I am pleased to introduce Dr. Chandrasekhar Paul to our readers as part of the BiocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More Expert. Dr. Paul is a soil microbiologist and biotechnologist at Hasta Eco, Sweden, whose work sits at the intersection of biochar chemistry, microbial processes, and nutrient transformation. His PhD research at the Czech University of Life Sciences, Prague, focused on understanding how phosphate-solubilising fungi can unlock tightly bound phosphorus in sewage-sludge biochar—an area that contributes directly to advancing circular nutrient use and sustainable fertiliser strategies.

Dr. Paul’s technical strengths include biochar production and physicochemical characterization, nutrient extraction and ICP-OES analysis, soil–biochar–microbe interaction studies, qPCR-based microbial quantification, and plant bioassays assessing biochar–microbe performance. His contributions to international research projects have explored how biochar influences root development, microbial activity, and nutrient cycling across diverse soil systems.

Through his combined experience in laboratory analysis, microbial ecology, and applied biochar research, Dr. Paul offers a distinctive perspective on how biochar can be engineered and biologically enhanced for soil health improvement. It is a pleasure to feature your insights here for our readers.

Shanthi Prabha: Dr. Paul, welcome to Biochar Today. Your work at the intersection of soil microbiology, biochar, and nutrient management is fascinating. Could you start by briefly defining your expertise—what is a Biochar–Microbe–Phosphorus Interaction, and why is this area so vital to the future of sustainable farming?

Dr. Chandrashekhar Paul: Thank you, Dr. Shanthi, for giving me the opportunity to share my thoughts on a respected platform like Biochar Today. I also feel fortunate that my PhD research allowed me to work deeply with biochar, soil microbiology, and plant nutrient dynamics.

In simple terms, the interaction between biochar and microorganisms, especially phosphorus-solubilising microbes, helps unlock phosphorus for plant use. During pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More, most of the phosphorus in the 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 becomes trapped in stable mineral forms within the biochar, making it difficult to solubilise and largely unavailable to plants. By using specific beneficial microorganisms, it is possible to mobilise and release this bound phosphorus.

It is equally important to evaluate how well microbes function on biochar, including their viability, their ability to produce secondary metabolites, and whether they can use biochar as a nutrient source. Additionally, the choice of feedstock and the pyrolysis conditionsThe conditions under which pyrolysis takes place, such as temperature, heating rate, and residence time, can significantly affect the properties of the biochar produced.   More, particularly temperature, play a critical role in shaping how effectively microbes can interact with the biochar. For this reason, selecting the right combination of biochar and microbial strains is essential.

SP: Your PhD focused on getting phosphorus (P) out of sewage sludge biochar. Why is P locked up in this type of biochar, and why is solving this P-availability paradox so crucial for sustainable agriculture?

CP: In sewage sludge biochar, most of the phosphorus becomes locked because the high-temperature pyrolysis process converts it into very stable mineral forms, mainly iron-, aluminium-, and calcium-bound phosphates. Since wastewater treatment commonly uses iron salts for P removal, much of the phosphorus in the sludge ends up as Fe–P, which becomes even less soluble after pyrolysis. These minerals are highly resistant to dissolution, so even though the biochar contains a high total P content, very little is immediately available to plants. Solving this P-availability paradox is crucial because sewage sludge is one of the most significant secondary phosphorus resources, and unlocking its P can reduce dependence on finite phosphate rock. Making this phosphorus plant-available would transform sewage sludge biochar into a safe, circular, and sustainable fertiliser for future agriculture.

SP: You highlight the role of beneficial microorganisms. How exactly do these microbes chemically or physically facilitate the release of tightly bound phosphorus from the biochar structure? What’s the main mechanism at play?

CP: Beneficial microorganisms release phosphorus from biochar mainly through the production of organic acids, enzymes, and chelating compounds that dissolve the stable mineral-P complexes formed during pyrolysis. These microbes lower the pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More in the microenvironment around the biochar, allowing organic acids such as citric, oxalic, and malic acid to break down iron-, aluminium-, and calcium-bound phosphates. They also produce phosphatase and phytase enzymes that mineralise organic P residues still present inside the biochar matrix. Some microbes can chelate metal ions like Fe³⁺ and Al³⁺, weakening the bonds that keep phosphorus immobilised. In addition, the porous surface of biochar provides an ideal habitat for microbial colonisation, allowing them to act directly on the P-rich sites. Overall, the main mechanism is acidification and ligand-mediated dissolution, which converts tightly bound phosphorus into soluble forms that plants can absorb.

SP: Sewage sludge is a challenging feedstock. What specific benefits does a biochar made from sewage sludge offer over, say, wood-based biochar, particularly in terms of nutrient content and waste valorization?

CP: Sewage sludge biochar offers a much higher nutrient content than wood-based biochar, particularly in phosphorus, nitrogen, calcium, magnesium, and trace elements essential for plant growth. Because sewage sludge naturally contains 3–6% total phosphorus, pyrolysis concentrates these nutrients even further, making the resulting biochar a slow-release source of nutrients. It also provides a sustainable solution for waste valorisation by transforming a problematic waste stream, often containing pathogens, organic pollutants, and moisture, into a stable, sanitised, and safe material. Pyrolysis significantly reduces contaminants and eliminates pathogens, allowing nutrients to be recycled rather than lost to landfilling or incineration. In contrast, wood-based biochar is primarily a carbon amendment with very low nutrient levels and limited fertiliser value. Therefore, sewage sludge biochar combines nutrient recycling with environmental safety, turning a disposal challenge into a valuable agricultural resource.

SP: You recently discussed quench water and its cleaning potential. From a soil science perspective, how does the high alkalinity of fresh biochar and quench water impact the soil microbiome and nutrient availability when applied to a field?

CP: Fresh biochar and its quench water are both highly alkaline, and when applied to soil, they can temporarily raise the soil pH, especially in acidic soils. This pH increase can reduce the activity of microbial groups that prefer acidic conditions but often promotes microorganisms that prefer neutral to slightly alkaline conditions, leading to a shift rather than a loss in microbial diversity. The rise in pH can also reduce aluminium and iron toxicity, which indirectly supports microbial activity and root growth. In terms of nutrients, higher pH can increase the availability of some elements, such as phosphorus, in acidic soils, while potentially decreasing the solubility of micronutrientsThese are essential nutrients that plants need in small amounts, kind of like vitamins for humans. They include things like iron, zinc, and copper. Biochar can help hold onto these micronutrients in the soil, making them more available to plants.   More like zinc or manganese. The alkalinity also helps suppress certain soil-borne pathogens, creating a cleaner microbial environment. Overall, the impact is a combination of short-term pH-driven shifts and longer-term improvements in soil biological stability, depending on soil type and application rate.

SP: Your work bridges microbiology and soil science. What is the biggest challenge when scaling up a successful biochar-microbe inoculation strategy from the lab (or rhizobox) to a farmer’s large-scale field application?

CP: The biggest challenge in scaling up a biochar-microbe inoculation strategy is maintaining consistent microbial performance under highly variable field conditions. In the lab, moisture, temperature, pH, and nutrient availability are carefully controlled; however, in a farmer’s field, these factors fluctuate daily and significantly influence microbial survival and activity. Ensuring that the inoculated microbes can successfully colonise biochar particles, compete with native soil microbes, and remain active long enough to release nutrients is often difficult. Another challenge is achieving uniform distribution of both biochar and microbes across large areas, since uneven application can lead to inconsistent crop responses. Storage, transport, and formulation stability of the microbe-biochar product also become critical at scale. Ultimately, translating laboratory success to the field requires optimising both the biological components and the practical application methods so that farmers see reliable, repeatable benefits.

SP: Based on your successful PhD experience, what is one piece of advice you would give to current doctoral researchers looking to contribute meaningful, publishable science in the complex field of biochar interactions?

CP: One key piece of advice is to recognise that biochar-microbe-soil interactions are highly complex, because biochar properties change with feedstock type and pyrolysis conditions, and soil types and microbial communities also vary widely. This means that results can differ greatly from one study to another, so careful experimental design and a clear understanding of previous work are essential. New researchers should invest time in reading deeply across disciplines, such as soil chemistry, microbiology, and biochar production, to understand how these factors interact with one another. It is also important to generate data systematically and consider using advanced tools, including AI-based systems, to organise and compare results so patterns become clearer. The field still requires large, well-structured datasets to identify the optimal combination of biochar and microorganisms for specific soils. My advice is to build on what has already been done, clearly identify the gaps, and design research that connects these pieces in a meaningful and publishable way.

SP: Now working in regenerative agriculture in Sweden, how are you applying your biochar-microbe expertise to help Hasta Eco build soil into a “living carbon sink?”

CP: At Hasta Eco in Sweden, my first step was to thoroughly understand the soil conditions, including both nutrient status and the existing soil microbiome, as our entire system is built on sustainable, zero-waste, and fully organic principles. Since we rely exclusively on natural inputs, treating waste biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More with beneficial microorganisms is essential to return nutrients back to the soil in a plant-available form. We are still in the development phase of optimising biochar-microbe interactions, testing how different biochar types and microbial consortia perform under our local soil and climate conditions. At the same time, we are planning the production of biochar types that can best support organic and bio-based farmers in the region. The broader goal is not only to improve our own fields but to create a chemical-free, nature-based farming system where soil becomes a true living carbon sink. In this way, food production can remain productive while also protecting the environment and enhancing long-term soil health.

SP: As an expert in characterizing biochar, what are the top three most important physicochemical parameters that a farmer or agronomist should look for on a Certificate of Analysis (CoA) to determine if a biochar is suitable for their specific soil type?

CP: When evaluating a biochar, the first parameter a farmer should check on the CoA is the pH, because it strongly influences how the biochar will interact with soil acidity and nutrient availability. The second key parameter is the ashAsh is the non-combustible inorganic residue that remains after organic matter, like wood or biomass, is completely burned. It consists mainly of minerals and is different from biochar, which is produced through incomplete combustion.   Ash Ash is the residue that remains after the complete More content and nutrient composition, particularly elements such as phosphorus, potassium, calcium, and magnesium, which determine whether the biochar contributes nutrients or primarily functions as a structural soil amendmentA soil amendment is any material added to the soil to enhance its physical or chemical properties, improving its suitability for plant growth. Biochar is considered a soil amendment as it can improve soil structure, water retention, nutrient availability, and microbial activity.   More. The third important parameter is surface area and 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, as these properties influence water retention, microbial habitat formation, and the biochar’s ability to enhance soil structure. Depending on the soil type (acidic, nutrient-poor, or sandy), each of these parameters can guide the correct choice of biochar. Farmers should always match the CoA values with their soil test results to ensure compatibility. Selecting the right biochar can significantly enhance soil function, plant growth, and long-term carbon sequestration.

SP:  You used qPCR for quantifying fungal and bacterial communities. How do molecular biology tools like qPCR and Illumina sequencing help us understand biochar’s long-term effect on soil health that we can’t see with traditional chemical tests?

CP: Molecular tools, such as qPCR and Illumina sequencing, enable us to detect changes in the soil microbial community that traditional chemical tests cannot capture. qPCR quantifies the actual abundance of bacterial and fungal groups, showing whether biochar increases or decreases key microbial populations over time. Illumina sequencing takes it a step further by revealing shifts in community composition, including which specific taxa thrive or decline in biochar-amended soils. These methods help us understand long-term ecological effects, such as how biochar influences beneficial microbes, nutrient-cycling organisms, or potential pathogens. Chemical tests only show nutrient levels, but molecular tools show the biological processes driving those nutrients. Together, they provide a complete picture of how biochar reshapes soil health at the microbial level.

SP:  Despite its promise, the adoption of biochar can be slow. In your experience, what is the single biggest misconception or challenge the industry needs to overcome to achieve widespread adoption?

CP: One of the biggest challenges slowing biochar adoption is the misconception that all biochars behave similarly, regardless of their production method. Biochar properties vary widely depending on feedstock and pyrolysis conditions, meaning a biochar that works well in one soil or crop system may perform very differently in another. This variability often leads to inconsistent field results, which can make farmers uncertain about investing in biochar. Another challenge is that many users expect immediate fertiliser-like effects, whereas biochar’s benefits are often long-term and linked to soil structure, microbial activity, and carbon stability. The industry needs clearer standards, better education, and more practical guidelines tailored to soil type and application goals. Once these gaps are addressed, farmers will have greater confidence, and adoption will increase more rapidly.

SP:  A funny one here-If the phosphate-solubilizing microbes could write a negotiation note to the locked-up phosphorus (P) in the biochar, what two-sentence note would they send to convince the P to be released and join the plant?

CP: “If you come out of that mineral cage, we will gently dissolve the bonds holding you and guide you straight to the plant roots, where you’ll finally be useful. Stop hiding in the biochar, your real purpose is to feed the plant, and we’re here to make it happen.”

SP:  To help our audience delve deeper into your work, could you share links to your most relevant publications on biochar and phosphorus release?

CP: I am happy to share a few key publications and my PhD thesis that best represent my work on biochar and phosphorus release.

• Woodchips biochar versus bone char in a one-year model soil incubation experiment: the importance of soil/char pH alteration on nutrient availability in soil
• The role of low molecular weight organic acids in the release of phosphorus from sewage sludge-based biochar

My PhD thesis is freely available online. Those interested in folowing my research can find me on LinkedIn and Research Gate.