Nitrous Oxide Emissions from Urban Green Spaces Substantially Offset Their Carbon Sequestration Benefits.

Urban green spaces (UGS) are a cornerstone of global climate adaptation strategies, widely regarded as Nature-based solutions (NbS) for their ability to sequester carbon and mitigate heat islands. However, their net climate benefit remains uncertain due to the poorly quantified release of nitrous oxide (N2O), a potent greenhouse gas and ozone-depleting substance. Here, we present the national-scale assessment of N2O fluxes from China's rapidly expanding UGS, based on high-frequency continuous monitoring and machine-learning extrapolation. We reveal that UGS function as intensive N2O sources, with a mean emission intensity of 12.5 kg·N·ha-1·yr-1, even though most monitored plots received no direct fertilizer application. These emissions are driven by a unique urban synergy where anthropogenic nitrogen enrichment interacts with irrigation pulses and the urban heat island effect to trigger explosive hot moments of denitrification. Crucially, when converted to CO2 equivalents, these N2O emissions offset 68% of the reported carbon sequestration benefits provided by China's UGS vegetation. Our findings demonstrate that ignoring N2O emissions leads to a significant overestimation of the net climate benefit of urban greening. To ensure cities contribute effectively to climate neutrality, urban planning must pivot from simply maximizing green cover to implementing precision nitrogen-water management.