In a recent research article published in the journal Biochar, lead author Sihang Deng and a team of researchers investigated how biochar and compost amendments impact different types of urban greenspaces. Urban parks and gardens are essential for city life, yet they frequently suffer from poor soil quality due to heavy use and construction. While adding organic materials like biochar and compost is a common way to try and fix this, the actual effectiveness of these treatments depends heavily on the existing condition of the soil. By conducting experiments across various locations with different baseline nutrient levels, the researchers discovered that the starting health of the soil determines whether these treatments will successfully store carbon or accidentally lead to its loss.

The results of the study indicate that the most significant improvements occur in nutrient-poor environments. In these areas, the combination of biochar and compost led to a dramatic increase in soil carbon and nitrogen, with effects that were over fourteen times stronger than those seen in richer soils. The science behind this involves the complex relationship between the soil and its microscopic inhabitants, specifically fungi and bacteria. In poor soils, the additions provided a much-needed boost that allowed fungi to thrive. These fungi are particularly good at building stable organic matter and holding onto nutrients over the long term. By increasing fungal richness and creating more stable underground fungal networks, the amendments effectively turned the soil into a long-lasting reservoir for carbon.

In contrast, the researchers found that applying the same treatments to nutrient-rich soils produced a very different and somewhat counterintuitive outcome. In these areas, the extra boost of organic matter favored bacteria rather than fungi. Because bacteria are fast-growing and prefer to consume easy-to-digest carbon, their increased activity actually accelerated the decomposition of the soil’s existing carbon pools. This led to a decline in fungal diversity and a weakening of the soil’s natural microbial stability. Instead of storing carbon, the nutrient-rich sites experienced a destabilization of their organic matter. This suggests that over-fertilizing or over-treating already healthy urban soils can actually be detrimental to climate goals like carbon sequestration.

The study also highlighted that the benefits of these treatments in poor soils were not just a temporary spike but represented a sustained improvement in soil fertility. Fungi were identified as the pivotal drivers in this restoration process. When fungi dominate the soil community, they help maintain the structural integrity of the earth and support the health of city plants and wildlife. On the other hand, the bacterial-driven carbon loss in rich soils was often accompanied by higher emissions of carbon dioxide, which can undermine the ecological purpose of greenspace management. These findings provide a clear conceptual model for city planners: the background state of the land is the most important factor in deciding where to invest in soil restoration.

Ultimately, this research provides a roadmap for more precise and effective urban land management. Rather than applying a one-size-fits-all approach to park maintenance, the data suggests that resources should be prioritized for the most degraded and nutrient-deficient areas. By focusing biochar and compost interventions on these poor sites, cities can maximize the ecological payoffs, ensuring that urban soils act as a sink for greenhouse gases rather than a source. This precision approach not only improves the success rate of soil restoration but also ensures that the environmental benefits of city greenspaces are realized more efficiently across the urban landscape.

Source: Deng, S., Gao, Q., Han, L., Tong, X., Shen, W., Liu, A., Lee, H., Ye, Z., Liu, S., Sun, K., Xia, X., & Yang, Y. (2026). Fungi enhance biochar and compost effects on carbon accrual in nutrient-deficient urban greenspace soils. Biochar, 8(1), 85.