Strap yourselves in for a lot of science in this one!
There is a great need to rethink our current energy paradigms in order to harmonize energy production with ecology. A potentially underutilized solution is to increase our use of an energy source known as "bioenergy,” essentially burning harvested grass for energy. Similar to how coal-fired power plants create energy via the combustion of ancient plant material, harvested biomass can be combusted to produce energy as well. Plant biomass is composed primarily of carbon, and the problem with burning any plant matter for energy, whether ancient or recently harvested, is that carbon dioxide (CO2) is released as a byproduct of the energy created. This subsequently gets trapped in our atmosphere and, because of the chemical structure of the CO2 molecule, absorbs a lot of the earth’s radiation, resulting in a warming of our atmosphere (like a greenhouse, hence “the greenhouse effect”; see box 1 for a greenhouse effect refresher). Critically, there is one crucial difference between combusting bioenergy crops and coal -- the CO2 released from burning the bioenergy plant biomass was recently drawn out of the atmosphere and into the tissues of the plant (carbon in, carbon out). Conversely, the carbon within the coal was extracted by the plants from an ancient atmosphere but is now reentering our modern atmosphere when we burn it (only carbon out). In this way, the growth and combustion of the bioenergy crop can act as a net-zero energy source in theory. In contrast, burning coal has drastically disrupted the energy balance of our atmosphere by pumping more planet-warming, ancient carbon, into it.
Quick energy balance lesson
But this only tells half of the story - the aboveground portion. Belowground, in the rooting zone of the bioenergy grasses, is where things get more interesting. Grasses have the critical ability to be able to transport and sequester large amounts of CO2 from the atmosphere and store it in the soil. As the plant draws CO2 out of the atmosphere for growth, a large portion of that carbon gets pumped into root growth. Roots are critical for acquiring the water and nutrients necessary for plants to grow, so investing large amounts of resources in them is highly beneficial to the plant. And just like the aboveground portion of the grasses seasonally withers and decay, so do parts of the roots. Both the dead above- and belowground materials slowly get incorporated and build up the carbon in the soil. In this way, bioenergy crops can go from a net-zero energy source to actually removing more CO2 from the atmosphere than we burn during the combustion process - net-negative energy!
If this sounds too good to be true, that’s because it currently is. The nutrients that the plants require (primarily nitrogen, but also phosphorus and potassium) are in short supply relative to the demand of an agricultural field, especially in semi-arid systems where soils tend to be less nutrient-rich compared to the central US or Midwest. Because of this, growers supplement their fields with nutrients in the form of fertilizer to increase yields and, ultimately, profits. It is via this process of obtaining and creating the fertilizer that the wheels fall off the net-negative energy bus. Currently, to create fertilizer, nitrogen is extracted from Earth’s atmosphere (nitrogen makes up ~80% of our atmosphere, so there is plenty), but this extraction process (known as the Haber-Bosch process, jointly named after a Nazi, Fritz Haber, and a Nazi-hater, Carl Bosch) is extremely energy intensive. For reference, another way nitrogen can be extracted from the atmosphere is via lightning - so it takes a lot of energy to extract nitrogen out of the atmosphere. Phosphorus and potassium are mined from the earth, which is also not very energy-efficient for obvious reasons. It is because of these energy-intensive reasons that bioenergy technologies cannot yet answer their promise of being net-zero or even net-negative energy solutions.
But what if you didn't need to extract nitrogen from the atmosphere using one of the most energy-consumptive processes on Earth? That is where this important new paper led by Dr. Justin Gay and published in Global Change Biology - Bioenergy comes in. This research examines and systematically tests two critical aspects of the bioenergy process that could increase the efficiency of the process, moving us closer to a net-zero energy solution. First, Gay et al. focused on developing and testing a more sustainable fertilizer type, and second, focused on the type of bioenergy crop (or, species of grass in this case) to see which crop could grow the best, produce the most biomass for energy production, and limit unintended environmental consequences. By pulling on these levers of the bioenergy system, they are able to test the feasibility of bioenergy carbon capture and sequestration, known as BECCS, in a semi-arid farming region.
First, the fertilizer type. The fertilizer tested in this experiment was a novel fertilizer known as cyanobacteria biofertilizer. This fertilizer is derived from nutrient-dense algae that, when grown and mixed into a slurry, can provide the requisite nutrients to support the growth of the grasses. Furthermore, previous work from this group highlighted that cyanobacteria function as a slow-release N-fertilizer compared to commonly used synthetic forms (Goemann et al. 2021) – this is important as it keeps N in the soil longer to better meet plant needs/demands. Thus, not only is cyanobacteria fertilizer less energy-intensive to create as compared to the Harbor-Bosch process but there are also several other beneficial agronomic factors involved with using this natural fertilizer.
Second, the crop type. Dr. Gay et al. compared two different species of grasses; the first, named switchgrass (Panicum virgatum), is a common bioenergy grass that has shown great promise in warmer, wetter areas of the US as a bioenergy source. However, in drier regions of the US, its applicability has not been tested. Second is a grass named tall wheatgrass (Thinopyrum ponticum). Fundamentally, these two plants have different photosynthetic machinery and incorporate CO2 from the atmosphere into their biomass in different ways. Tall wheatgrass uses C3 photosynthesis, named so because the first carbon compound produced during photosynthesis has three carbon atoms. In this process, CO2 enters the plant through pores on the plant’s leaves, where a series of complex reactions and a protein called Rubisco transform the CO2 into biomass and energy for the plant’s growth. Yet C3 plants are victim to two key slowdowns: first, Rubisco can accidentally mistake oxygen molecules for CO2, which costs the plant unnecessary energy. Second, when CO2 enters the pores on the plant’s leaves (known as stomata), precious water escapes from the plant. So, the plant has to balance this CO2-in, H2O-out tradeoff – a critical tradeoff that has governed their success and adaption to different environments.
However, plants have evolved another form of photosynthesis to help reduce these losses, known as C4 photosynthesis, where a four-carbon compound is produced. Switchgrass makes use of this form of photosynthesis. In C4 photosynthesis, CO2 is stored within the plant, allowing the plants to retain water through the ability to continue fixing carbon while stomata are closed. Similarly, this stored CO2 shields Rubisco from oxygen molecules, so excess energy isn’t wasted by Rubisco when it mistakes oxygen for CO2. C4s downside? Given sufficient water, temperature, and sunlight, C3 plants can outproduce C4 plants, but in harsher environments, C4s prevail. Taken together, switchgrass (a C4 plant) is more suitable in warmer climates but potentially suggests that tall wheatgrass (a C3 plant) may be more suited to cooler and drier climates of the northern semi-arid regions.
So, are we on track for a net-negative bioenergy cropping system in a region once thought to be too cold and dry for this form of energy production? The findings from this paper point to exciting and promising results. Gay et al., 2022 show that the cyanobacteria fertilizer led to high growth of both types of grass, no different from the more common, energy-intensive fertilizer. As for the species of grass that did best, the tall wheatgrass grew surprisingly well at the test farm, producing a whopping 98% more biomass than the switchgrass. Furthermore, the results also point to the wheatgrass pumping more carbon into the soils than the switchgrass. Finally, from a climate mitigation standpoint, the cyanobacteria fertilizer did not produce more nitrous oxide when applied to either crop. CO2 isn’t the only earth-warming greenhouse gas; nitrous oxide is a powerful greenhouse gas that is a major contributor to global warming, especially in agricultural systems, which is no laughing matter (nitrous oxide is also used in medical settings and is known as laughing gas). Often nitrous oxide production as a byproduct of crop fertilization is not considered from a climate mitigation perspective – an important tradeoff to determine when making agronomic recommendations. Thus, that Gay et al. do not see higher emissions of nitrous gas in the cyanobacteria-fertilized treatment is a very promising finding.
I have only begun to scratch the surface with the results in this paper. Be sure to check it out for yourself using the links below.
Authors: Justin D. Gay, Hannah M. Goemann, Bryce Currey, Paul C. Stoy, Jesper Riis Christiansen, Perry R. Miller, Benjamin Poulter, Brent M. Payton, E.N. Jack Brookshire
Online link: https://onlinelibrary.wiley.com/doi/full/10.1111/gcbb.13001. Access to PDF here.