E-Letter responses to:

reports: David Tilman, Jason Hill, and Clarence Lehman Carbon-Negative Biofuels from Low-Input High-Diversity Grassland Biomass Science 2006; 314: 1598-1600 [Abstract] [Full text] [PDF]

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Published E-Letter responses:

Tilman et al. Reply to Cassman

David Tilman, Jason Hill, Clarence Lehman   (19 April 2007)

The Experimental Low-Input High Diversity Biofuel System

Kenneth G. Cassman   (19 April 2007)

Tilman et al. Reply to Cassman 19 April 2007
David Tilman Department of Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, MN 55108, USA, Jason Hill, Clarence Lehman Respond to this E-Letter: Re: Tilman et al. Reply to Cassman	Cassman misinterprets our study and seems unaware of the results of long-term studies that refute his assertions. We carefully considered numerous studies of the similarities and differences between autumn or spring harvesting of prairie biomass as a bioenergy crop and burning as a surrogate for such harvesting [e.g., (1-8)]. We realized, as does Cassman, that limiting nutrients recycled via in situ burning would need to be applied when biomass is removed. Although we showed that nitrogen (N) fixation by legumes more than compensated for N exports in harvested biomass, we stated that “application of P [phosphorus] or other nutrients may be needed if initially limiting or to replace nutrient exports.” We estimated the P removed in harvested prairie biomass, and explicitly included this amount in all of our calculations as a fertilizer input and as an energy cost, for both its production and its application [figs. 2 and 3 and supporting online material (SOM)]. We also suggested that prairie biomass should be harvested in autumn to “both yield greater biomass and decrease ecosystem loss of N, P, and other nutrients” (our SOM), since herbaceous perennials have translocated nutrients to roots and lost aboveground nutrients to leaching by that time (8). We also note that nutrient-addition experiments have shown that, even on our degraded sandy soils, prairie perennials are not limited by P, K, Ca, or Mg (9). Contrary to Cassman’s claim, we never proposed that biomass could be produced sustainably without nutrient addition. Rather we explicitly asserted, even in the title of our paper ("Carbon-negative biofuels from low-input high-diversity grassland biomass"), that low inputs were needed. We contrasted our "low-input high-diversity" approach with the high inputs currently used for corn, which is the source of U.S. transportation ethanol. U.S. corn receives average annual inputs of 148 kg ha-1 of N, 23 kg ha-1 of P, and 50 kg ha-1 of K (10). In contrast, replacement application rates for our harvested prairie biomass are 0 kg ha-1 of N, 4 kg ha-1 of P, and 6 kg ha-1 of K (8, 11). It is not unique for low-input high-diversity grasslands to have sustainable yields and plant compositions despite nutrient removal via harvesting (12-14). Total amounts of many soil minerals are immense relative to amounts removed in hay or late-harvested prairie biomass. This likely is why yields of annually hayed high-diversity grasslands have been sustainable without fertilization for 55 years on 11,000 hectares of native prairie in Woodson County, Kansas (12) and for 150 years in control plots of the Park Grass Experiment (13, 14). We did not emphasize such "no-input" results in our paper, but rather named our method one of "low inputs" to recognize that the long-term sustainability of ecosystem productivity requires that cycles of limiting nutrients be closed by returning removed nutrients to the soil. In total, high-diversity prairie plantings have the potential to produce high levels of biofuels on marginal lands receiving low inputs. LIHD biomass production merits serious consideration, as do all other approaches that may increase the sustainability, net energy gains, and environmental benefits of biofuels. David Tilman,1 Jason Hill,1,2 and Clarence Lehman1 1Department of Ecology, Evolution, and Behavior, 2Department of Applied Economics, University of Minnesota, St. Paul, MN 55108, USA. References 1. P. Adler, M. Sanderson, A. Boateng, P. Weiner, H-J. Jung, Agronomy Journal 98, 1518 (2006). 2. D. Tix, I. Charvat, Restoration Ecology 13, 20 (2005). 3. S. Collins, A. Knapp, J. Briggs, J. Blair, E. Steinauer, Science 280, 745 (1998). 4. L. Hulbert, Ecology 50, 874 (1969). 5. B. Glaser, W. Amelung, Global Biogeochem. Cycles 17, 1064 (2003). 6. X. Dai, T. Boutton, B. Glaser, R. Ansley, W. Zech, Soil Biol. Biochem. 37, 1879 (2005). 7. R. Ansley, T. Boutton, J. Skjemstad, Global Biogeochem. Cycles 20, 3006 (2006). 8. Samson et al., Crit. Rev. Plant Sci. 24, 461 (2005). 9. D. Tilman, Oikos 58, 3 (1990). 10. Agricultural Chemical Usage 2005 Field Crops Summary (National Agricultural Statistics Service, U.S. Department of Agriculture, Washington, DC, 2006). 11. M. Koelling, C. Kucera, Ecology 46, 529 (1965). 12. Shortridge, Geogr. Rev. 63, 533 (1973). 13. Jenkinson et al., J. Agric. Sci. 122, 365 (1994). 14. Silvertown, M. Dodd, K. McConway, J. Potts, M. Crawley, Ecology 75, 2430 (1994).
The Experimental Low-Input High Diversity Biofuel System 19 April 2007
Kenneth G. Cassman Department of Agronomy and Horticulture, University of Nebraska Respond to this E-Letter: Re: The Experimental Low-Input High Diversity Biofuel System	In their Report “Carbon-negative biofuels from low-input high-diversity grassland biomass” (8 Dec. 2006, p. 1598), D. Tilman et al. conclude that low-input high-diversity (LIHD) prairie grasslands on degraded soils sequester significant amounts of carbon in soil organic matter and produce more net energy than corn grain ethanol systems. Unfortunately, these conclusions are not defensible given the methods used in their field study. A striking feature of these LIHD systems was an ability to sustain net productivity during a 10-year period without fertilizer inputs on an agriculturally degraded sandy soil. Energy yield was calculated from estimates of aboveground plant biomass taken in August each year. However, less than 3% of the standing biomass was actually removed to make these estimates; the remaining biomass stayed on the field for about 7 months until it was burned off in spring. Under such a regime, most of the phosphorus and nearly all of the potassium, calcium, magnesium, and essential nutrients other than nitrogen and sulfur would be returned to soil in the ash. Even a substantial amount of carbon would be recycled to soil through leaf and stem drop and in situ decomposition during a 7-month weathering period. These nutrient inputs would represent a substantial proportion of available nutrients in a sandy soil because such soils have small nutrient storage capacity, given their large particle size and small total surface area, to hold nutrients. Therefore, it is unlikely that net productivity of the LIHD system could be maintained if all of the aboveground biomass and nutrients it contained were removed as would occur in a real-world LIHD biofuel production system. Comparison of the experimental LIHD biofuel system in the absence of biomass removal to ethanol made from corn under conditions in which the grain and associated nutrients are removed from the field is like comparing apples and oranges. Kenneth G. Cassman Department of Agronomy and Horticulture, University of Nebraska, Lincoln, NE 68583–0724, USA.