Is More Really Less?

Overfertilizing can degrade quality
By Morgan Gallagher | April 18, 2011

Plant lignin content remains a major hurdle for the cellulosic ethanol industry. Pretreatment of feedstock to remove or break down lignin so that enzymes can access the cellulose is the most costly and slowest step in the process of making cellulosic ethanol. In our research at Rice University and Michigan State University, we have found that managing the amount of nitrogen (N) fertilizer applied to a crop, may be another way to improve feedstock quality and potentially speed up the conversion process.

Rather than look at only the total mass yield of either the corn grain or the crop residue, we took a different approach. We measured the mass yield of key biochemicals such as carbohydrate, protein, lignin or lipids, analyzing the change in the corn grain or corn crop residue depending on the amount of N fertilizer applied. This approach would allow farmers to fine-tune their agricultural management techniques to produce a crop optimized for specific purchasers. If corn grain is intended for food use, N fertilizer would be applied to maximize the protein yield. On the other hand, if grain is being sold to be used to make corn grain ethanol, farmers would want to maximize the amount of carbohydrate in the grain. Farmers choosing to sell part of the crop residues for cellulosic ethanol would want to maximize the carbohydrate yield while minimizing the lignin yield.

We looked at the biochemical yields of corn grown under seven N fertilizer rates ranging from 0 to 202 kilograms per hectare (0 to 180 pounds N per acre) at the Kellogg Biological Station-Long Term Ecological Research site in southwest Michigan. In the corn grain, we found that both carbohydrate and protein stocks increased with fertilizer between 67 to 101 kg per ha until these yields plateaued. In other words, adding fertilizer at higher rates higher gave no additional return in carbohydrate or protein yields. In the crop residue, both carbohydrate and lignin yields increased, but at different rates. Carbohydrate yields increased by 21 percent, from the lowest to the highest fertilizer rates, while lignin increased by 40 percent. This implies that fertilizing at higher rates will produce a poorer quality cellulosic ethanol feedstock

Farmers will always grow corn crops with the grain as the primary commodity. Harvesting part of the crop residue may become a secondary commodity, if soils can tolerate the loss of some residue. Our results show that farmers can use less N fertilizer, between 67 to 101 kg per ha, at our Michigan field site to maximize their grain yields, whether they sell the grain to be used for food or fuel, and still produce a corn crop residue that is a better quality with more carbohydrates, less lignin, than if they had applied higher fertilizer rates. 

Even if farmers decide not to sell their residues and leave them in the field to improve soil quality for the next year’s crop, lower fertilizer rates are still beneficial. Crop residues grown at lower fertilizer rates have biochemical compositions that are harder for microbes and bacteria to decompose than if they had been grown at higher fertilizer rates. Lower fertilizer rates therefore produce a crop residue that has a higher potential to sequester carbon in the soils. In other words, the crop residue grown at lower fertilizer rates is less likely to be decomposed, which would release that carbon to the atmosphere as carbon dioxide.

Knowing the appropriate amount of N fertilizer that will maximize crop biochemical yields depending on their intended use has several benefits. First, it allows farmers to be more precise in their fertilizer applications and thereby save money on their fertilizer purchases. Second, not overfertilizing their fields will reduce environmental impacts such as nitrate leaching into groundwater and emissions of greenhouse gases. Third, the ability to produce better quality feedstocks for the biofuel industry may save companies time and money in the conversion process. Understanding how N fertilizer impacts specific biochemicals, such as carbohydrates, lignin and protein, can not only help farmers save money and reduce environmental impacts, but can also help the industries that they support by allowing farmers to produce better quality crops while maintaining their yields. 

Author: Morgan Gallagher
Postdoctoral Researcher, Rice University/Michigan State University
(713) 348-4114
morgan.gallagher@rice.edu