Urban parks, campus lawns, residential gardens, and other greenspaces are vital parts of city life. They cool neighborhoods, support biodiversity, store carbon, and provide places for people to relax. Yet many urban soils are losing organic matter and nutrients under the pressure of rapid urbanization. A new study published in Biochar suggests that the success of common soil restoration practices may depend strongly on one often-overlooked factor: the balance between fungi and bacteria already living in the soil.
Researchers conducted a field experiment across three representative urban greenspaces in Beijing, China, including a university campus greenspace, a park greenspace, and a residential greenspace. These sites differed in their initial soil fertility, allowing the team to test whether biochar, compost, or a combination of both would work equally well across different urban soil conditions.
The answer was clear. Biochar and compost increased soil carbon and nitrogen storage most strongly in nutrient-poor greenspace soils, with amendment effects up to 14.4 times greater than those observed in nutrient-rich soils. In nutrient-poor sites, the amendments promoted fungal richness, strengthened fungal network connectivity and stability, and increased the fungal-to-bacterial richness ratio. These fungal-driven changes were closely linked with higher soil carbon and nitrogen accumulation.
"Our study shows that urban soil restoration is not simply a matter of adding more organic materials," said corresponding author Dr. Qun Gao. "The original nutrient status of the soil, together with the microbial community, can determine whether biochar and compost help store carbon or whether that carbon is quickly consumed."
The team found a contrasting pattern in nutrient-rich greenspace soils. In these soils, biochar and compost did not deliver the same carbon-building benefit. Instead, the treatments reduced fungal diversity and weakened fungal network stability, while promoting faster bacterial growth. This shift may have accelerated carbon consumption and destabilized soil carbon pools. In one nutrient-rich site, combined biochar and compost even reduced total carbon and total nitrogen.
These findings challenge the assumption that richer soils always respond better to added resources. Instead, the study suggests that nutrient-poor urban soils may offer the greatest opportunity for carbon gains because they have more room to accumulate and stabilize new organic matter. In such soils, fungi appear to play a key role by supporting carbon retention and improving fertility.
"Fungi are not just passive members of the soil community," said corresponding author Dr. Ling Han. "They can act as important ecological engineers, helping nutrient-deficient urban soils retain carbon and recover fertility after amendment."
The study also has practical implications for city managers, landscape planners, and policymakers. Biochar and compost are increasingly used to recycle organic waste, improve soil quality, and support climate-friendly urban management. However, the new research suggests that these amendments should be applied strategically rather than uniformly.
Prioritizing nutrient-poor greenspaces for biochar and compost interventions could maximize ecological benefits, improve soil fertility, and enhance urban carbon storage. In contrast, nutrient-rich greenspaces may require more careful evaluation, since additional organic inputs could shift microbial activity toward faster carbon turnover rather than long-term storage.
As cities expand worldwide, improving the health of urban soils will become increasingly important for climate resilience, biodiversity, and public well-being. This study highlights that the smallest organisms in the soil may help decide the fate of carbon beneath our feet.
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Journal Reference: Deng, S., Gao, Q., Han, L. et al. Fungi enhance biochar and compost effects on carbon accrual in nutrient-deficient urban greenspace soils. Biochar 8, 85 (2026).
https://doi.org/10.1007/s42773-026-00599-8
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About Biochar
Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field.
