What if the key to feeding the world didn't come from a factory, but from a wastewater treatment plant?
In a groundbreaking leap for sustainable agriculture, two leading Chinese research teams have turned one of the most overlooked waste streams, sewage sludge, into a powerful new resource for farming. Not just fertilizer, but precision-engineered fertilizer.
Published on September 17, 2025, in the open-access journal Carbon Research, this innovative study reveals how modified hydrochar, a carbon-rich material made from treated sewage sludge, can be fine-tuned to deliver phosphorus (P) to crops in a smarter, more sustainable way. And the secret lies in a simple choice: calcium or magnesium.
Led by Dr. Wei Guo from the National Engineering Laboratory for Advanced Municipal Wastewater Treatment and Reuse Technology at Beijing University of Technology and Dr. Xiaohui Liu from the Key Laboratory of Marine Environment and Ecology at Ocean University of China, this research bridges environmental engineering, soil science, and microbiology to tackle one of agriculture's biggest challenges: phosphorus scarcity.
The Phosphorus Problem—And a Waste-Based Solution
Phosphorus is essential for plant growth. But global reserves of phosphate rock, the source of most commercial fertilizers—are dwindling, and excessive use leads to pollution and eutrophication. Meanwhile, sewage sludge, rich in organic matter and nutrients like phosphorus, is often discarded or incinerated.
This study flips the script: instead of waste, it's raw material.
By treating sewage sludge at 260°C for 2 hours in water (a process called hydrothermal carbonization), the team created hydrochar—a stable, soil-enhancing material. But they didn't stop there. They boosted its performance by adding calcium (Ca) or magnesium (Mg) salts, CaO, CaCl₂, MgO, and MgCl₂, before processing.
The result? Two distinct types of hydrochar, each with a unique phosphorus personality.
Calcium vs. Magnesium: The Great Phosphorus Divide
- Calcium-based hydrochars formed slow-release phosphorus compounds like hydroxyapatite and chlorapatite—rock-stable minerals that lock in P for the long haul. These compounds increased by 48.6% to 86.3% compared to untreated sludge, making Ca-hydrochars a long-term reservoir of phosphorus, perfect for building soil fertility over years.
- Magnesium-based hydrochars, especially those with MgO, produced easily soluble phosphorus forms like Mg₃(PO₄)₂. Though their total P increase was lower (0–50.7%), they released nutrients rapidly—ideal for giving crops a quick boost.
"This isn't just recycling, it's reprogramming," says Dr. Wei Guo of Beijing University of Technology. "We're not just returning phosphorus to the soil. We're controlling how and when it becomes available."
Plants, Microbes, and Real-World Results
To test their hydrochars, the team grew mung beans (Vigna radiata) in pot experiments and used the advanced DGT (Diffusive Gradients in Thin-films) technique to measure bioavailable phosphorus in real time.
The findings were clear:
- Mg-modified hydrochars, especially MgO, led to faster plant growth, higher chlorophyll levels, and better photosynthesis. Mung beans grown with Mg-hydrochar absorbed more phosphorus—proof that the nutrients were not just present, but usable.
- Ca-modified hydrochars didn't push rapid growth, but they transformed the soil microbiome. They enriched bacteria like Skermanella and RB41, known for breaking down organic matter and cycling minerals—helping unlock phosphorus over time.
- In contrast, Mg-hydrochars boosted P-solubilizing powerhouses like Pseudomonas and Bacillus, which actively dissolve phosphorus into plant-usable forms.
"It's a beautiful synergy," explains Dr. Xiaohui Liu from Ocean University of China. "The hydrochar doesn't just feed the plant, it feeds the soil microbiome, which in turn feeds the plant. We're seeing a whole ecosystem response."
A Smart Strategy for Sustainable Farming
This study doesn't just offer a new fertilizer, it offers a strategy:
- Use Ca-based hydrochars for long-term soil restoration, carbon sequestration, and slow nutrient release.
- Use Mg-based hydrochars when crops need a fast start—like in degraded soils or early growth stages.
It's a dual-path approach to phosphorus management, born from waste, guided by science, and ready for real-world impact.
Beijing University of Technology and Ocean University of China: Leading the Green Revolution
At the heart of this innovation are two of China's top environmental research institutions.
- The National Engineering Laboratory for Advanced Municipal Wastewater Treatment and Reuse Technology at Beijing University of Technology is pioneering the transformation of urban waste into high-value resources.
- The Key Laboratory of Marine Environment and Ecology at Ocean University of China is expanding the frontier of nutrient cycling and soil-microbe interactions in coastal and agricultural systems.
Together, their collaboration shows how interdisciplinary science can turn pollution into productivity.
The Future of Fertilizer is Circular—and Smart
So next time you flush, think beyond waste. Think resource. Think phosphorus cycling. Think soil health.
Thanks to visionary scientists like Dr. Wei Guo and Dr. Xiaohui Liu, and their teams at Beijing University of Technology and Ocean University of China, we're not just closing the loop on waste, we're opening a new chapter in sustainable agriculture.
One hydrochar pellet at a time, we're building a world where nothing is wasted, and everything has purpose.
Stay tuned for more breakthroughs from these dynamic research groups, where sewage becomes soil gold, and science grows a greener future.
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- Title: Soil–plant-microbial evidence for the available phosphorus generation and utilization of Ca/Mg salts conditioned hydrochar from sewage sludge
- Keywords: Sewage sludge; Hydrothermal carbonization; Calcium/magnesium salts; Phosphorus species; Plant growth; Microbial community
- Citation: Zhao, Q., Guo, W., Zhu, Y. et al. Soil–plant-microbial evidence for the available phosphorus generation and utilization of Ca/Mg salts conditioned hydrochar from sewage sludge. Carbon Res. 4, 64 (2025). https://doi.org/10.1007/s44246-025-00228-2
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About Carbon Research
The journal Carbon Research is an international multidisciplinary platform for communicating advances in fundamental and applied research on natural and engineered carbonaceous materials that are associated with ecological and environmental functions, energy generation, and global change. It is a fully Open Access (OA) journal and the Article Publishing Charges (APC) are waived until Dec 31, 2025. It is dedicated to serving as an innovative, efficient and professional platform for researchers in the field of carbon functions around the world to deliver findings from this rapidly expanding field of science. The journal is currently indexed by Scopus and Ei Compendex, and as of June 2025, the dynamic CiteScore value is 15.4.
