The world generates vast amounts of wastewater, much of which is rich in valuable nutrients, such as nitrogen (N) and phosphorus (P), derived from human urine. Instead of treating it as a polluting waste product, agricultural scientists are looking for ways to capture and recycle these nutrients. A new study published in Scientific Reports by Samukelisiwe P. Vilakazi, Pardon Muchaonyerwa, and Alfred O. Odindo investigated one such recycling solution, using readily available pine bark (PB) materials to create a solid, stable fertilizer from human urine. Their work focused on understanding how pine bark feedstockFeedstock refers to the raw organic material used to produce biochar. This can include a wide range of materials, such as wood chips, agricultural residues, and animal manure. More, biochar (pyrolyzed at 350°C and 650°C), and ash influence the crucial step of nutrient release in soil.

The goal was to mix urine with a solid medium, dehydrate it to remove water, and produce a nutrient-dense, easily transportable fertilizer. The team created various urine-based products by mixing urine with these PB derivatives and dehydrating them at temperatures of 45∘C and 60∘C, or under glasshouse conditions (G). Once produced, the materials were incubated with soil at a standard N application rate of 100 kg N ha−1 for spinach to track how the nitrogen and phosphorus became available over 56 days.

The results showed that the type of solid material used dramatically influenced nitrogen retention and release. For nitrogen recovery—the measure of how much N was captured from the urine before it was applied—the products made with biochar pyrolyzed at the highest temperature (650°C) were the most successful, showing over 100% recovery. This impressive recovery is likely due to the high-temperature biochar’s superior surface area, porosityPorosity of biochar is a key factor in its effectiveness as a soil amendment and its ability to retain water and nutrients. Biochar’s porosity is influenced by feedstock type and pyrolysis temperature, and it plays a crucial role in microbial activity and overall soil health. Biochar More, and aromaticity, which create more internal pores and functional groups to adsorb and hold the N-rich urea product.

However, when these products were added to soil, the materials with the highest recovery didn’t necessarily release the nitrogen fastest. Nitrogen is released in two key forms: ammonium-N (NH⁴⁺) and nitrate-N (NO³⁻). All treatments showed a sharp rise in ammonium-N initially (within the first seven days) due to urea hydrolysis (the breakdown of urea by moisture), followed by a rapid decline as nitrification converted the ammonium-N to nitrate-N.

The urine-based product made with ash provided the highest amount of plant-available nitrogen. The treatments using ash dehydrated at 45°C (ash 45) and ash dehydrated under glasshouse conditions (ash G) maintained the highest levels of nitrate-N. The ash G treatment reached a maximum nitrate-N concentration of 38.7 mg kg⁻¹ in the soil. This concentration is significant because it could supply an equivalent of 92.9 kg N ha⁻¹ for the crop. Given that spinach requires about 100 kg N ha⁻¹, this ash product effectively provides almost all the necessary nitrogen, suggesting it can significantly reduce the need for synthetic nitrogen fertilizers. This effectiveness is likely because ash does not adsorb the N like biochar does, allowing the urea to remain free and readily dissolve and hydrolyze once applied to the soil.

In contrast, the biochar products, while excellent at capturing N, had significantly lower soil nitrate-N levels than the ash and feedstock treatments. The scientists suggest that biochar’s stable carbon structures may have locked the N in a form less accessible to the microorganisms that break it down, slowing its release.

For phosphorus (P), a critical plant nutrient, the pattern was more complex. Extractable P generally decreased initially before increasing toward the end of the 56-day incubation. This initial drop is likely due to P fixation to acidic sites in the soil, which was exacerbated by the temporary drop in pH caused by the nitrification process.

In conclusion, this research highlights a potent, sustainable strategy for nutrient recycling. While high-temperature biochar excels at N recovery, the ash provides the most readily available nitrogen to the soil, nearly fulfilling the N requirements for crops like spinach. The findings suggest that by using local pine bark waste, farmers in South Africa and beyond can transform a significant environmental pollutant into a valuable, organic fertilizer, creating a blueprint for more resource-efficient agriculture. Future work must now test these urine-based products under real-world field conditions to fully understand their potential.

Source: Vilakazi, S. P., Muchaonyerwa, P., & Odindo, A. O. (2025). Nitrogen and phosphorus release from dehydration products of urine mixed with pine bark feedstock, biochar types and ash. Scientific Reports, 15, 39508.