One of the greatest scientific and political challenges of the twenty-first century is to regulate global soil carbon (C) sequestration, which supports multiple ecosystem services ranging from soil biodiversity to nutrient cycling. Although city parks and gardens promote important ecosystem services that benefit humankind, including recreational services and mental and physical health, their role in storing large global soil C stocks remains poorly understood. Unraveling the amount of C stored in our parks and the main drivers behind it is essential to support the sustainability of these important urban ecosystems and their capacity to contribute to climate change mitigation.

To address this knowledge gap, we conducted a global study of 56 paired urban greenspaces and adjacent natural ecosystems from locations in 17 countries and 6 continents across environmental gradients. We collected data on soil organic C concentration (SOC), microbial aspects (i.e., functional genes associated with C cycling), and soil C persistence. Together, we aimed to tease apart commonalities and differences between the environmental drivers of soil C and its sensitivity to warming in urban and natural ecosystems.

Our study indicated that surface soils in urban and natural ecosystems sustain similar SOC concentrations and soil C persistence. Despite the smaller global area covered by urban greenspaces compared to natural ecosystems, our results imply that urban greenspaces play a crucial role as reservoirs of surface soil C across the globe. This finding highlights the potential of urban greenspaces as natural climate solutions and underscores the need to incorporate them into future management policies aimed at mitigating C footprints.

Moreover, our study sheds light on the environmental drivers that regulate soil C stocks in both urban and natural ecosystems. First, we found that both urban and natural ecosystems are associated with mean annual temperature, with negative correlations between temperature and soil C worldwide. Second, our findings indicate that while SOC concentration is correlated with net primary productivity (litter inputs) in natural ecosystems, soil microbes emerge as the most important driver in urban ecosystems.

In summary, our study underscores the importance of urban greenspaces as global reservoirs of soil C worldwide, with soil microbes playing a crucial role in regulating soil C and its response to warming. Specifically, our analyses revealed that urban greenspaces support an important functional microbiome associated with C mineralization. The message for scientists, policymakers, and concerned organization is therefore clear: urban management strategies should consider the soil microbiome to maintain soil C and ecosystem sustainability.