Biochar (biochar) is defined as a solid, carbon-rich product resulting from the process pyrolysis – thermochemical decomposition of biomass with limited or completely excluded access to oxygen.

1. Why are we studying it?

In current environmental research, biochar represents a key tool for carbon sequestration (C). Its application to the soil enables long-term storage of atmospheric CO₂ in a stable form, thereby directly contributing to climate change mitigation.

An important fact for environmental practice is that biochar is not a uniform material. Its physicochemical properties are the result of the interaction between feedstock a maximum processing temperature (HTT). These parameters determine the stability of biochar in the soil and its ability to affect the original organic matter. To understand this effect, we must first look at the molecular metamorphosis that biomass undergoes during thermal treatment.

2. Molecular Metamorphosis: How Temperature Changes the „Building Blocks“

During pyrolysis, progressive aromatization of carbon chains occurs. With increasing temperature, oxygen functional groups and hydrogen are released, which leads to the formation of stable aromatic nuclei. In the scientific literature, we encounter two slightly different temperature ranges (e.g. 400/600/800 °C for molecular modeling vs. 350/550/750 °C for incubation experiments). For didactic purposes, we understand these values as representatives of low, medium and high intensity thermal processing.

Synthesis and insight: The key trend is an increase in stability (recalcitrance). Higher temperature leads to an increase in sizes of aromatic domains, which are the basic "building blocks" of biochar with island-like architecture. An important detail is the presence of non-hexagonal rings (five- and seven-membered). These introduce into graphene-like layers curvature. This curvature prevents perfect layering (graphitization), which explains why biochar is amorphous and has a lower density than pure graphite. It is this complex, porous architecture that is responsible for biochar's interactions with soil organic matter.

3. The „Priming Effect“ Phenomenon: Why Does Soil Respond to Biochar?

Adding biochar to soil induces priming effect (PE) – a change in the rate of mineralization of the original soil organic matter (SOM). The mechanism of this phenomenon occurs in two time phases:

1. Early phase (Positive priming): Short-term acceleration of the decomposition of the original SOM. It is driven by the release biochemically available substrates (labile DOC) and volatile substances that stimulate microbial activity.
2. Later phase (Negative priming): Long-term slowing of the decomposition of the original SOM. Biochar acts as a stabilizer here.

Sequence of processes leading to sequestration:

1. Biochar application: A stable aromatic matrix with high specific surface area (SSA) and porosity.
2. Microbial activation: Short-term increase in CO₂ due to metabolization of labile biochar components.
3. Physical stabilization (Adsorption): The original organic matter (SOM) molecules are adsorbed onto the surface of the biochar and penetrate its micropores. This „trapment“ makes them inaccessible to the extracellular enzymes of microorganisms.
4. Formation of organo-mineral complexes: The interaction of biochar, minerals, and SOM leads to the formation of stable aggregates.

However, the intensity of these processes is determined by the genetic makeup of the original biomass.

4. Battle of the Trees: Schima superba vs. Cunninghamia lanceolata

To distinguish CO₂ originating from biochar from CO₂ from soil, the experiments use a method isotopic labeling (13C). Comparison of deciduous trees (S. superba) and conifer (C. lanceolata) brings the following insights:

Synthesis and insight: Biochar from C. lanceolata Biochar exhibits better sequestration properties because its low content of labile carbon and nutrients does not support excessive microbial activity. Biochar from S. superba on the contrary, it is more susceptible to decomposition due to its higher nitrogen content, which can paradoxically lead to the loss of soil carbon in the initial phase.

5. Microbial life under the influence of biochar

We monitor the reaction of microbial biomass using phospholipid fatty acids (PLFA). Research shows that while pyrolysis temperature (HTT) significantly changes the total biomass, it has an even stronger effect on the community structure than the type of wood used.

Key microbial reactions:

• Inhibition at low temperatures (350 °C): Biochar produced at low temperatures often reduces the total PLFA biomass. This is due to the presence of toxic residues (phenols, dioxins, furans) from incomplete pyrolysis.
• Structural shift: HTT determines the proportion of Gram-positive (G+) vs. Gram-negative (G-) bacteria and fungi. G+ bacteria often appear to be more resistant to physicochemical changes in pH and salinity after biochar application.
• Mushroom sensitivity: Fungi and actinomycetes respond sensitively to changes in soil micro-architecture and the availability of nutrients bound in biochar.

The general trend is that medium and high pyrolysis temperatures (above 500°C) create an environment that is less stressful for microbial communities than „labile“ low-temperature biochars.

6. Recipe for effective sequestration

In the search for optimal biochar, we must balance carbon stability and the prosperity of the soil ecosystem.

Golden mean: Temperature 550 °C Temperature around 550°C represents a technological and ecological optimum. At this temperature, biochar reaches the high aromaticity and specific surface area required for negative priming, while avoiding the toxicity of low-temperature products (350 °C). Although biochars produced at 750–800 °C are even more stable, the energy costs of their production and only a marginal increase in negative priming make them 550°C the most effective option for managing soil carbon storage. JRi&CO2AI