What if the key to revolutionizing global agriculture lies not in conventional factories, but within the untapped potential of wastewater treatment plants? This provocative question forms the cornerstone of an innovative study by two leading Chinese research groups, who have transformed sewage sludge—a ubiquitous, often discarded byproduct of urban waste—into a precision-engineered fertilizer with unparalleled potential for sustainable farming.
Published in the esteemed open-access journal Carbon Research on September 17, 2025, this groundbreaking research explores how hydrochar, a carbon-rich material derived from hydrothermal carbonization of sewage sludge, can be chemically modified to optimize phosphorus availability to plants. Phosphorus—the critical nutrient underpinning healthy plant growth—remains one of the most challenging elements in agricultural management. Global reserves of phosphate rock, the primary source for conventional fertilizers, are depleting rapidly, while inefficient phosphorus application contributes to environmental degradation via eutrophication. The novel approach presented shifts focus: phosphorus is no longer merely a fertilizer supplement, but a carefully controlled nutrient delivery system engineered at the molecular level.
Hydrothermal carbonization, conducted by heating sewage sludge to 260°C for two hours in an aqueous environment, produces hydrochar—a stable, carbon-dense solid with soil amending properties. The revelation in this research lies in the strategic conditioning of these hydrochars with divalent salts of calcium or magnesium prior to the carbonization process. By incorporating calcium oxide (CaO), calcium chloride (CaCl₂), magnesium oxide (MgO), or magnesium chloride (MgCl₂), researchers have effectively ‘reprogrammed’ the phosphorus forms within the hydrochar, creating two distinct phosphorus profiles tailored for different agricultural needs.
Calcium modification encourages the formation of slow-release, highly crystalline phosphate minerals, predominantly hydroxyapatite and chlorapatite. These mineral phases act as phosphorus reservoirs, releasing nutrients gradually into the soil environment and thereby supporting sustained soil fertility. Quantitative analyses indicated that these minerals increased substantially, by approximately 48.6% to 86.3%, relative to untreated sludge. This slow nutrient release paradigm facilitates long-term soil restoration and carbon sequestration, simultaneously addressing nutrient cycling and climate resilience.
Conversely, magnesium-conditioned hydrochars, particularly those prepared with MgO, show a propensity for generating rapidly soluble phosphorus forms such as Mg₃(PO₄)₂. Though the total increase in phosphorus content ranges from 0 to 50.7%, the bioavailability of this phosphorus markedly enhances, providing plants with a swift nutrient boost. This trait is especially advantageous during initial crop growth phases or in nutrient-depleted soils, where immediate phosphorus accessibility directly translates to improved photosynthetic efficiency and biomass accumulation.
The precision of these phosphorus delivery systems was demonstrated through meticulous pot experiments with mung beans (Vigna radiata). Utilizing the advanced Diffusive Gradients in Thin-films (DGT) technique allowed the real-time assessment of bioavailable phosphorus dynamics in soil-plant interfaces. Hydrochars modified with magnesium salts notably accelerated plant growth metrics, including chlorophyll concentration and photosynthetic rate, underscoring the immediate utility of the soluble phosphorus released.
Intriguingly, the influence of these hydrochar modifications extends beyond nutrient availability to reshape the soil microbial community. Calcium-based hydrochars fostered the enrichment of bacterial taxa such as Skermanella and RB41, genera known for their roles in organic matter degradation and mineral nutrient cycling. These microbial shifts underpin a longer-term enhancement of phosphorus mobilization from soil organic pools. Meanwhile, magnesium hydrochars selectively augmented populations of phosphorus solubilizing bacteria like Pseudomonas and Bacillus, further reinforcing the fast-release nutrient effect through increased biological mediation.
This dual-path strategy for phosphorus management heralds a paradigm shift in sustainable agriculture. Instead of a one-size-fits-all fertilizer product, the nuanced application of calcium or magnesium hydrochars allows precise tailoring of fertilizer regimes to crop developmental stages and soil health status. Employing calcium-based hydrochars aligns with goals of soil ecological restoration and carbon storage, delivering phosphorus gradually for extended fertility. Alternatively, magnesium-enriched hydrochars serve immediate crop nutrient demands, providing a timely and biologically supported phosphorus pulse.
This research exemplifies the transformative potential of interdisciplinary collaboration, bridging environmental engineering, soil chemistry, and microbial ecology. The National Engineering Laboratory for Advanced Municipal Wastewater Treatment and Reuse Technology at Beijing University of Technology, alongside the Key Laboratory of Marine Environment and Ecology at Ocean University of China, synergize expertise to convert waste into a resource of immense agricultural value. This work not only closes the nutrient loop but also creates a blueprint for integrated circular economy strategies in agronomy.
As Dr. Wei Guo of Beijing University of Technology aptly summarizes, “We are not merely recycling phosphorus; we are redesigning its bioavailability and synchronizing it with plant life cycles.” Meanwhile, Dr. Xiaohui Liu from Ocean University of China highlights the soil microbiome’s central role: “This system orchestrates a symbiotic relationship between soil microbes and plants, amplifying the bioavailable phosphorus in a self-sustaining manner.”
Looking beyond the scientific intricacies, the implications for global food security and environmental health are profound. With phosphate rock reserves declining and environmental concerns mounting, transforming sewage sludge into smart fertilizers signifies an ingenious and ecologically responsible solution. It leverages an abundant waste stream to reduce dependency on finite mineral resources and minimizes damaging runoff effects associated with traditional fertilizers.
This novel approach suggests a future where agriculture operates within natural biogeochemical cycles, enhanced by advanced chemical engineering and microbial ecology insights. In this emerging framework, fertilizer production is decentralized, waste valorization becomes standard practice, and nutrient management is adaptive and finely tuned to ecosystem dynamics.
With ongoing advancements, the vision of sustainable, circular agriculture grows more tangible. The pioneering work of these Chinese research teams paves the way for further development and widespread adoption, promising large-scale agricultural productivity gains coupled with responsible environmental stewardship.
In summary, these calcium and magnesium-modified hydrochars redefine phosphorus fertilization. They offer a smart, multifaceted tool for farmers, environmentalists, and scientists seeking a world where agricultural inputs are efficient, sustainable, and integrated within broader ecological cycles. By literally turning sewage into soil gold, this innovation exemplifies how science can propel a greener, more resilient future one pellet at a time.
Subject of Research: Not applicable
Article Title: Soil–plant-microbial evidence for the available phosphorus generation and utilization of Ca/Mg salts conditioned hydrochar from sewage sludge
News Publication Date: 17-Sep-2025
Web References:
References:
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).
Image Credits: Qian Zhao, Wei Guo, Yuhan Zhu, Dongyue Li, Xiaohui Liu, Minda Yu, Dongyang Li, Xiang Gao, Xishi Tai & Jun Li
Keywords
Sewage sludge; Hydrothermal carbonization; Calcium/magnesium salts; Phosphorus species; Plant growth; Microbial community
