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Potential Impacts of Biofuel Production on Soils
By the Alberta Agriculture and Rural Development. This document is part of the Capturing Feed Grain & Forage Opportunities 2007 Proceedings - "Farming for Feed, Forage and Fuel".
Take Home Message
Biofuel production can offer better ability to recycle nutrients and carbon back into soil when the by-products of production are returned to the soil either indirectly through livestock feeding and manure production, or through direct application to the soil.
Introduction
Expansion of the biofuel industry is expected to have implications for the fertility and productivity of soils in Western Canada. Grain – based ethanol provides distillers grains as a byproduct that, when fed to cattle, give the opportunity to recycle some of the nutrient in the grain back to the field in manure, that otherwise would leave the region if grain is exported. Some of the distiller’s grains may also end up applied to the land directly. Production of ethanol from straw that is harvested from farm fields represents an additional export of carbon and nutrients from the soil that needs to be considered in nutrient budgets. Biodiesel produced from canola results in glycerin as a by-product. The value of by-products in all possible uses, including feed, fuel, industrial, processing, and land application should be explored.
As a rapidly emerging industry, many aspects of the relationship of biofuel production to the environment have not yet been clearly established. Maintenance of soil quality and production capacity is recognized as one of the pillars of a sustainable agricultural production system. The measurement of soil attributes that influence the ability of the soil to sustain plant growth and act as an environmental filter is a common approach to assessing the impact of some practice on soil quality (Greer and Schoenau, 1997). At present, there is a limited body of information upon which to draw to make predictions as to, for example, how the land application of manure from distillers grain fed cattle may affect soil phosphorus fertility compared to manure from cattle finished on barley, or how glycerin applied to soil might affect soil organic matter and biological activity. In this paper, consideration is given to potential impacts of biofuel production on soils. The topics addressed hopefully will provide a framework for identification of key issues and research needs.
Recycling nutrients in grain-based ethanol production systems
The means by which to maximize efficiencies in inputs that are used to produce grains for ethanol production require some consideration at the outset. Traditionally, when viewing grains for human or livestock consumption, emphasis has been placed on protein yield per acre or yield per pound of added nitrogen fertilizer. There will be a need in future plant breeding programs and fertilizer management strategies to consider how to specifically increase the yield of carbohydrate per acre of land and per unit of nitrogen fertilizer, as the carbohydrate is the key ingredient in fermentation. Recycling of the nutrients added will also be an important issue to address.
Recognizing that about 70% of the nitrogen and phosphorus in a mature cereal crop is contained in the grain is important when considering ability to recycle nutrient in grain that goes towards ethanol production. As the carbohydrate is consumed and converted to ethanol in fermentation, the distiller’s grains that are left behind after the fermentation process retain much of this nutrient. In cattle feeding, only about 10% of the nitrogen and only about 20% of the phosphorus in the feed is retained in the beef animal, with the rest excreted in dung and urine. The extent to which these retentions may be influenced by distillers grain feed versus other feeds is not known yet. However, much of the nutrient that originally went to the distillation facility as grain can ultimately make its way back to land as manure, if cattle feeding of the distillers grain is part of the system. This is also why the continued development and evaluation of strategies to improve efficiencies of manure handling, transportation, application and crop recovery are needed.
Under some circumstances, direct application of distiller’s grain to soil, especially wet distillers grain may be required, owing to problems that may occasionally arise in the drying and distribution system. The effects of application of distiller’s grain directly to the soil as a soil amendment are largely unknown. However, some preliminary research work this fall has suggested that high rates may initially have some detrimental effects on germination and early plant growth, but that in later stages, the plant recovers and the amendment is an effective source of plant nutrient, promoting yield. Much remains to be learned about potential phytotoxic effects associated with organic acids and other constituents of the distillers grain, and its net effect on available nutrients, soil biological activity and organic matter. More detailed research into these aspects is planned in the coming year.
Straw harvesting and use
The production of ethanol from high cellulose materials such as straw and wood by-product is gaining interest due to advances in production technology. Straw has traditionally been harvested from cropland for use as a lower quality feed, and as bedding, and is returned to the soil when bales are laid out in the field, or hauled out of pens along with manure, adding to soil nutrients and organic matter (Jungnitsch et al. 2005).
The removal of straw from the field for ethanol production represents an alternative use that has implications in soil fertility and quality. Cereal straw contains about 50% carbon, 0.5% nitrogen, 1% potassium, 0.5% phosphorus and 0.1% sulfur, with part per million concentrations of micronutrients. Straw carbon contributes to maintenance of soil organic matter, as soil organic matter is mainly comprised of carbon. When straw is returned to the soil, the soil biota (microbes, earthworms etc) decompose the straw. About 70% of the straw carbon is respired to the atmosphere as carbon dioxide gas in the year of application, but a portion, albeit small, enters into the soil each in the form of “black humus” that is stabilized soil organic matter.
If straw carbon is removed from the field and not returned, this represents a reduction in the annual input of organic matter to the soil. To maintain the balance, a reduction in output from other sources is required. This could be in the form of adoption of reduced tillage that would reduce the decomposition rate. A change in cropping system such as more frequent use of forages in the rotation could also compensate by increasing the organic matter inputs to the soil. Soils that are low in organic matter to begin with, such as sandy eroded knolls, will be most sensitive to reduction in carbon input due to straw removal. Lower slope positions and sloughs would be more desirable locations in the landscape for straw removal. Soils of high inherent organic matter content appear to be quite resilient to straw removal. For example, a study at the Agriculture Canada Research Station (Black Chernozem soil) at Indian Head SK using long-term fertilized fallow-wheat-wheat rotation plots with and without straw removal by baling suggested that removal of straw over a number of years did not adversely affect soil organic carbon levels (Campbell et al., 1989).
When straw is removed from a system, in addition to carbon, nutrients like nitrogen, phosphorus, sulfur and potassium are removed. For example, the straw from a 40 bushel /acre wheat crop contains on average about 25 lbs N, 4 lbs P, 46 lbs K and 5 lbs S per acre. Several years of repeated straw removal will reduce the total soil amount and supply rates of these nutrients, unless these removals are balanced off by additional nutrient supplied as fertilizer or manure.
Land application of glycerin by-product from canola biodiesel production
Glycerol, commonly called glycerin, is a three carbon sugar alcohol made up of carbon, hydrogen and oxygen. During manufacture of biodiesel, about 100 kg of glycerin is produced for every 1 tonne of biodiesel. While many potential uses for glycerin exist, such as in food, industrial chemical and pharmaceutical preparations, surplus glycerol resulting from large recent increases in production is disposed of by incineration (The Glycerol Challenge, 2007). Research is underway to investigate alternative uses, including transformation into other value-added molecules. Application of glycerin to soil is another alternative use that has received little attention. This approach could be more carbon neutral than burning as some of the carbon may be sequestered in the soil, and also contribute to soil quality by increasing soil organic matter content. Potential issues surrounding application of glycerin to soils include its effect on soil microbial activity and nutrient availability, as the compound contains no nutrient itself and would create potential for significant immobilization of nitrogen.
To provide some information on the effects of adding glycerin to prairie soils, in the summer of 2007 glycerin (methanol stripped) from canola biodiesel production in Saskatchewan was added to a Brown Chernozemic soil at four rates (0, 100, 1000, and 10,000 kg per ha) in a growth chamber. Wheat plants were grown on the soil for six weeks and the plants were harvested and analyzed for dry matter yield and nitrogen uptake. At the end of the growth period, soils were analyzed for available N and P, and organic carbon concentration. The results are shown in Table 1.
The biomass yield of the wheat tended to be increased with the addition of glycerin, with the highest yield at the 1000 kg / ha application rate. Some microbial immobilization of available nitrogen and phosphorus induced by addition of the carbon source is evident at the higher rates of addition. There was a trend towards increased total soil organic carbon content measured at the end of the experiment. These results suggest that land application of glycerin may be a beneficial alternative to incineration. Future studies are planned that will assess in more detail the effects of glycerin amendment on soil fertility and plant growth in prairie soils.
| Table 1. Effect of glycerol addition to a Brown Chernozem on wheat biomass yield, nitrogen uptake and soil characteristics in a growth chamber experiment (Qian and Schoenau 2007 unpublished data) | |||||
| Rate Glycerol kg/ha |
Wheat Yield g/pot |
Wheat N Uptake mg/pot |
Soil Avail. N mg/kg |
Soil Avail. P mg/kg |
Soil Organic C % |
|---|---|---|---|---|---|
| 0 | 0.13b1 | 5.8bc | 27a | 8.5a | 1.96b |
| 100 | 0.17ab | 7.3a | 27a | 8.7a | 2.11b |
| 1000 | 0.20a | 6.2ab | 11b | 7.6b | 2.05b |
| 10000 | 0.16ab | 4.8c | 7b | 6.8b | 2.40a |
| 1Values in a column followed by the same letter are not significantly different at p≤0.05. | |||||
Conclusion
An expanded grain ethanol industry in western Canada offers the potential for more recycling of nutrients back into our soils compared to when grain is exported out of the region. Nutrients contained in distiller’s grains that are fed to animals can enhance the fertility of soils through sound management of the manure nutrients that are produced. Straw harvesting for cellulose based ethanol production should be done carefully to ensure that the additional carbon and nutrients removed in the straw are compensated for, to avoid gradual depletion of soil nutrients and organic matter. The return to the soil of biofuel processing by-products, either directly as in the example of glycerol amendment, or indirectly via manure from cattle fed by-products, represents an important way that soil productivity can be maintained in value-added systems. Research is needed now to document the impacts and better understand the relationships so as to most effectively manage the byproducts as resources rather than waste products.
References
C.A. Campbell, K.E. Bowren, G. Lafond, H.H. Janzen and R.P. Zentner. 1989. Effect of crop rotations on soil organic matter in two Black Chernozems. Proceedings of the 1989 Soils and Crops Workshop, Saskatoon, Saskatchewan, 368-378.
K.J. Greer and J.J. Schoenau, 1997. Toward a Framework for Soil Quality Assessment and Prediction. In E.G. Gregorich and M.R. Carter (eds), Soil Quality for Crop Production and Ecosystem Health, 313-321. Amsterdam: Elsevier.
P. Jungnitsch, H.A. Lardner and J.J. Schoenau, 2005. The Effect of Winter Feeding Systems on Soil Nutrients, Forage Growth, Animal Performance and Economics. Proceedings of Growing the Livestock Industry Conference, October, Saskatoon, Saskatchewan, 52-60.
The Glycerol Challenge, 2007. Available at http://theglycerolchallenge.org (verified 31 Oct 2007).
