Biochar and its impact on the carbon cycle

[Geoengineering News]

https://www.sciencedirect.com/science/article/pii/S0961953425007767

Authors: David Chiaramonti, Franco Berruti, Johannes Lehmann, Ondrej Masek, Henrik Ingermann Petersen, Manuel Garcia Perez, Hamed Sanei, Francesco Primo Vaccari

https://doi.org/10.1016/j.biombioe.2025.108365

12 September 2025

A critique to biochar-based carbon dioxide removal is that it draws carbon from the biosphere without reducing atmospheric CO2, but just moving this from one to another storage form (reservoir, or pool). This perspective reflects a misunderstanding of ecosystem dynamics, photosynthesis and carbon fluxes. The aim of this work is to provide correct scientific framing on biochar as CO2 removal (CDR) solution, based on scientific evidence, and clarify the underlying carbon-cycle accounting and timescales.

The biosphere, atmosphere, and geosphere are connected through the global carbon cycle. Carbon continuously moves between these reservoirs (or pools) via biological, chemical, and geological processes. The distinction between the geosphere and the biosphere is however not simply a matter of depth. Soils are at the surface, yet they are also part of the geosphere, just as biological activity can occur at depths of 2000 m or more and still be considered part of the biosphere. These spheres are best defined as process-based rather than spatial divisions. For example, when carbon is stored in soil in a form that is no longer easily available for short-term biological activity, it effectively belongs to the geosphere, even though it remains at the surface. In fact, the geosphere begins with soils and sediments at the surface. An entire scientific field of biogeochemistry exists, which deals with biological processes within the geosphere. So, in order to apply an accurate definition, the key element that count is that once carbon becomes inaccessible to biological degradation, it is functionally preserved and enters the geological carbon cycle, regardless of its physical position.

In the biosphere, which comprises living organisms, plant residues, and soil organic matter, organic carbon typically exhibits short residence times, ranging from weeks to decades, with some exception as wood buried in clay [1]. In contrast, pyrogenic carbon stored in the geosphere can persist for centuries to millions of years as proven by Earth's coal reserves. So, even though some of the non-charred/pyrolyzed organic matter could remain for centuries in some very specific circumstances, on average biomass residues such as leaf litter will degrade in a short period of time compared to pyrolyzed biogenic material and its most persistent carbon fraction. Biochar can thus be framed as shifting the probability distribution of residence times: pyrolysis converts a share of biogenic turbostratic C into more condensed aromatic structures (at a more or less disordered level depending on process conditions) and relocates it to reservoirs with negligible or very low near-term reversal effects, increasing expected persistence. Storing well-pyrolyzed biomass in the soil is the only way to offer long-term storage. Moreover, the amount of the durable carbon share in biochar can be determined in various way, meaning it can be predicted from its properties and does not require costly quantification of carbon remaining.

The biochar value chain, like other nature-based pathways, works downstream of photosynthetic uptake to fix CO2 into biomass. Carbonization reactions then converts the organic matter into a more durable form of carbon, slowing down the return of a portion of this carbon to the atmosphere by microbial activity in soil and sediments. In fact, Carbonization reactions creates condensed aromatic structures with slower mineralization than unpyrolyzed biomass. Such structures are similar to those found in coal materials, a material with well documented stability. The biochar value chain is therefore a two-step process, combining thermochemical conversion with the upstream photosynthesis that on its own does not ensure long-term durability of carbon in soil.

As a soil amendment, biochar, if applied properly, has the potential to improve soil health, resilience and productivity, making the whole agronomic system photosynthetically more efficient: in fact, reported agronomic co-benefits (soil health, resilience, productivity) are context-dependent and secondary to the permanence mechanism. A substantial literature documents the agronomic effects of biochar across different types of crops, soils, and management practices (among others: [[2], [3], [4], [5], [6], [7]]). Biochar can also be used as an additive for the formulation of advanced fertilizers. It is however not the main goal of this editorial to focus on the agronomic impacts from biochar addition, but to clarify the role of biochar as a permanent carbon removal system in the global carbon cycle.

The organic carbon in the biosphere amounts to more than 6000 Gt of carbon, while the geosphere (lithosphere) stores more than 15 millions Gt of carbon on a multi-million-year timescale [8,9] At 427 ppm concentration of CO2 in the atmosphere, 3341 Gt of CO2 are present, corresponding to 910 Gt of carbon.

Of course, carbon cannot be removed from one reservoir without affecting others. The global carbon cycle is a closed system on human timescales, and all interventions, whether through photosynthesis, combustion, decay, or sequestration, result in carbon flowing from one pool to another.

What defines carbon dioxide removal is not simply the depletion or accumulation of carbon in one reservoirs (soil or atmosphere), but rather the combination of the direction of the carbon shift (from the atmosphere to the soil reservoir) and the conversion of a large share of this carbon to a long-term durable state, then stored in the receiving pool (soil).

CO2 moved from the atmosphere to the geosphere either through inorganic or organic pathways: both generating long-term removals. The inorganic pathway is driven by geochemical reactions, where CO2 is transformed into dissolved ions and ultimately precipitated and stored permanently in carbonate minerals. In contrast, the organic pathway depends on biological fixation of CO2 via photosynthesis into biomass, followed by selective preservation, organic carbon maturation, and carbonization into stable macerals that persist in the geosphere [10]. In the organic carbon pathway, therefore, atmospheric CO2 is first assimilated into the biosphere through photosynthesis, where it is stored temporarily as biomass. When biomass is subjected to thermal alteration (carbonization), it undergoes progressive dehydration aromatization and condensation of organic structures, the preferential loss of hydrogen and oxygen is in form of H2O, resulting in a enrichment in durable carbon. Through this process, organic matter is transformed from a relatively short-lived, biologically active pool into highly durable macerals that persist on geological timescales. In this framework, the directional transfer of carbon from atmosphere to biosphere, soil and ultimately to geosphere does not merely represent a change in spatial domain, but rather reflects a series of chemical and molecular transformations that progressively increase the mean residence time of carbon.

In the case of biochar and its use in soil, a large share of the biogenic carbon is thus converted to a very durable form via thermochemical conversion at pyrolysis temperatures typically at 550–600 °C [11,12]. Carbon is moved from a short-lived biospheric form to the geosphere time scale, with mean residence times that are significantly higher and re-release minimal in the timeframes considered by current legislations (orders of hundreds years). This redirection represents a shift in carbon residence time and enhances long-term climate change mitigation.

There is actually no danger of a sudden carbon re-release for biochar: the carbon fraction in biochar that is less or more persistent can be well predicted and measured. Therefore, in BCR (Biochar Carbon Removal), there is no real risk of unexpected and sudden re-relase, but rather the need to ensure an accurate measurement, rigorous characterization, and tracing. Several studies have shown that the dominating factor for forming an inert, condensed polyaromatic carbon structure with very high permanence is maximum carbonization temperature (e.g., Refs. [[13], [14], [15], [16]]. In this way the carbon is moved from a short-lived biospheric form to the geosphere time scales, with mean residence times that are significantly higher and risk of re-release minimal over the timeframes considered by legislations (in the order of hundreds of years).

Most scientific studies showing significant degradation over short timeframes have used insufficiently carbonized biochar (<550 °C; see decay data in Ref. [17] or the applied decay models have been incorrectly parameterized [12]. Long-term field experiments also confirmed the unaltered presence of biochar most permanent fractions even under f15 years of full tillage and conventional farming [18]. There is abundant archaeological evidence of biomass-derived carbon materials in soils for thousands of years.

In conclusion, in sustainable systems, harvested biomass is replaced through regrowth, maintaining the biosphere's capacity to absorb atmospheric carbon. By thermochemically converting a fraction of this biomass into biochar and storing it in soils or other stable environments, a portion of the carbon is diverted away from the atmosphere via the combined action of photosynthesis and pyrolysis, and transferred into the long-term geosphere pool. This constitutes a net atmospheric carbon removal.

When implemented in combination with sustainable land management, biochar does not diminish the biosphere's role as a carbon sink but instead creates an additional and long-term storage pathway, a genuine net reduction of atmospheric CO2.

To fight global warming, it is critical to take full advantage of Earth's green engine (photosynthesis) working in parallel with advanced carbonization technologies.

Source: ScienceDirect