Aging of biochar

Aging of Biochar in Soils and Its Implications

Biochar does not remain the same once it enters the soil. From the moment it's applied, it begins to interact with its surroundings—physically, chemically, and biologically. These changes, known as "aging," alter its properties and affect how it performs. Some changes are fast and surface-deep. Others are slow but reach into the internal structure of the material. Understanding how biochar ages is essential to predict its behavior in real-world applications—especially in soils.

What Happens When Biochar Ages?

Aging refers to the set of transformations that occur to biochar after it has been exposed to natural conditions. This includes changes to its surface chemistry, structure, and interactions with nutrients, water, and microbes. These changes are driven by three main processes:

1. Abiotic oxidation – exposure to oxygen, water, and other reactive compounds in the environment, especially right after application.

2. Interactions with soil minerals and organic matter – these can lead to coatings, blockages, or the formation of organo-mineral complexes.

3. Biological activity – microbial colonization and enzymatic processes slowly alter the surface and sometimes internal chemistry of the biochar.

The result is that biochar evolves. Its nutrient retention properties change, its surface becomes more polar and reactive, and it can even gain or lose the ability to interact with certain contaminants or metals.

Physical Changes: From Porous to Coated

Fresh biochar tends to be porous and hydrophobic, especially if it’s been produced at high temperatures. After some time in soil, however, its pores begin to fill with minerals, organic compounds, or microbial biomass. This isn’t always a bad thing. For example, the clogging of pores may reduce leaching or improve nutrient retention, depending on the context.

At the same time, the surface becomes more hydrophilic. This is due to the formation of oxygen-containing groups like carboxylic acids and phenols, especially during the early stages of abiotic oxidation. These changes enhance water interaction and nutrient binding capacity, which may improve plant growth in the short to medium term.

Chemical Aging: Surface Oxidation and Functionalization

One of the most consistent observations across studies is that biochar becomes more oxidized over time. The surface gains oxygen-containing groups, which change its charge, reactivity, and ability to bind cations or anions.

This oxidation can be quite fast—occurring within weeks or months—and is more pronounced in low-temperature biochars that have more reactive carbon sites. Over time, biochars with higher initial aromaticity (made at higher temperatures) also oxidize, though more slowly.

These functional groups influence several properties:

• They increase cation exchange capacity (CEC), making biochar more effective at retaining nutrients like ammonium or potassium.

• They enhance binding of heavy metals and organic pollutants, which can be useful for remediation.

• They may also affect microbial colonization, since surface polarity influences microbial adhesion and biofilm formation.

Interactions with Soil Particles and Organic Matter

As biochar particles remain in soil, they begin to form coatings of mineral or organic material. These coatings can either enhance or block the biochar’s original properties.

For example, calcium and iron from the soil can precipitate on the surface, forming stable complexes. These changes often happen alongside sorption of dissolved organic matter, which can enter the pores and create new microenvironments.

These coatings may reduce accessibility of internal surfaces—but they can also improve stability and integration with the surrounding soil matrix. In some cases, aged biochar becomes less mobile, less likely to leach, and more likely to form part of soil aggregates.

Microbial Effects: Colonization and Biotransformation

Microbes interact with biochar surfaces, especially as they become more oxidized and hydrophilic. Fungi and bacteria can colonize the biochar, use it as a habitat, and—in some cases—modify its chemistry through enzymatic activity.

Microbial aging tends to proceed more slowly than abiotic aging, but over time it can lead to structural changes, especially in lower-temperature biochars with more degradable carbon. However, for most biochars, especially those made above 500°C, microbial decomposition is minimal even over several years.

How Aging Affects Biochar Function

Stability
Biochar is generally stable in soil over long periods, especially when produced at high temperatures. But aging can increase reactivity on the surface while leaving the core structure intact. This means the material can become more functionally active without losing its carbon sequestration value.

Nutrient retention
As surface oxidation increases, so does the ability of biochar to retain nutrients via cation exchange. Aging improves nutrient holding, especially for biochars that were originally low in surface reactivity.

Contaminant binding
Aged biochars can have higher sorption capacity for some heavy metals and organic pollutants due to the formation of specific functional groups. This can improve their performance in water or soil remediation over time.

Microbial activity
Aging creates a more favorable surface for microbial colonization. In some systems, this may promote beneficial microbial communities that contribute to plant health or nutrient cycling.

Hydrophobicity
Most biochars start out water-repellent, especially if produced at high temperatures. Aging tends to reverse this, making the material more water-friendly and improving its integration into soil moisture dynamics.

Takeaways and Uncertainties

Aging tends to follow a recognizable pattern. The most significant transformations—such as surface oxidation and increased wettability—occur in the first few weeks or months after the biochar is applied. Over longer timescales, slower processes driven by mineral interactions and microbial activity continue to modify the material.

In general, aging improves nutrient retention and increases the number of functional groups on the surface, enhancing the biochar’s ability to interact with its environment. At the same time, some pores may become less accessible as coatings or organic matter accumulate. High-temperature biochars remain more structurally resistant to both chemical and biological changes, but even these materials gradually evolve in soil.

The exact trajectory of aging depends heavily on the local soil conditions—factors like moisture, temperature, pH, and biological activity all influence the speed and extent of change. While we understand the main directions in which biochar tends to evolve, predicting its precise behavior in a given context remains challenging. This is due not only to the complexity of soil systems but also to the interplay between the physical, chemical, and biological processes involved in aging.

Still, one thing is clear: biochar is not inert. It is a dynamic material, and in many cases, aging enhances rather than diminishes its beneficial effects.