Concrete jungles vs. carbon sinks: How urbanization weakens forests’ hidden climate superpower

A landmark open-access review published on November 28, 2025, in Carbon Research (Volume 4, Article 70) delivers a sobering yet actionable insight: urban forests are losing their capacity to store stable soil carbon, not because they’re gone—but because the soil beneath them is being silently transformed by human activity.

Led by Dr. Xiankai Lu of the Guangdong Provincial Key Laboratory of Applied Botany, South China Botanical Garden, Chinese Academy of Sciences, and the Key Laboratory of Plant Ex-Situ Protection and Utilization in South China, National Forestry and Grassland Administration, this synthesis pulls together global evidence to map how carbon stability shifts across the urban–rural forest gradient.

The Hidden Crisis Beneath City Trees

Soil isn’t just dirt—it’s Earth’s largest terrestrial carbon reservoir. And within it, soil organic carbon (SOC) exists in different forms, some fleeting, others locked away for decades or centuries. The most stable fractions—particulate organic carbon (POC) and mineral-associated organic carbon (MAOC)—act like long-term vaults, shielding carbon from microbial breakdown.

But in urban forests, this vault is cracking.

Dr. Lu’s review shows that SOC stability is consistently lower in urban forest soils compared to suburban and rural counterparts. Why? Because urbanization brings soil compaction, erosion, pollution, and disrupted inputs of leaf litter and root exudates—the very ingredients needed to build and maintain stable carbon pools.

Even MAOC, typically considered highly resistant to decomposition, suffers in cities. “It’s not just about how much carbon is there,” explains Dr. Lu, “but whether the conditions allow it to stay there.” In compacted, nutrient-imbalanced, or polluted urban soils, the processes that bind organic matter to minerals break down—leaving carbon vulnerable to release as CO₂.

Suburban Forests: The Unexpected Carbon Champions

Surprisingly, suburban forests often outperform both urban and rural sites in certain stability metrics. These transitional zones tend to accumulate more non-readily oxidizable carbon (NROC)—a chemically stable form that resists degradation over time.

They also benefit from moderate organic inputs, less extreme disturbance than city centers, and sometimes enhanced management (like mulching or reduced tillage). The result? A sweet spot where human presence doesn’t overwhelm natural processes—but may even support them.

Microbes: More Diverse, But Less Effective

Urban soils often show higher bacterial diversity, likely due to nutrient runoff, varied plantings, and physical disturbance introducing new microbes. But diversity alone doesn’t equal function.

In fact, microbial biomass carbon (MBC)—a key indicator of active, carbon-processing life in soil—declines significantly in urban settings, especially where compaction limits oxygen and root growth. Enzyme activities linked to carbon cycling become erratic, and total microbial abundance drops in polluted or heavily trafficked areas.

“Soil microbes are the engine of carbon stabilization,” says Dr. Lu. “When their habitat is degraded, even a diverse community can’t perform its climate-regulating role.”

Two Worlds, Two Sets of Rules

Perhaps the most crucial insight from this review is the divergence in drivers across the landscape:

• In urban forests, SOC stability is controlled mainly by anthropogenic stressors: air pollution, altered microclimates, soil sealing, and chemical contamination.
• In rural forests, natural factors dominate: litter quality, vegetation type, climate, and soil fauna.
• In suburban forests, it’s a mix—where thoughtful design can tip the balance toward resilience.

“This means we can’t manage all forests the same way,” emphasizes Dr. Lu. “Urban greening strategies must go beyond planting trees—they must protect and restore the living soil beneath them.”

A Call to Rethink Urban Greening

Based at the South China Botanical Garden in Guangzhou—a national hub for biodiversity conservation and ecosystem research—Dr. Lu’s work bridges fundamental science and urban policy. His review doesn’t just diagnose the problem; it maps a path forward, identifying critical research gaps and urging integrated approaches that consider soil health as central to climate mitigation.

And because the article is published open access, city planners, landscape architects, ecologists, and policymakers worldwide can use these findings to design truly climate-smart urban forests—ones that don’t just look green, but function as enduring carbon sinks.

The Bottom Line

Trees in cities do more than beautify—they cool neighborhoods, filter air, and support mental health. But if we ignore the soil, we risk turning these green oases into carbon liabilities.

Thanks to the rigorous synthesis by Dr. Xiankai Lu and his team at the Chinese Academy of Sciences, we now understand not only why urban forest soils falter—but where hope lies: in suburban buffers, in restored soil structure, and in reimagining urban ecology from the ground up.

The next time you walk through a city park, remember: the real climate action might not be in the canopy above—but in the quiet, complex world beneath your feet.

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Journal reference:  Tobiloba, L.H., Mustafa, A., Lu, X. et al. How urbanization reshapes soil organic carbon stability in urban forests: a critical review. Carbon Res. 4, 70 (2025).   

https://doi.org/10.1007/s44246-025-00230-8  

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About Carbon Research

The journal Carbon Research is an international multidisciplinary platform for communicating advances in fundamental and applied research on natural and engineered carbonaceous materials that are associated with ecological and environmental functions, energy generation, and global change. It is a fully Open Access (OA) journal and the Article Publishing Charges (APC) are waived until Dec 31, 2025. It is dedicated to serving as an innovative, efficient and professional platform for researchers in the field of carbon functions around the world to deliver findings from this rapidly expanding field of science. The journal is currently indexed by Scopus and Ei Compendex, and as of June 2025, the dynamic CiteScore value is 15.4.

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