The scientific community has long viewed biochar as a reliable method for sequestering carbon and mitigating the release of nitrous oxide, a greenhouse gas significantly more potent than carbon dioxide. A study published in the journal Biochar by Shumin Guo and a team of researchers from Nanjing Agricultural University challenges the assumption that these benefits are permanent. By examining acidic tea plantation soils in China, the authors discovered that the impact of biochar on the environment undergoes a dramatic shift as the material ages within the soil. While the initial years of application show great promise for climate goals, the long-term legacy effects suggest a more complex and potentially counterproductive outcome for atmospheric health.

Nitrous oxide emissions from agricultural soils are a major driver of global climate change and ozone depletion. Acidic soils are particularly prone to high emission rates, making them a primary target for biochar amendments. In the early stages, specifically between three and five years after application, biochar effectively suppresses these emissions. The researchers found that during this period, the material enhances the soil environment by raising the pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More and increasing available organic carbon. These conditions favor specific bacteria, such as Rhodanobacter and Gemmatimonas, which possess the genetic ability to transform nitrous oxide into harmless nitrogen gas. This microbial shift ensures that the nitrogen cycle in the soil remains balanced and that fewer harmful gases escape into the atmosphere.

However, the study reveals a starkly different reality when looking at soil where biochar has persisted for nine years. At this stage, the material is no longer a tool for mitigation but rather a catalyst for increased emissions. The cumulative release of nitrous oxide was found to be four times higher in long-term biochar-treated soils compared to those treated only with conventional fertilizers. This reversal is primarily attributed to the aging process of the biochar itself. Over time, the material loses its ability to buffer soil acidity and its porous structure becomes clogged or degraded. Instead of raising the soil pH, the aged biochar was associated with a decline in pH, which creates a hostile environment for the beneficial bacteria that typically reduce gas emissions.

The microbial mechanisms driving this long-term increase involve an imbalance between how the gas is produced and how it is consumed. While biochar continued to suppress some pathways of gas production even after nine years, it inhibited the reduction of that gas much more severely. The researchers observed a significant decrease in the abundance of the nosZ gene, which is the functional marker for the final step of the nitrogen cycle where nitrous oxide is converted to nitrogen gas. Furthermore, the aged biochar led to a depletion of dissolved organic carbon, which acts as the necessary fuel for the microbes responsible for cleaning up greenhouse gases. With less fuel and a more acidic environment, the soil’s natural ability to mitigate its own emissions is crippled.

Another critical finding in the long-term analysis was the shift toward fungal denitrification. Unlike many bacteria, most fungi lack the genetic machinery to complete the nitrogen cycle, meaning their metabolic processes often stop at the production of nitrous oxide. The study showed that as biochar aged, the contribution of fungi to the nitrogen cycle increased. This shift, combined with the suppression of gas-reducing bacteria, created a bottleneck where nitrous oxide was produced but had no biological pathway for removal. Consequently, the gas accumulated and was released from the soil at much higher rates than in the years immediately following the biochar application.

These findings serve as a vital reminder that the climate impacts of soil amendments are not static. The temporal divergence observed in this research suggests that the current models used to predict the benefits of biochar may be overestimating its long-term effectiveness if they do not account for microbial and chemical changes over decades. For biochar to remain a sustainable strategy for climate-smart agriculture, management practices must evolve. This might include the periodic re-application of fresh biochar to maintain soil pH or the use of specific microbial interventions to support the bacterial communities that keep greenhouse gas emissions in check.

Ultimately, the work of Guo and the research team underscores the necessity of long-term field monitoring. While biochar remains a powerful tool for waste management and initial carbon sequestration, its role in nitrogen cycling requires careful oversight. Understanding that the legacy effects of biochar can reverse from mitigation to stimulation allows scientists and policymakers to develop more nuanced strategies. By incorporating a temporal perspective into soil management, the agricultural sector can better align its practices with global climate targets and ensure that today’s environmental solutions do not become tomorrow’s challenges.

Source: Guo, S., Lin, H., Li, Z., Han, Z., Wu, J., Bo, X., Shen, M., Zhang, Z., Liu, S., Wang, J., & Zou, J. (2026). Divergent legacy effects of biochar on nitrous oxide emissions in acidic soils driven by altered microbial N pathways. Biochar, 8(1), 40.