Biochar transport in terrestrial ecosystems

Biochar Transport in Terrestrial Ecosystems: Fate and Impact

Once biochar is applied to soil, it doesn’t just sit there unchanged. It moves—vertically, horizontally, and even in some cases, out of the ecosystem entirely. Understanding how biochar is transported within and across terrestrial systems helps us evaluate its long-term stability, environmental safety, and effectiveness in carbon sequestration or soil improvement. In this chapter, we focus on what is currently known—beyond reasonable doubt—about how biochar moves, where it ends up, and what that means for applications in the field.

How Biochar Moves

The physical movement of biochar in terrestrial environments is governed by a few primary pathways:

1. Vertical transport within soil profiles

2. Lateral transport over land

3. Losses to aquatic systems via runoff or erosion

4. Redistribution by biotic activity (mainly soil organisms and plant roots)

Each of these processes depends on the characteristics of the biochar (e.g., particle size, density, hydrophobicity), the properties of the soil (e.g., texture, structure, water content), and external forces such as rainfall, tillage, or wind.

Vertical Transport: How Deep Does Biochar Go?

Biochar can migrate downward in the soil column over time. This movement is generally slow and occurs through two mechanisms: mechanical mixing (e.g., by tillage, root growth, or bioturbation) and infiltration-driven movement during heavy rain or irrigation events.

In the absence of disturbance, vertical transport is typically limited. Studies in undisturbed soils show that most biochar remains within the top 10 cm of soil even after several years. However, in sandy soils or under high rainfall conditions, finer biochar particles can move deeper. For example, field data indicate that biochar may reach depths of 20–30 cm within a year under intense leaching conditions, particularly when particle size is small (\<0.5 mm) and soils are coarse-textured.

The density and hydrophobicity of the biochar also matter. Low-density, hydrophobic particles are more mobile in the early stages after application but tend to stabilize once they age and become more hydrophilic.

Lateral Transport and Erosion

Surface runoff and erosion can carry biochar particles laterally across the landscape, especially before the material is fully incorporated into the soil. This is particularly relevant in sloped fields, bare soils, or during intense rainstorms.

The extent of lateral transport depends on surface cover, rainfall intensity, and particle size. Fine, light particles are more likely to be mobilized. Quantitative studies suggest that 1–5% of surface-applied biochar can be lost through runoff in a single storm event under unfavorable conditions. These losses are highest immediately after application and tend to decrease significantly once the biochar becomes embedded in the soil matrix.

In agricultural systems, practices like no-till or cover cropping help reduce this type of transport. In forest or grassland systems, surface organic matter provides a natural buffer.

Losses to Aquatic Systems

When biochar is transported out of the terrestrial environment—through runoff into ditches, rivers, or groundwater—it ceases to contribute to soil improvement or carbon storage on land. While there is limited field data quantifying long-term export to aquatic systems, experimental studies suggest that most losses occur during the first few heavy rain events following application.

However, biochar’s fate in aquatic environments is not necessarily negative. Once in sediments, it can be quite stable and even provide benefits such as contaminant sorption or nutrient retention. Still, for projects focused on soil function or terrestrial carbon sequestration, these losses represent a reduction in effectiveness.

Biological Redistribution

Soil organisms can move biochar particles both vertically and horizontally. Earthworms, ants, and other burrowing animals transport small particles through their tunnels or castings. Root growth can also displace biochar, especially in active rhizospheres where root turnover is high.

These biotic movements are slower than physical erosion or leaching but are relevant over multi-year timescales, especially in systems with high biological activity. The redistribution is usually localized but can help biochar integrate more evenly into the soil profile over time.

Implications for Carbon Accounting and Soil Management

For carbon sequestration, the primary concern is whether biochar remains in place and retains its carbon content. Most applied biochar does stay in the soil, especially once incorporated. However, early-stage losses due to runoff or wind erosion can reduce the net carbon retained.

These transport mechanisms also affect where biochar exerts its effects. If material migrates from the topsoil to deeper layers, it may become less available for plant roots or nutrient interactions but more protected from microbial degradation. If it moves into aquatic systems, it might influence water chemistry or sediment dynamics.

From a soil management perspective, understanding biochar transport helps with placement decisions. Incorporating biochar into the topsoil, rather than surface broadcasting, minimizes erosion risk. Timing application before heavy rainfall, or combining it with organic matter or mulch, further reduces mobility.

Final Notes

Biochar isn’t fixed in place. While most of it remains in or near the application zone—especially when applied carefully—some movement is inevitable. What matters is knowing when and how that movement happens, and what it means for the system where biochar is being used.

If the goal is soil improvement or carbon sequestration, minimizing early transport is key. Once stabilized in the soil matrix, biochar is relatively immobile. But getting it there—and keeping it there—requires attention to particle size, weather, soil type, and how the material is handled on the ground.