The power of poop: How human waste can boost crop yields and cut greenhouse gas emissions

August 13, 2025 by Paul Arnold, Phys.org

Collected at: https://phys.org/news/2025-08-power-poop-human-boost-crop.html

Human waste usually gets flushed away, but instead of going down the toilet, it can help the planet and solve global fertilizer shortages, according to a new study published in the journal Proceedings of the National Academy of Sciences.

The solution isn’t to spread human excrement directly on fields but rather to make a charcoal-like substance called biochar from it. It’s created by heating biomass, such as sewage, at high temperatures in a low-oxygen environment. This stable form of carbon can be added to soil to improve its quality by enhancing its ability to retain water and nutrients.

Global fertilizer shortage

Currently, the world is facing a global fertilizer shortage, primarily because of supply chain disruptions and rising energy costs. The consequence of this is increased food prices and a threat to food security.

The study claims that biochar made from solid human excrement can provide up to 7% of the phosphorus used around the world every year. And while the current process is for solid waste, nutrients from urine could be added, which would more than double the annual phosphorus application, according to the research.

Powerful tool for fighting climate change?

The study, a meta-analysis of other papers, also showed that long-term use of biochar would produce significant and lasting benefits. When applied for four or more years, it recorded a reduction in greenhouse gas emissions, with methane (CH₄) emissions dropping by 13.5% and nitrous oxide (N₂O) emissions falling by over 21%. Biochar also improved the soil’s health by increasing its organic carbon content by more than 52%. However, a single application of biochar was nowhere near as effective.

“Long-term annual applications sustain and even enhance the benefits on crop yields, GHG (greenhouse gas) mitigation, and SOC (soil organic carbon) sequestration. In contrast, those benefits diminish over time under single applications,” wrote the researchers.

Biochar sounds like an elegant solution, but there is a potential problem. Treated sewage sludge containing human waste is already spread onto farmland, but its use is controversial because it can contain microplastics, toxic waste and forever chemicals. These chemicals are used in products such as cosmetics, baking equipment, food packaging and floor polish, and are so called because of their persistence in the environment. However, the scientists behind the study argue that the biochar process avoids this problem by separating the waste at source.

Ultimately, they believe that this innovative approach to waste management creates a circular economy that transforms a common waste product into a valuable resource.

More information: Johannes Lehmann et al, Biochar in the circular bionutrient economy, Proceedings of the National Academy of Sciences (2025). DOI: 10.1073/pnas.2503668122

Journal information: Proceedings of the National Academy of Sciences