Spatial and temporal variability of CO₂, N₂O and CH₄ fluxes from an urban park in Denmark

Abstract. With the rapid worldwide increase in urbanization, urban green spaces are becoming increasingly important in regulating biogeochemical cycles and associated greenhouse gas (GHG) fluxes on regional and global scales. However, the existing data and research on the potential roles of urban green spaces remain limited. In this study, we conducted in situ measurements of nitrous oxide (N₂O) and methane (CH₄) fluxes, as well as ecosystem carbon dioxide (CO₂) respiration, at 56 sites in a temperate urban park with a hilly landscape during the vegetation and frost-free period as well as the freeze–thaw period. Based on the arithmetic mean of all the measurements, the soil acted as a source of N₂O (23.8 ± 1.7 μg N m⁻² h⁻¹) and a weak sink of CH₄ (-0.26 ± 2.14 μg C m⁻² h⁻¹). Over the entire observation period, the mean ecosystem CO₂ respiration was calculated to be 228 ± 18.5 mg C m^-2 h^-1. High spatial and temporal variability was observed for all three GHGs fluxes, with the coefficient of variation ranging from 45.6–259 % for N₂O, 3154–4962 % for CH₄ and 40.3–49.3 % for CO₂, respectively. This variability was primarily associated with changes in soil and environmental factors, including vegetation structure, soil hydrothermal conditions, pH, and the availability of soil carbon and nitrogen.  Moreover, random forest models combining the in situ measured data and landscape parameters demonstrated a high probability of identifying spatial patterns and hot or cold spots of GHG fluxes across this heterogeneous landscape. However, the models' performance was limited by the lack of high-resolution soil and vegetation data. Overall, our study provides valuable insights into scaling GHG fluxes in urban green spaces more effectively, enabling a more accurate assessment of how urbanization changes landscape fluxes.