How fertilizer may be accelerating global climate change and how a new process may help slow it down

Part 2: Where else can fertilizer come from?

In the first part of this series, the need for an alternative to synthetic fertilizer was described. In this second part, I discuss possible alternatives to the Haber-Bosch process which has outlived its usefulness. The Haber-Bosch process made sense in the days of cheap energy but those times have passed.

Organic fertilizers or so-called biofertilizers have long been proposed as alternatives to synthetic fertilizers. These include the bat guano mentioned in Part 1. Other alternatives that have been much studied include the legumes (soybeans, peanuts, etc.) and the Azolla fern — plants that can “fix” nitrogen from the air. These plants can fix nitrogen because they have a unique, symbiotic relationship with certain kinds of bacteria, which are the actual nitrogen “fixers.” The legumes serve as a host to these bacteria, which then take the nitrogen from the air and convert them into soluble nitrogen compounds which are then made valuable nutrients available to the host. This is a classic example of a symbiotic relationship.

It makes sense therefore to propose that crop rotation or intercropping with these legumes might have beneficial effects on the soil. Indeed, an ancient practice in China is to grow the Azolla fern between seasons and plow it into the soil, thus providing a natural fertilizer for rice. For various reasons, however, these seemingly common-sense sustainable strategies have not been widely adopted. Conspiracy theories aside, perhaps it really is too difficult to use organic fertilizers on a large scale. 

Is there another way perhaps to obtain the needed nitrogen fertilizers from the air? Perhaps the answer lies with the microorganisms that give the legumes their ability to fix nitrogen. “Blue-green algae” (more properly, cyanobacteria) are among the earliest living things to appear on earth and are largely credited for creating the present form of the earth’s atmosphere. Since cyanobacteria are photosynthetic, they produce their own food from the sun and produce their own fertilizer. They have long been proposed as a source of fertilizer but harvested and dried directly. 

What if, instead of using the cyanobacteria in this crude form, nitrogen-based compounds are obtained directly from the cyanobacteria while simultaneously using the rest of the cyanobacteria for fuel? The results from applying this idea to a specific cyanobacteria and nitrogen compound (ammonium sulfate) using the tools of life-cycle assessment are dramatic: nearly 90 percent reduction in non-renewable energy usage and nearly 60 percent reduction in global warming impact. The results from the study have been published on pages 339-346 of Vol. 107 of the Elsevier journal, Bioresource Technology. While the results are from theoretical calculations only, the assessment puts together technologies that are already existing. No technology breakthroughs are necessary.

There were several principal ideas that were explored and elucidated in this study. The first is that cyanobacteria can be used as the feedstock for nitrogenous bulk chemicals. The second is that the algal biomass products do not need to be energy products in order to have an impact on the energy problem. They may also be substitutes for energy-intensive chemicals like ammonia and its products. Thus the microalgae provide a means for saving energy, in addition to supplying fuels which provide energy. The third idea is that significant energy usage and greenhouse gas reductions are achieved because more than one product is made from the microalgae: biogas and ammonium sulfate. An added bonus: ammonium sulfate is not explosive unlike the more common forms of fertilizer like ammonium nitrate.

Some caution may be appropriate. As with any sunlight-based energy source, there may be a trade-off in land use. A much larger land area would be necessary in comparison to the Haber-Bosch process. Likewise, those familiar with the fertilizer problem may note that this study actually addresses only half the problem. Besides the environmental impact of making the fertilizer, there is the actual impact from using the fertilizer. However, as noted earlier, there may not be many viable large-scale alternatives to using synthetic chemicals. Until these are found, a more environmentally friendly solution might be the ones described in this study.

As the idea for a biorefinery starts taking root, it is likely that the process we studied may be an important component of such a refinery.

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Dr. Luis F. Razon is a full professor of Chemical Engineering in De La Salle University. He graduated from De La Salle University in 1980 with a B.S. in Chemical Engineering (magna cum laude). He obtained his M.S. and Ph.D. in Chemical Engineering from the University of Notre Dame in Notre Dame, Indiana under the direction of Prof. Roger Schmitz in 1985. Prior to returning to the academe in 2001, he worked in the food industry for 14 years. He has performed research on a variety of topics, primarily chemical reactor stability and dynamics and, over the past four years, biofuels and life cycle assessment. He is the author or co-author of five out of the18 Scopus-listed publications from the Philippines about biodiesel.