesn't distort the soil organic carbon (SOC) cycle (image, click to enlarge).

An ongoing, five-year study by the Agricultural Research Service analyses how much residues can be removed from fields for the production of cellulosic ethanol, without reducing too much soil organic matter. Energy crops such as switchgrass, willow, and poplar, are targeted as sources of bioenergy, but crop residues, especially corn stover and wheat straw, have been identified as a source of cellulosic biomass as well.

The amount of crop residue needed to protect soil from erosion and to sustain soil organic carbon stores constrains residue removal for bioenergy. The researchers now made a first estimate of just how much residues must be kept on the field in order to ensure soil conservation: the U.S. at best has to cut in half the amount of cornstalks that can be harvested to produce cellulosic ethanol in a sustainable manner.

Research over the past century has shown conclusively that crop production practices result in loss of SOC. Typically loss of SOC has detrimental effects on soil productivity and quality. The objectives of the research - part of the Renewable Energy Assessment Project (REAP) - are to determine the amount of residue needed to protect the soil resource, compare economic implication based on the value of stover as bioenergy and carbon source, and provide initial harvest rate recommendations and guidelines.

Jane Johnson, a soil scientist with the ARS North Central Soil Conservation Research Laboratory in Morris, Minnesota, found that twice as many cornstalks have to be left in the field to maintain soil organic matter levels, compared with the amount of stalks needed only to prevent erosion.

This doesn't mean harvesting cornstalks for cellulosic ethanol isn't feasible - just that when you add soil organic matter concerns to erosion concerns, it reduces the amount of cornstalks available for conversion to ethanol. For example, 213-bushel-per-acre corn yields leave farmers an average of 4 tons per acre of cornstalks after harvest. Farmers could then harvest about 2 tons of cornstalks per acre for conversion to ethanol - but only from land with low erosion risks, using little or no tillage:
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If the same farmers rotate with soybeans as recommended, they can remove half again as much biomass for ethanol production, or just 1 ton per acre, to compensate for the lower biomass left by soybeans.

Johnson also explored the use of a byproduct of ethanol fermentation as an organic additive to soils. This is an example of the innovations needed to support residue removal.

The estimates are part of the Renewable Energy Assessment Project, formally created in 2006, although she and a core group of colleagues worked on these measurements for several years prior. REAP was formed to ensure that cellulosic ethanol programs will be sustainable. Most participants work with corn, but others work on switchgrass for cellulosic ethanol. When cellulosic ethanol is made from corn, it uses cornstalks as well as grain. Nine ARS locations are participating in REAP in eight states, from Alabama to Indiana to Oregon.

Products from the current program on sustainable residue removal will be:

1. guidelines for management practices supporting sustainable harvest of residue,
2. algorithm(s) estimating the amount of crop residue that can be sustainably harvested, and
3. decision support tools and guidelines describing the economic trade-off between residue harvest and retention to sequester soil C.

Delivery of this knowledge and these products to farmers and the biomass ethanol industry will promote harvest of stover and crop residues in a manner that preserves the capacity our soil to produce food, feed, fiber, and fuel.

Meanwhile, other scientists analysing the feasibility of the production of carbon-negative biofuels, are discovering that storing agrichar or biochar (obtained from the pyrolysis of biomass) in soils can boost their health (earlier post). Contrary to conventional agriculture which depletes SOC, 'terra preta' farming techniques increase carbon matter in soils. However, the long-term effects of the practise are unknown, and much research remains to be done on the commercial feasibility of this form of agriculture.

More information:
Jaradat, A.A., Johnson, J.M., Karlen, D.L., Wilhelm, W.W. 2006. Sustainable Production of Crop Residue as a Cellulosic Ethanol Feedstock. REAP - Renewable Energy Assessment Project sponsored by USDA & DOE. Oct 10-12 2006, St Louis, MO. Meeting Abstract.

Detailed project description: IMPACT OF RESIDUE REMOVAL FOR BIOFUEL PRODUCTION ON SOIL - RENEWABLE ENERGY ASSESSMENT PROJECT (REAP), Location: Agroecosystem Management Research, Project Number: 5440-12210-009-00, Start Date: Jun 01, 2006 - End Date: May 31, 2011

Maps on SOC can be found at the US Geological Survey: Assessing Carbon Stocks in Soils