In a long-term field study published as a preprint in Research Square, authors Imrul Kayes, Md Abdul Halim, Wenxi Liao, Md Rezaul Karim, Melanie A. Sifton, and Sean C. Thomas explored how biochar influences greenhouse gas exchange in urban environments. The research focused on extensive green roofs, which are increasingly common in cities to manage stormwater and building temperatures. While these engineered systems are known to sequester some carbon, their ability to regulate non-carbon dioxide gases like methane has remained largely unquantified. By tracking gas fluxes over a five-year period from 2020 to 2024, the team demonstrated that adding biochar to green roof substrates fundamentally shifts their role in the urban methane cycle.

The researchers found that biochar acts as a powerful catalyst for methane consumption. In the spring of 2023, the modules treated with biochar showed methane uptake rates that were nearly five times higher than those of the control modules. This consistent performance turned the green roof substrates into reliable methane-absorbing systems across different seasons. Even as the substrates aged, the biochar-treated sections maintained significantly higher uptake rates than the unamended controls. This suggests that biochar creates a long-lasting physical and biological environment that favors methanotrophy, the process by which specialized microbes consume methane as an energy source.

A critical finding of the study was the decoupling of methane and carbon dioxide fluxes. Environmental scientists often worry that amendments intended to solve one problem might inadvertently worsen another, such as increasing overall carbon losses. However, this study confirmed that the enhanced methane uptake did not lead to an increase in carbon dioxide emissions. This indicates a net climate benefit, as methane is a far more potent greenhouse gas than carbon dioxide in the short term. The biochar appeared to stabilize the carbon balance of the organic-rich substrate while simultaneously supercharging its ability to act as a methane sink.

The mechanisms behind this improved performance are tied to how biochar modifies the physical structure of the substrate. Biochar is highly porous and has a large surface area, which helps maintain aerobic microsites within the soil even when moisture levels are high. The study’s integrated pathway analysis revealed that methane uptake was strongly associated with substrate moisture and water vapor flux. By stabilizing moisture availability and enhancing gas diffusivity, biochar ensures that oxygen can reach the microbes responsible for methane oxidation. In a rooftop environment where soil is often shallow and prone to cycles of extreme wetting and drying, this structural stability is vital for maintaining microbial activity.

Furthermore, the research highlighted the role of evaporation in methane cycling. Biochar addition significantly increased evapotranspiration through enhanced surface flux. This increased movement of water vapor is thought to improve the connectivity of pores in the near-surface layers of the substrate. This improved connectivity facilitates better oxygen diffusion into the soil, which in turn supports the metabolic needs of methane-oxidizing bacteria. Essentially, the biochar helps the roof “breathe” better, allowing it to process more atmospheric methane while still performing its primary duties of stormwater retention and plant support.

These results have broad implications for the design of future urban green infrastructure. By viewing biochar as a multifunctional design component rather than just a carbon additive, engineers can create green roofs that are passive, high-performance systems for greenhouse gas mitigation. The study suggests that incorporating biochar into engineered substrates allows urban planners to address climate goals without sacrificing the hydrological or aesthetic functions of green roofs. As urban populations grow, leveraging every available surface for climate resilience becomes essential, and biochar-enhanced green roofs represent a practical, long-term strategy for turning city skylines into active participants in global biogeochemical cycles.

Source: Kayes, I., Halim, M. A., Liao, W., Karim, M. R., Sifton, M. A., & Thomas, S. C. (2026). Biochar enhances methane uptake in engineered green roof substrate. Research Square.