| 18 Nov 2025
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.
This manuscript addresses the current limitations in the understanding of biogeochemical carbon and nitrogen cycles in urban green spaces and the relatively low accuracy in estimating greenhouse gas (GHG) budgets. To this end, 56 representative sampling sites with diverse vegetation types and landscape positions were selected within urban parks to conduct long-term, seasonally cross-sectional observations of greenhouse gas fluxes. Through systematic measurements of soil methane (CH4), nitrous oxide (N2O), and carbon dioxide (CO2) fluxes, the study reveals the dynamic patterns and driving mechanisms across temporal and spatial scales. Based on the observational data, the random forest (RF) model was developed to predict the probability of GHG hot spots and/or cold spots. The findings provide critical data support for a deeper understanding of carbon and nitrogen cycling processes in urban ecosystems, and lay a scientific foundation for improving the accuracy and predictive capability of GHG budget models for urban green spaces. Although the manuscript presents some valuable findings, I do not believe the authors are adequately prepared for this manuscript to be published. The manuscript contains numerous basic errors that require careful revision and correction. The main issues are outlined below:
In figure 3b, a clear emission hotspot is observed near the stream. However, based on the sampling point distribution in figure 1, the number of plots located around the stream is limited, which raises concerns about the accuracy of this result. A similar issue is also present in figure 3d. The authors are advised to re-examine the data and verify the accuracy of the results.