Effects of biochar and irrigation on greenhouse gas exchange in hemiboreal urban green areas

As cities worldwide pursue carbon neutrality, developing a comprehensive understanding of the contributions of urban vegetated areas to climate change mitigation is required. Significant knowledge gaps remain regarding the full range of urban biogenic greenhouse gas (GHG) dynamics, particularly concerning GHGs other than carbon dioxide (CO2) and the possibly crucial role of urban soils. Existing studies suggest that urban soils often exhibit higher respiration rates, increased nitrous oxide (N2O) emissions and reduced methane (CH4) uptake compared to natural ecosystems. This study investigates how irrigation practices and biochar amendment influence soil GHG fluxes in urban lawns and tree growing media, and examines the potential impacts of biochar on tree growth and leaf-level CO2 exchange. Ultimately, we aim to provide information on whether sensible use of irrigation and biochar could help to enhance the climate change mitigation potential of urban green areas.The study is based on a measurement campaign using manual dark chambers to quantify soil CO2, CH4 and N2O fluxes and leaf CO2 exchange measurements made in Helsinki, Finland, in summer 2025. Irrigation impacts on urban lawn GHG exchange were studied on 10 controlled, non-fertilized plots located along a footpath in Kumpula botanic garden. Half of the plots were irrigated weekly, while the other half functioned as unirrigated controls. Flux measurements were complemented with manual and automatic measurements of soil moisture and temperature. To provide a comparison with less managed vegetation, a nearby meadow was also measured using the same protocol without irrigation. Effects of biochar on soil and tree GHG exchange were investigated in an urban park in Eastern Helsinki, where trees planted in 2023 grow in standardized growing media. First half of the trees received biochar at planting, while the other half served as controls. Soil and leaf GHG fluxes were measured alongside with soil moisture, temperature and tree health assessments.While data analysis for the summer 2025 measurements is still ongoing, preliminary results indicated a significant reduction in CH4 uptake under irrigation. CO2 and N2O showed no consistent response, with especially N2O fluxes exhibiting high variability across plots and measurement days. In the biochar experiment, biochar appeared to suppress the largest N2O flux events from soil, but no significant effects on CO2 and CH4 fluxes were detected. CH4 fluxes showed pronounced spatial variability across the study site. While most plots acted as CH4 sinks, one section of the park exhibited notable emissions, possibly reflecting local anoxic conditions in the soil. As part of the ongoing analysis, net soil GHG balances for the studied vegetation types, expressed as CO2 equivalents, will be calculated for the measurement period to provide an integrated assessment of their climatic impacts.