No-till and biofuel crops

“America is addicted to oil” as President George W. Bush acknowledged in his 2006 State of the Union Speech. And, it is not just a US problem, nor is the addiction only to oil. Oil, coal and natural gas are the fossil reserves which power our planet, but now the spotlight is on crop biomassDescription Mass of organic matter of non-fossil biological origin which can be exploited for energy purposes. Authoritative On-line References and Resources http://www1.eere.energy.gov/biomass The US Department of Energy's Office of Energy Efficiency and Renewable Energy has a Biomass Program working with industry, academia and US National Laboratories on research into biomass feedstocks and conversion technologies. The goal is cost competitive, high performance biofuels, bioproducts, and biopower. to provide a significant alternative source of energy and materials.

No-tillDescription Also known as conservation tillage or zero tillage is a way of growing crops from year to year without disturbing the soil through tillage ie cultivating the soil usually with tractor-drawn implements. Authoritative On-line References and Resources http://www.no-till.com/index.htm A portal for on-line information about no-till farming. farming and paraquat have a vital role to play in producing enough biomass while sustaining food production and protecting the environment.

At present, biofuelsDescription Fuel derived from biomass. Authoritative On-line References and Resources http://www.biofuelstp.eu/ The European Biofuels Technology Platform is upportd by the European Commission and aims to help develop cost-competitive, world class biofuel technology, contribute to the creation of a European biofuels industry and to identify the research needed to achieve this. are manufactured from the parts of crops otherwise harvested for food, eg grain. This leads to two problems:

1. Not enough fuel
2. Potentially not enough food

The yield of fuel – biodieselDefinition Mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats for use in diesel engines. It refers to pure fuel before blending with conventional diesel fuel. Blends are denoted as, "BXX" with "XX" representing the percentage of biodiesel contained in the blend (ie: B20 is 20% biodiesel, 80% petroleum diesel). Authoritative On-line References and Resources http://www.eere.energy.gov/afdc/fuels/biodiesel.html The US Department of Energy's Office of Energy Efficiency and Renewable Energy has an Alternative Fuels and Advanced Vehicles Data Center containing key information on all biofuels. or bioethanol – from the oils or starch found in seeds is relatively low. With the economic and environmental motivation to grow more crops for biofuels, in future, they may take up valuable land that should be used for growing food, especially in poor Third World countries. Already, in Mexico the rising price of corn tortillas, a staple food for many poorer people, has been a problem. This has been due to the higher price of US corn, driven-up by the demand for ethanol.

To address both fuel and food issues, it would be much more attractive to use unharvested parts such as corn stover or wheat straw for biofuel production. However, as will be explored in this article, this will have potentially serious environmental consequences without appropriate changes in farming practice.

What are Biofuels?

A variety of vegetable materials, technically called ‘biomass’, can be squashed, cooked, and fermented into biofuels. Biodiesel is the product of vegetable oils derived from soybeans, sunflowers, rapeseed, palm, coconuts, and other more exotic sources such as jatropha tree seeds. Bioethanol, a.k.a. ethanol, comes from the fermentation of corn, sugar beet and sugar cane, switchgrass, and wheat, as well as other “cellulosic” vegetable matter.

Ethanol use far outstrips biodiesel globally, as ethanol has more mature markets and infrastructure. In the U.S., where it is mostly fed by corn, ethanol has been blended with gasoline for many years.

The high fossil fuel energy input to manufacturing and for transport to market currently undermines the promise of U.S. corn-based ethanol as a solution to global warming and energy security. Some schemes to set-up ethanol production in fact depend on coal and diesel energy to run ethanol plants. Ethanol made in Brazil from sugar cane, on the other hand, stands on an efficient process honed since the 1970’s oil crises, and reduces greenhouse gas emissions by 80%.

Biodiesel on the whole remains more fuel-efficient and less emissions-intensive than ethanol. Studies show that biodiesel on average is about four times as efficient as petroleum diesel when considering all energy inputs and fuel mileage and carbon uptake of plants. Ethanol is about twice as efficient.

Production and use of biodiesel not only results in a reduction in carbon dioxide emissions, but, unlike petroleum diesel, also avoids emission of lead, sulfur or toxic aromatics such as benzene, toluene, and xylene. In Europe, where diesel technology is more advanced and widespread, biodiesel currently has more potential for commercial and civilian transport than it does in the U.S. Biodiesel production in Europe far outstrips production in the U.S. The ease with which biodiesel can be blended with conventional petrodiesel is perhaps its best selling point. Vehicles with conventional diesel engines can run on 20% blends without need for modifications.

Within the next decade or so, cereal straw and corn stover are expected to make a major contribution to the production of biofuels. At present, grain and other sources of starch and sugars are used to make ‘first generation’ bioethanol. Soon, the process technology to make ‘second generation’ biofuels, from cellulose in stover and straw will be implemented in ‘biorefineries’.

Biofuel Crop Management

Although straw and stover are seen as potentially attractive sources of second generation biofuels. far from being waste materials, crop remains have an essential role in maintaining or improving soil quality. Roots add organic matter to soils, but stubble and straw or stover – sometimes called ‘trash’- are important too. Care must be taken not to over-harvest these to produce biofuels. Trash management is an important part of conservation agriculture - which aims for profitable crop production while protecting soil and water, and enhancing biodiversityDescription The variety of life in all its forms, levels and combinations. Includes ecosystem diversity, species diversity, and genetic diversity (IUCN, UNEP and WWF, 1991). Authoritative On-line References and Resources http://earthtrends.wri.org/ EarthTrends is a comprehensive online database, maintained by the World Resources Institute, that focuses on environmental, social, and economic trends. Statistics on biodiversity indicators are available.. Stubble and straw, which is best chopped and spread evenly over the field, guard against erosion by wind and rain, provide a habitat for wildlife and ultimately help to maintain soil organic matter levels.

No-till techniques, where crops are sown into uncultivated stubbles, are the ideal way to establish crops under conservation agriculture. No-till was pioneered by using paraquat as a non-selectiveDescription A chemical product used for eliminating all types of weeds (annual and perennial grasses and broadleaved weeds). Authoritative On-line References and Resources http://www.weeds.iastate.edu/ An invaluable source of contemporary information about herbicides and weeds from Iowa State University. burndown herbicide to control weeds no longer buried by ploughing. Although glyphosate has become established as an alternative non-selective herbicide, paraquat has advantages when fast action in colder weather and rainfastness are important. Using paraquat is also an important part of resistanceDescription The inherited ability of a plant/weed to survive a dose of a herbicide normally lethal to that species. Authoritative On-line References and Resources http://www.weedscience.org/in.asp The International Survey of Herbicide Resistant Weeds monitors the evolution of resistant species and assesses their impact. All confirmed instances of new cases are listed. management strategies as glyphosate succumbs to weed resistance.

Recently, the US Department of Agriculture and Department of Energy (USDA & DOE) reported on work assessing the feasibility of meeting targets for replacing oil, coal and natural gas with biomass¹. By 2030, the replacement of 30% of the country’s petroleum demand with biofuels is envisaged.

To achieve this target, over one billion tonnes of biomass feedstock are reckoned to be needed each and every year. This must be consistent with not only satisfying the need for food, animal feed and fiber, but also compatible with sustaining soil quality and productivity.

A number of assumptions about technical progress are necessary, but by 2030 more than 350 million tonnes of biomass could come from forestry and nearly one billion tonnes from agricultural land.

The breakdown of where these billion tonnes could come from is shown in the chart opposite and includes 430 million tonnes of field crop remains, mainly corn stover.

Assumptions include: increases in yield in-line with current trends, improvements in harvesting technology to enable greater recovery of stover and other straws, and the management of all cropland with no-till methods.

USDA and DOE acknowledge that converting all cropland to no-till may be unrealistic, but, nevertheless, point out that a strong market for bioenergy could be a forceful driver for large increases in the no-till acres which are key to meeting the targets.

The importance of increasing the area of no-tilled cropland and recognizing this in policy is noted in the 2007 Farm Bill Theme Paper Agriculture and Energy².

“There is a significant opportunity to realize immediate economic and environmental gains through energy conservation activities ... The measures include: Doubling of no-till acreage (from 25 to 50 million hectares), saving 821 million liters of diesel fuel (217 million gallons) and $500 million each year; …”

Why is no-till so important to the fast developing biofuels industry and how paraquat can help? This article goes on to look at the potential of biofuels, the issues involved and the solutions offered by no-till and paraquat.

Potential of Biofuels

Estimated known reserves of oil would last for about 40 years if used at the current rate, gas and coal for longer, but the demand for energy is growing³.

• In 2007, every day the US uses more than 20 million barrels of oil. The EU uses 15 million barrels per day. Compare these figures to China’s current consumption of only 6 million barrels per day and India’s 2 million and think how fast those vast countries’ economies are growing.
• Look at a map of where the major fossil reserves are located and see that perhaps more than two-thirds lie under politically unstable regions.
• Feel the warmer temperatures which most climate scientists believe are linked to greenhouse gas emissions and note that the concentration of carbon dioxide in the atmosphere has increased by 25% in 50 years and will double by the end of the century.

No wonder fuel prices are rising and there is fast growing interest and investment in bioethanol, biodiesel, biopolymers and other non-food product from crops. In the short-term, the US aims for an annual production of 28.4 billion liters (7.5 bn gallons) of bioethanol by 2012 ² and the EU aims to be using 5.75% of road transport fuels from biorenewables by 2010⁴.

The idea of biofuels is not new. The diesel engine was originally designed to run on ‘vegetable’ oils, first being fuelled by peanut oil, and Henry Ford called ethanol “the fuel of the future”. However, it is only in recent years that bioethanol and biodiesel have been catching on. Brazil took an early lead with its investment in the Proalcool program to produce ethanol from sugarcane in 1975. In 2005 the US produced 15 billion liters (4 bn gallons) of ethanol from corn, accounting for 14% of all domestic corn production². In Europe bioethanol is made from wheat and sugarbeet, currently in much smaller volumes than in the US or Brazil. Biodiesel is produced from oilseeds. In the US, soybeans are the feedstock and in Europe oilseed rape (canola) is mainly used.

Energy from the sun is captured by photosynthesis in 57 billion tonnes of new plant material on land every year⁵. Mankind uses about 4 billion tonnes from agriculture and 7.5 billion tonnes from forestry annually. The rest, of course, is essential to maintain the planet’s ecosystems.

The greatest potential production of biodiesel is from tropical oilseeds, especially oil palm (Elaeis guineensis) in Malaysia and Indonesia, and jatropha (Jatropha curcas) in India. Not only are large areas of these tropical perennial crops planned to be grown, but the intense sunlight and plentiful water allow high productivity.

The global demand for food is forecast to double by 2050 and many more people will want to eat meat. Food energy in beef, for example, requires up to eight times more crop land than a vegetarian diet equivalent. So, more than ever, the pressure is on to increase agricultural productivity and to use it wisely. Protecting the soil is essential for sustainable agricultural production.

Biofuel Issues

The bioethanol and biodiesel currently available are ‘first generation’ biofuels, made from starch or oil (triglycerides) that plants store in seeds, or sucrose stored in roots or cane. Research is now focused on improving the chemical and biological processes necessary to make ‘second generation’ biofuels from the more abundant materials plants use for structure, as found in, eg corn stover and wheat straw, particularly cellulose.

There are three key problem areas with first generation biofuels:

1. First, there is simply not enough land available. It has been calculated that if all US corn grain went to make bioethanol and all soybeans went to make biodiesel then that would satisfy only 12% of current US gasoline demand and 6% of current diesel demand⁶. Using cellulose from straw and stover left from grain crops grown under no-till management will support much greater and more sustainable feedstock supply.
2. Second, large amounts of energy are used in the production of biofuels from starch, sucrose or oil, especially in making bioethanol. Tractors and harvesters consume fossil diesel, nitrogen fertilizer production requires energy, and substantial amounts of heat and power are used in ethanol refineries. No-till can increase the net energy contribution of biomass by reducing energy used in crop production.
3. Third, these activities and processes which consume energy also release carbon dioxide. Few people realise that transport fuels are a lesser contributor to greenhouse gas emissions than either agriculture or deforestation, ie the burning of residues (see chart below)⁷. Clearing forests to grow oil palm for biodiesel is not sustainable, and this was one of the reasons prompting the formation of the Roundtable on Sustainable Palm Oil in S.E. Asia. No-till can reduce greenhouse gas emissions by using less fuel, but crucially by building-up soil organic matter to sequester carbon dioxide.

Agriculture, however, has the opportunity to play a pivotal role in providing biomass for energy and in reducing greenhouse gas emissions. ‘Cellulosic biomass’ from straw and stover for second generation biofuels will be a much more abundant feedstock. Within the next decade, innovative farming methods together with improved chemical and biological process technologies will make biomass more positive in net energy contribution and give a substantial further reduction in carbon dioxide emissions. No-till farming, using non-selective herbicides like paraquat for weed control, will form the platform for success through a sustainable supply of biomass.

No-Till Benefits

No-till farming has developed since the introduction of paraquat more than 40 years ago made weed control possible without ploughing. The many benefits of no-till are listed in the table below. There are practical solutions to the few issues involved, but skill and appreciation of the soil’s needs are vital to successful no-till.

Summary of benefits and issues usually associated with no-till

Worldwide there are over 90 million ha of no-tilled cropland, most in the Americas⁸.

Of the 32.4 million ha (80 million acres) of corn grown in the US in 2004, 20% were no-tilled and a further 18% were ridge- or mulch-tilled on land where it is not believed feasible to no-till, but where many of the benefits of no-till are still achievable⁹. Some 15% of the 30.4 million ha of small grains such as wheat (75 million acres) were no-tilled.

More soybeans are grown under no-till in the US (38% of 76 million acres, or 47% under all conservation tillageDescription Any tillage and planting system that covers 30 percent or more of the soil surface with crop residue after planting to reduce soil erosion by water. Authoritative On-line References and Resources http://www2.ctic.purdue.edu/Core4/CT/Definitions.html Core4Conservation is part of the Purdue University-based Conservation Tillage Information Centre. techniques), but soybeans leave less residues than corn. However, breeding programs are pursuing higher biomass soybeans¹.

No-till Solutions: 1. Providing Sustainable Biomass

How much biomass can be removed from corn fields without compromising the benefits from leaving a cover of stover over the soil?

The answer depends on many factors including crop yield level, climate, topography, soil type and soil management. The Soil Conditioning Index (SCI) is a tool to predict the qualitative effects of farming practice on soil organic matter levels:  will they remain stable, increase or decline under particular circumstances? Worked examples are included in the USDA National Conservation Resources Service (NCRS) National Agronomy Manual¹⁰.

Surface trash has been shown to be less important under no-till¹¹. There are a number of reasons for this, but when the soil is not disturbed, there is less susceptibility to erosion.  Longer runs of no-tillage give more opportunity for levels of organic matter to have increased and for soil structure and stability to have improved.

Attempts have been made to estimate the amount of crop residual biomass which could be sustainably removed¹². These use methodologies based on the Revised Universal Soil Loss Equation (RUSLE) and the Wind Erosion Equation (WEQ) to insure that the amount of soil loss by wind and rain would not exceed a tolerable limit. These led to the calculations of sustainable levels of removal in the USDA/DOE report on meeting the US targets for biomass use¹.

Using paraquat for weed control also helps. Paraquat is not translocated in plants, so only shoot growth contacted by spray is destroyed and the roots remain intact for some time. This provides an anchoring effect for the soil. Similarly, because paraquat is inactivated by extremely strong adsorption immediately on contact with soil it has no residual effect. New flushes of germination are, therefore, unaffected. A naturally regenerated and managed weed flora provides a vegetative cover to the soil performing a similar function to crop remains spread over the field.

In cases where removal of crop remains would seem unsustainable, there are alternatives¹³. These include growing cover cropsDescription Cover crops are primarily planted not to be harvested for food but to reduce soil erosion, control weeds and improve soil quality. They are usually plowed or tilled under before the next food crop is planted, in which cases the "cover crop" is used as a soil amendment and is synonymous with "green manure crop." Authoritative On-line References and Resources http://attra.ncat.org/attra-pub/covercrop.html ATTRA is the US National Centre for Appropriate Technology's Sustainable Agriculture Information Centre. between the main crops in arable rotations, or growing dedicated perennial energy crops such as switchgrass (Panicum virgatum) or Miscanthus (Miscanthus giganteus). Paraquat already has well established uses in the management and desiccation of cover crops. Perennial energy crops are usually woody species. In perennial crops paraquat is ideal for broadspectrum weed control because it can not penetrate bark, nor does it have any residual activity in soil. Any small amount of spray contact with leaves is not important because paraquat, unlike glyphosate, is a non-systemic herbicide.

No-till Solutions: 2. Increasing Net Energy Contribution of Biomass

Energy is needed to grow biomass and then to transport and process it. An analysis of farm energy inputs into growing corn for bioethanol and soybeans for biodiesel points to the importance of two inputs: diesel fuel and nitrogen fertilizer ⁶. 

Soybeans have an advantage in that as a legume, they receive little or no nitrogen fertilizer, making biodiesel significantly more energy positive than bioethanol.

However, over the whole rotation, diesel consumption is by far the biggest drain on energy balance. In this analysis no account was taken of specific agronomic practices. Clearly, more field operations will burn more diesel. There are many other factors including soil type and condition, depth of cultivations, tractor power, etc. A breakdown of fuel use under various cultivation systems in Illinois showed that, although some gains from lower fuel use under no-till are clawed-back by more fuel use to plant and spray, no-tilling corn used 14% less fuel, and no-tilling soybeans used 49% less fuel¹⁴.

Fuel used in growing corn and soybeans in Illinois under conventional and no-till systems¹⁴

Other analyses have suggested even larger savings in fuel from no-till. For example, the USDA 2007 Farm Bill Theme Paper ‘Energy and Agriculture’ states:

“During the past couple of decades, NRCS has helped farmers adopt no-till practices on about 25 million hectares of cropland. Assuming an average savings of 33.13 liters/ha (3.5 gallons per acre) in diesel fuel, this amounts to a savings of 821 million liters of diesel fuel per year with cost savings to farmers of about $500 million per year.”

Although non-selective herbicides such as paraquat must be used in no-till systems for weed control, and this necessitates burning fuel to make both the herbicide itself, transport it and to spray it, these are very small compared to the savings from not ploughing or cultivating.

No-till Solutions: 3. Reducing Greenhouse Gas Emissions

The amount of carbon dioxide in the atmosphere is increasing exponentially. Estimates vary, but are always in billions of tonnes added every year. In 2000, an estimated 32 billion tonnes were released - 77% of all greenhouse gas emissions as calculated by their global warming effects⁷.

The other significant greenhouse gases are methane (14%), nitrous oxide (8%) and chlorinated hydrocarbons (1%). These are produced in much smaller quantities, but have more potent effects on global warming than carbon dioxide (approximately 20, 300 and several thousand times more, respectively).

Around 80% of greenhouse gas emissions come from the use of fossil fuels. The rest, together with most of the emissions of methane and nitrous oxide come from agriculture and the burning of unwanted wood from deforestation.

There are two ways in which no-till can reduce emissions of carbon dioxide:

• Less fuel burned
• More organic matter conserved

Reduction in fuel used in crop production will obviously reduce emissions, but this is a relatively small effect compared to the enhanced potential of soils under no-till to sequester carbon in organic matter.

In Europe (the continent, excluding the former USSR), it has been estimated that if all cropland were to be no-tilled, there would be potential to sequester more than 150 million tonnes of carbon dioxide per annum¹⁵. In addition, savings in diesel fuel consumption would reduce emissions of carbon dioxide by nearly 12 million tonnes every year. To put this into perspective, a family car typically emits about 4 tonnes of carbon dioxide in a year’s travel.

In the US, the Consortium for Agricultural Soil Mitigation of Greenhouse Gases¹⁶ CASMGS, pronounced ‘Chasms’) is a government funded alliance of ten universities and institutes challenged with providing

“ … the tools and information needed to successfully implement soil carbon sequestration programs so that we may lower the accumulation of greenhouse gases in the atmosphere, while providing income and incentives to farmers and improving the soil. Such benefits include an increased and stable agricultural production and an overall reduction of soil erosionDescription Displacement of solids (soil, mud, rock and other particles) usually by the agents of currents such as, wind, water, or ice by downward or down-slope movement. Authoritative On-line References and Resources http://soilerosion.net/ This site brings together reliable information on soil erosion from a wide range of disciplines and sources. It aims to be the definitive internet source for those wishing to find out more about soil loss and soil conservation. and pollution by agricultural chemicals and fertilizers.”

CASMGS estimate that agricultural soils in the US have the potential to sequester between 275 and 730 million tonnes of carbon dioxide each year. If a carbon trading scheme were to be introduced whereby industries emitting more pollutants pay those who emit less, then US agriculture could benefit from a $5 billion market for carbon sequestration. No-till would be key to this. The Iowa Farm Bureau is working to aggregate carbon credits from Iowa farmers for sale on the Chicago Climate Exchange under a pilot project¹⁷.

One note of caution is that wet, compacted soils may encourage the conversion of fertilizer or soil sources of nitrate to harmful emissions of nitrous oxide¹⁸. Although the type of fertilizer also has an effect, no-till must be done well to avoid such adverse conditions.

Biofuels Need No-Till and Paraquat

Cereal straw and corn stover are expected to make a major contribution to the production of biofuels. At present, starch and sugars from grain and other sources are used to make ‘first generation’ bioethanol. Within perhaps 10 years the process technology to make ‘second generation’ biofuels, from cellulose in stover and straw will have been implemented in ‘biorefineries’. Increasingly, straw is also burned to produce energy for heat and power.

However, straw and stover are not waste products and already play an important role in cropping systems. Crop material left in fields after harvest, helps to prevent erosion, provides habitats for wildlife, and increases the level of soil organic matter.

It ultimately adds to the organic matter which is important for the structure, stability, and productivity of soil. In the near future, therefore, there will be a conflict between using crop remains for soil management and collecting it as biomass to produce biofuels.

The solution to this dilemma is to increase the amount of cropland under no-till cultivation. Large increases in the area of no-till in the US have been recommended in order to meet the country’s future annual requirements estimated at over one billion tonnes of biomass. No-till has three key benefits in biofuels production:

1. No-tilled soils are more stable and less susceptible to erosion allowing more straw to be removed,
2. No-till means substantially less fuel is consumed in crop production, increasing the net energy contribution of biofuels,
3. Partly because less fuel is used, but much more significantly because soil organic matter levels increase, no-till soils sequester large amounts of carbon dioxide – with important implications for its role as a greenhouse gas in global warming.

As weeds are not controlled by ploughing in no-till, its success relies on the use of non-selective herbicides like paraquat. Paraquat is the best choice when fast action and rainfastness are needed. Paraquat has no soil residual activity and contributes to minimizing soil erosion by only destroying shoot growth. Roots are left intact and provide an anchoring effect. In addition, paraquat is an essential component in rotating herbicides to avoid weed resistance.

References

1. USDA/US DOE (2005). Biomass as Feedstock for a Bioenergy and Bioproducts Industry: The Technical Feasibility of a Billion-Ton Annual Supply.
2. USDA (2006). Energy and Agriculture. 2007 Farm Bill Theme Paper.
3. Energy Information Administration: http://eia.doe.gov.
4. European Commission (2006). An EU strategy for biofuels.
5. Imoff, M L et al (2004). Global patterns in human consumption of net primary production. Nature, 429, 870-873.
6. Hill J et al (2006). Environmental, economic, and energetic costs and benefits of biodiesel and ethanol biofuels. Proceedings of the National Academy of Science, 103, 11206-11210.
7. Baumert K A et al (2005). Navigating the numbers: greenhouse gas data and international climate policy. World Resources Institute.
8. Jones, C A et al (2006). Conservation Agriculture in Europe. SOWAP: UK
9. Conservation Tillage Information Center: http://www.conservationinformation.com
10. USDA National Conservation Resources Service: National Agronomy Manual subpart 508C: Soil management (2002).
11. Wilson G V et al (2004). Tillage and residues effects on run-offDescription The occurrence of surplus liquid (like rain) which originates up-slope and is collected beyond the ability of the soil to absorb it. The surplus liquid then flows away over the surface to reach the nearest surface water (pond, lake, river). Authoritative On-line References and Resources http://www.sowap.org/index.htm SOWAP (Soil and Water Protection) is a collaboration between industry, NGOs, universities and farmers to test a range of site-specific soil management methods, based on the concept of conservation tillage. It has looked at economic and environment aspects including effects on soil erosion and run-off. and erosion dynamics. Transactions of the American Society of Agricultural Engineers, 47, 119-128.
12. Nelson, R. G. 2002, Resource assessment and removal analysis for corn stover and wheat straw in the Eastern and Midwestern United States - rainfall and wind-induced soil erosion methodology. Biomass & Bioenergy, 22, (5):349–363.
13. Andrews, S S (2006). Crop residue removal for biomass energy production: effects on soils and recommendations. USDA Natural Resource Conservation Service white paper.
14. University of Illinois (2006). Farmdoc Newsletter April 19 2006. http://www.farmdoc.uiuc.edu/manage/newsletters/fefo06_07/fefo06_07.html
15. Smith P et al (1998). Preliminary estimates of the potential for carbon mitigation in European soils through no-till farming. Global Change Biology, 4, 679-685.
16. Consortium for Agricultural Soil Mitigation of Greenhouse Gases: http://www.casmgs.colostate.edu
17. Iowa Farm Bureau: http://www.iowafarmbureau.com/special/carbon/default.aspx
18. Venterea R T, et al (2005). Nitrogen oxide and methane emissions under varying tillage and fertilizer management. Journal of Environmental Quality, 34, 1467-1477.