Constraining urban biogenic CO₂ fluxes: Composition, seasonality and drivers from radiocarbon and inventory analysis

Urban areas play a pivotal role in achieving net-zero emissions to limit global warming to 1.5 °C, given their high carbon footprint and mitigation potential. Accurate quantification of urban CO2 sources is essential for effective carbon budgeting and targeted climate action. While fossil fuel CO2 (CO2ff) emissions are extensively studied, biogenic CO2 (CO2bio) dynamics remain poorly constrained. Here, we separate fossil and biogenic contributions to CO2 enhancements above background using Δ14C and CO2 measurements in Shenzhen, a humid subtropical Chinese megacity potentially subject to substantial biomass burning influence. We calculate human/livestock metabolic emissions (CO2HLM) at 9.32 Mt/6.22 kt per year from population/livestock data and respiratory/excretory rates, and estimate biomass burning emissions (CO2BB) at 5.05 Mt/yr using an inventory encompassing both open and domestic combustion. The residual CO2bio component is attributed to the terrestrial biosphere (CO2bio'). Integrating Δ14C with multi-source data reveals annual CO2bio contributions relative to fossil fluxes: CO2HLM (17.8 ± 3.1 %), CO2BB (9.2 ± 1.5 %), and CO2bio' (73.0 ± 3.5 %). Key findings demonstrate the terrestrial biosphere component acts as a year-round net carbon sink with significant seasonality (11.5 ppm amplitude), driven primarily by atmospheric temperature (1–2 months lag; r = –0.80, p = 0.01) rather than precipitation. This study establishes human metabolic emissions as the dominant biogenic CO2 source (17.8 % vs. 9.2 % from BB) in megacities, yet shows that concurrent biospheric sequestration can offset 63 % of fossil emissions during growing seasons, advancing understanding of urban carbon budgets.