A study led by researchers from Rice University has found that overfertilizing corn grown for ethanol is bad for the production of cellulosic biofuel as well as for the environment.
In a study published in the ACS journal Environmental Science & Technology, the team found that increases in biomass in response to fertilization are not uniform across biochemical classes (carbohydrate, protein, lipid, lignin) or tissues (leaf and stem, grain, reproductive support). Although corn grain responds vigorously and nonlinearly to high levels of fertilization (25% increase with 202 kg N ha-1), corn residue shows only modest increases in carbohydrate yields in response.
Furthermore, the researchers determined that lignin yields in the residue increase almost twice as much as carbohydrate yields in response to nitrogen, implying that the quality of the residue—what would be the biomass feedstock for use in the production of cellulosic ethanol—declines as more fertilizer is applied. Fertilization also increases the decomposability of corn residue, implying that soil carbon sequestration becomes less efficient with increased fertilizer.
Our results suggest that even when corn is grown for grain, benefits of fertilization decline rapidly after the ecosystem’s N demands are met. Heavy application of fertilizer yields minimal grain benefits and almost no benefits in residue carbohydrates, while degrading the cellulosic ethanol feedstock quality and soil carbon sequestration capacity.—Gallagher et al.
The implicit assumption has always been that the response of plant cellulose to fertilizer is going to be the same as the grain response, but we’ve showed this assumption may not always hold, at least for corn.—Morgan Gallagher
Lignin breaks down slowly via bacterial enzymes, and it is expensive to remove by chemical or mechanical processes that create a bottleneck in cellulosic ethanol production.
The ideal cellulosic ethanol crop has no lignin—except you can’t have a plant without it, because it would fall over. Plants need some lignin to maintain structure. What we want is a low lignin-to-cellulose ratio.—co-author Bill Hockaday
Reducing fertilizer to the minimum serves that purpose. Gallagher also notes that improving the yield of feedstock for cellulosic ethanol leaves more corn for food. The researchers hope their methods can be transferred to other crops grown for ethanol. Gallagher, who recently earned her doctorate at Rice, plans to quantify the effects of nitrogen fertilization on switchgrass.
The National Science Foundation and its Long-Term Ecological Research Program at the Kellogg Biological Station and MSU’s AgBioResearch supported the research.
Morgan E. Gallagher, William C. Hockaday, Caroline A. Masiello, Sieglinde Snapp, Claire P. McSwiney, Jeffrey A. Baldock (2011) Biochemical Suitability of Crop Residues for Cellulosic Ethanol: Disincentives to Nitrogen Fertilization in Corn Agriculture. Environmental Science & Technology 45 (5), 2013-2020 doi: /10.1021/es103252