How Animal Manure Could Help Reduce Agriculture’s Carbon Footprint

What if we could decrease the negative impacts of climate change through sources derived from animal manure? A system that uses these sustainable resources to reduce agriculture’s reliance on fossil carbon resources and the resultant carbon dioxide emissions is one solution proposed by researchers in the colleges of Natural Resources and Agriculture and Life Sciences at NC State.

This reduction of carbon dioxide emissions, also known as decarbonization, could be done through a system in which organic waste-derived biogas — a mixture of methane and carbon dioxide gases — is converted into dietary protein and ammonia fertilizer. Biogas can be produced from raw bioresources — in this case, animal manure. 

Carbon dioxide emissions from farms are caused in part by fossil-intensive fertilizers and land-use change, both of which are driven partly by increasing demand for dietary protein. This proposed system would produce low-carbon fertilizer and high-quality protein to address these primary drivers of agricultural emissions. 

“We think the impact of this research is to show the potential of using this organic waste for making chemicals and agricultural products that are difficult to decarbonize,” said William Joe Sagues, an assistant professor in the Department of Biological and Agricultural Engineering and a recent P.h.D. graduate from the Department of Forest Biomaterials. 

Sagues, who is interested in alternative protein sources, especially algae, is currently seeking opportunities to further the research that he started as an intern for the Advanced Research Projects Agency-Energy (ARPA-E) in the U.S. Department of Energy. He is working alongside Sunkyu Park, an EJ Woody Rice associate professor and university faculty scholar in the Department of Forest Biomaterials. 

During this internship, Sagues discovered that about 90% of ammonia produced in the United States is used in fertilizer, which helps sustain food production for people worldwide. This piqued Sagues’s curiosity into whether agricultural wastes could be used as feedstock to make ammonia. 

If implemented across the United States, the proposed system would use organic waste from municipal wastewater, landfills, animal manure and commercial operations. This could replace 30% of dietary protein intake and 127% of ammonia use, reducing greenhouse gas emissions, land use and water consumption. There is also the potential to use organic waste feedstocks from pulp and paper and convert it into ammonia and protein using Sagues’ proposed system. 

“The pulp and paper industry produces a lot of waste material that we call paper sludge,” Park explained. “Most of the paper sludge ends up in landfills. Paper sludge typically contains 50-80% carbohydrates, and the rest of it is mostly ash. We need to develop an efficient technology to convert paper sludge into ammonia and other valuable products.”

Challenges facing the decarbonization of agriculture 

As with any innovative solution to address global warming, the process of decarbonizing chemicals like ammonia comes with its challenges. “You can’t simply do it with renewable energy like solar and wind, so there needs to be more creativity in designing integrated systems,” Sagues said.

The biggest challenge with decarbonizing ammonia is that there is only one way to make it inexpensively. “That’s the Haber-Bosch process, which was developed in the early 1900s and sparked the green revolution,” said Sagues. “Without this innovation, without making synthetic ammonia as a synthetic fertilizer, we wouldn’t have almost eight billion people on the planet. Over 40% of the nitrogen in your food is synthetic, made through this process.”

The agricultural industry relies heavily on fossil fuels, making it difficult to make something inexpensive that doesn’t rely on a material other than fossil fuel. Because fossil fuels are so rich in chemical energy, it is easier to transfer that chemical energy from fossil fuel to ammonia. According to Sagues, there needs to be more innovative systems thinking on decarbonizing ammonia in the agriculture industry. 

With alternative protein sources, cost also poses a challenge. But lack of social acceptance is a steeper barrier, according to Sagues. 

“People are so used to eating animal meat or dairy as their protein source,” he said.”With trying to make protein from algae, there’s a social barrier because it doesn’t taste the same and it’s weird to people. The nutrient profiles of algal proteins, theoretically, should be suitable for the human metabolism, but additional long-term evidence demonstrating this is needed.”

Some algae, such as spirulina, consist of up to 70% protein. Each year, approximately 30 million tons of the alternative protein source are produced worldwide. 

Recent data also reveals that only approximately 5% of adults in the U.S. consider themselves vegetarians, following a strict plant-only diet. This percentage has stayed about the same over the past few decades, with 6% of Americans identifying as vegetarians in 1999.

Despite challenges, researchers are hopeful that they can one day create more sustainable agricultural resources to combat climate change. 

According to Sagues, there is only so much land available as people continue to increase the demand for animal-derived protein, which is expected to double by 2050. The amount of land currently used for animal agriculture is substantial, with one-third of land suitable for growing crops being used to grow livestock feed. With increased demand, agricultural costs will increase, and there will be a turning point where alternative protein sources will become more cost-effective. Continued research into the social acceptance and associated health benefits of these alternative proteins, including algal protein, is needed.