The Trouble With Biochar: Why Turning Agricultural Waste Into Carbon-Rich Soil Additives Isn't Enough

Agricultural waste can be transformed into carbon-rich soil amendments, but producing biochar with promising chemical properties does not necessarily translate into better crop performance. Research examining agricultural and forestry residues reveals a more complicated relationship between biomass conversion, plant development, fertilizer use and environmental safety, challenging assumptions that waste-derived carbon automatically improves agricultural productivity.

The study, "Evaluation of Biochar from Agricultural and Forestry Residues: A Comparative Study of Olive and Poplar Prunings," published in the journal Energies, was conducted by Monica Carnevale, Alberto Assirelli, Francesco Gallucci, Enrico Paris, Claudio Beni and Enrico Santangelo. Working with olive pruning and poplar biomass from Central Italy, the researchers investigated whether converting locally available residues into biochar could support renewable energy production and sustainable soil management.

Their findings reveal pronounced differences between the two materials. Poplar-derived biochar contained more carbon and produced consistently favourable seed-germination responses, while olive-derived biochar generated a higher production yield but raised concerns about certain metal concentrations. Neither material significantly improved lettuce biomass when applied without additional fertilization.

Combining biochar with organic fertilizer produced approximately 10% more fresh lettuce biomass than fertilization alone, although the increase was not statistically significant.

From Agricultural Residues to Carbon-Rich Products: Why Feedstock Selection Changes Everything

Agricultural and forestry residues represent an underutilized resource in production systems seeking to reduce waste and recover energy. Olive pruning and fast-growing poplar biomass are particularly relevant in Mediterranean landscapes, where substantial quantities of woody material arise from cultivation and forestry management.

Instead of leaving these residues underused or disposing of them through open-field burning, thermochemical conversion offers opportunities to recover energy while producing biochar. Pyrolysis heats biomass under oxygen-limited conditions, generating a carbon-rich solid alongside gaseous and condensable products.

The researchers examined this process using a pilot-scale fixed-bed reactor operating at 700°C. Applying comparable processing conditions to both feedstocks allowed them to assess how differences in the original material influenced the resulting biochar.

The comparison revealed that poplar biochar contained 92% carbon, significantly exceeding the 79.9% recorded for olive-derived biochar. Poplar also produced higher fixed carbon content, while olive biochar contained considerably more ash.

However, carbon concentration was only one part of the production picture. Olive pruning generated a biochar yield of 41.9%, compared with 31.7% for poplar. Consequently, the material producing the more carbon-rich product did not deliver the largest quantity of biochar.

Energy-related outputs also differed. Olive residues generated synthesis gas with a lower heating value of 7.3 megajoules per normal cubic metre, compared with 6.7 for poplar. These differences show how feedstock characteristics influence the distribution and properties of thermochemical products.

The results suggest that evaluating biochar production requires more than measuring carbon concentration. The quantities of recovered material, chemical composition and potential agricultural applications must also be considered when assessing different biomass resources.

A Carbon-Rich Material Is Not Necessarily a Better Soil Amendment

Biochar's agricultural performance depends on interactions between its chemical characteristics, soil conditions and plant species. The researchers examined these relationships through germination tests involving pea, tomato, sorghum and sunflower, followed by lettuce cultivation experiments.

The germination results exposed a substantial difference between the two biochars. Poplar-derived material produced germination indices exceeding 100% across all four tested species, indicating favourable early biological responses and potential phytostimulatory properties under the experimental conditions.

Olive-derived biochar produced less consistent outcomes. Its aqueous extracts showed potentially phytotoxic effects on tomato and sunflower, demonstrating that materials produced under identical thermal conditions can behave differently when exposed to developing plants.

These findings highlight the value of preliminary biological screening. Germination tests offer a relatively rapid method for identifying potentially unfavourable plant responses before more extensive agricultural experiments are undertaken.

However, successful germination does not establish that biochar will increase crop productivity. The lettuce experiments demonstrated the limits of translating favourable laboratory indicators into measurable harvest improvements.

Researchers tested biochar application rates equivalent to 5, 10, 20 and 30 tonnes per hectare. Across the tested treatments, differences in lettuce growth and productive characteristics remained limited.

Average fresh biomass reached 48.1 grams with poplar biochar and 47.8 grams with olive biochar, compared with 47.3 grams in the control soil. These differences were not statistically significant.

The physiological measurements produced a similarly restrained picture. Although some responses varied across application rates, the study did not demonstrate consistent improvements in photosynthetic performance or plant development attributable to biochar alone.

Fertilizer Combinations Reveal Promise, but Environmental Safety Adds Another Constraint

The researchers subsequently examined whether biochar performed differently when combined with organic fertilizer. This experiment compared fertilized soil, biochar-only treatments, untreated soil and combinations of fertilizer with each biochar.

The strongest average lettuce biomass values emerged from the combined treatments. Fresh weight reached 107.88 grams with olive biochar and organic fertilizer and 107.08 grams with poplar biochar and organic fertilizer, compared with 97.53 grams under fertilization alone.

Both combinations therefore produced approximately 10% higher average fresh biomass than fertilizer alone. However, the increases did not reach statistical significance, preventing a firm conclusion that biochar enhanced fertilizer effectiveness.

Photosynthetic measurements showed favourable trends in the combined treatments, including higher values for certain indicators of plant functioning. These differences were also generally not statistically significant, leaving the underlying mechanisms unresolved.

The experiment nevertheless identified an important research direction: biochar may need to be evaluated within complete fertilization systems rather than as an isolated agricultural input. Its interactions with nutrients and soil conditions warrant further investigation.

Chemical safety presented another important consideration. The researchers found that olive-derived biochar contained higher concentrations of zinc, copper and iron, while poplar-derived material contained more calcium and magnesium.

Zinc concentration in olive biochar reached approximately 1,250 milligrams per kilogram, exceeding the 800-milligram-per-kilogram limit considered under European Union fertilizing-product requirements. Copper and zinc concentrations also exceeded relevant European Biochar Certificate reference values for agricultural applications.

These results demonstrate why converting biomass into biochar does not automatically establish agricultural suitability. Mineral elements can become concentrated during processing, creating potential restrictions even when the original material comes from conventional agricultural activities.

Soil carbon dioxide measurements introduced further complexity. Emissions varied across treatments and sampling periods, reflecting changing biological activity and soil conditions. The observations did not establish a consistent reduction in soil respiration attributable to biochar.

Importantly, soil respiration cannot be interpreted as a direct measure of carbon sequestration. The study therefore provides insufficient evidence to establish long-term climate benefits from applying either material.

Beyond the Pilot Plant: What Biochar Needs Before Wider Agricultural Adoption

The research contributes to biochar assessment by connecting pilot-scale production with chemical characterization, germination screening, crop development and soil respiration measurements. This integrated approach provides a broader understanding of how biomass origin influences agricultural suitability.

Its findings also expose the limitations of treating biochar as a uniform product. Carbon concentration, production yield, biological compatibility and regulatory compliance represent separate performance considerations that may not move in the same direction.

For agricultural applications, the results favour feedstock-specific evaluation and careful assessment of fertilizer interactions. Chemical screening is equally important, particularly where elevated metal concentrations could restrict the intended use.

Nevertheless, the experimental evidence remains limited to selected feedstocks, one geographical setting and short-duration cultivation trials. Lettuce responses observed under controlled pot conditions cannot establish how biochar will perform across different crops, soil environments or successive growing seasons.

The researchers also acknowledge missing measurements, including electrical conductivity and additional contaminants. Further investigations are required to establish more comprehensive assessments of agronomic safety.

Long-term field experiments could clarify whether biochar influences soil fertility, nutrient dynamics and plant productivity differently as the material interacts with soil over time. Repeated monitoring would also improve understanding of soil carbon behaviour and biological responses.

The findings provide a basis for exploring integrated energy–agriculture value chains in which biomass residues become potential resources rather than discarded materials. However, the study does not establish commercial viability or demonstrate that pilot-scale results can be reproduced economically at larger production capacities.

The broader significance lies in the need for evidence-based decisions about biomass conversion. Producing a carbon-rich material is technically feasible, but its agricultural value depends on chemical composition, crop responses, fertilization practices and environmental suitability.

Biochar may contribute to more integrated residue management and sustainable farming systems, provided its performance is assessed under relevant conditions. The research eventually shows that the pathway from agricultural waste to productive soil amendment requires more than successful conversion: it requires measurable agronomic benefits, credible safety assessment and long-term validation.