Microplastics May Skew Estimates of Biochar’s Climate Benefits in Agricultural Soils

Biochar has become one of agriculture’s most promising tools for removing carbon from the atmosphere. Produced by heating plant material in a low-oxygen environment, this carbon-rich material can be added to soil to improve water retention, support plant growth, and potentially lock carbon away for decades or even centuries. But a new scientific review warns that biochar is entering agricultural soils alongside another persistent form of carbon: microplastics. When the two materials meet, the result may complicate both soil chemistry and the way climate benefits are measured.

Published in Agricultural Ecology and Environment, the review examines how biochar, microplastics, and naturally occurring soil organic carbon interact across several physical and biological scales. The researchers describe agricultural soil as a complex network of pores, mineral surfaces, aggregates, water films, and microbial habitats. Biochar and microplastics can occupy many of the same spaces, meaning their effects may overlap, reinforce one another, or change over time as particles weather and move through the soil.

Biochar can influence the soil carbon cycle in several ways. Its porous structure provides surfaces that can adsorb dissolved organic matter, including compounds that would otherwise be rapidly consumed by microbes or transported away with water. Biochar may also encourage the formation of soil aggregates, in which organic material becomes physically protected from decomposition. In addition, its surfaces can promote associations between organic molecules and soil minerals. These processes may slow the breakdown of carbon and alter the availability of nutrients and water.

Microplastics, however, can disrupt the same soil architecture. Tiny plastic particles change pore size and connectivity, potentially affecting the movement of water, oxygen, dissolved organic matter, and microorganisms. Their impact depends on the type of polymer involved, as well as particle shape, concentration, size, weathering, and the chemical properties of the surrounding soil. Some microplastics may stimulate microbial activity by providing surfaces for biofilms, while others can limit oxygen diffusion, alter moisture conditions, or interfere with microbial communities responsible for decomposing organic matter.

The review emphasizes that the combined effect of biochar and microplastics cannot be predicted simply by adding together their separate effects. Biochar may partially reduce some disturbances associated with microplastics by improving aggregation or offering additional surfaces onto which plastic-associated chemicals and dissolved organic compounds can attach. This could reduce the mobility of certain contaminants or change their availability to soil organisms. Yet the authors caution that the protective capacity of biochar may decline as both materials age, fracture, become coated with organic matter, or fill available sorption sites.

This aging process is especially important because soil is not a static environment. Rainfall, repeated wetting and drying, root growth, freeze-thaw cycles, and microbial activity can gradually alter biochar surfaces and break larger plastic fragments into smaller particles. Weathered microplastics may become more chemically reactive or develop cracks and oxygen-containing functional groups. At the same time, aged biochar may lose some of its original surface characteristics while gaining new mineral and microbial coatings. These transformations could change how carbon is stored, transported, and decomposed over years or decades.

The most immediate concern raised by the researchers involves carbon accounting. Standard soil organic carbon tests generally measure the amount of carbon in a soil sample, but they may not reliably distinguish among carbon derived from plants, carbon transformed by fire and added as biochar, and carbon contained in fossil-fuel-based plastic polymers. That distinction matters because these carbon pools have different origins, chemical structures, environmental behaviors, and implications for climate mitigation. A soil sample containing microplastics could therefore appear to hold more organic carbon even when part of that measurement represents persistent synthetic material rather than newly sequestered atmospheric carbon.

The potential scale of this problem is substantial. According to the review, if microplastic-derived carbon is not separately identified, concentrations equivalent to approximately 0.1% to 0.5% carbon in the upper 20 centimeters of an agricultural plough layer could contribute roughly 3 to 15 megagrams of carbon per hectare to routine soil carbon measurements. The estimate does not mean that every field contains this amount, nor that all measured polymer carbon would be counted as climate mitigation. Instead, it illustrates how synthetic carbon could create a false-positive signal in monitoring systems, especially where projects receive credits for increasing soil carbon stocks.

That issue directly affects measurement, reporting, and verification, or MRV, systems used by soil carbon programs and carbon removal markets. The authors propose an evidence-tiered framework combining polymer-specific analyses with techniques capable of separating pyrogenic carbon from native soil organic carbon. Such methods could include chemical and spectroscopic approaches that identify polymer signatures, assess the structure of fire-derived carbon, and track changes in carbon pools over time. Improved sampling strategies will also be necessary because microplastics and biochar are unlikely to be distributed evenly through a field; they may accumulate near soil surfaces, in irrigation pathways, or within particular aggregate fractions.

The review concludes that long-term field studies are urgently needed. Much of the existing evidence comes from short laboratory experiments using high concentrations of relatively uniform plastic particles and freshly produced biochar. Real agricultural soils contain weathered plastics of different sizes and compositions, mixed with roots, minerals, microorganisms, fertilizers, and changing moisture conditions. Future research will need to follow these systems over multiple growing seasons while measuring greenhouse-gas emissions, microbial activity, carbon chemistry, particle movement, and crop responses. The central message is clear: agricultural soils increasingly contain biogenic, pyrogenic, and synthetic carbon at the same time, and credible climate accounting will depend on telling those carbon sources apart.

News Publication Date: 28-May-2026

Web References: https://doi.org/10.48130/aee-0026-0014

References: Yang Z, Simarani K, Zhang X, Di Martino A, Chen Y, et al. 2026. “Biochar-microplastic co-occurrence in agricultural soils: interfaces, effects on soil organic carbon, and implications for measurement and verification.” Agricultural Ecology and Environment 2: e017. DOI: 10.48130/aee-0026-0014

Keywords

Biochar, microplastics, agricultural soils, soil organic carbon, carbon sequestration, soil carbon accounting, climate mitigation, pyrogenic carbon, synthetic carbon, measurement reporting and verification, soil microbiology, greenhouse gases, carbon removal, soil aggregates