Characteristics and Controlling Factors of Annual Soil CO₂ Emissions and CH₄ Uptakes from Urban Turfgrasses in Beijing

With accelerating urban expansion and increasingly growing city population density, urban ecosystems are becoming the hotspots of global climate change. Although urban turfgrasses are a vital part of cities, their effects on soil–atmosphere exchanges of carbon dioxide (CO₂) and methane (CH₄) remain unclear. In this study, we performed year-round field measurements of soil respiration (CO₂ fluxes) and CH₄ fluxes, associated with environmental factors, from three typical urban turfgrasses (i.e., warm-season turfgrass (WT) dominated by C₄ plant species(in photosynthesis, CO₂ is fixed as three-carbon compounds and four-carbon compounds) and cool-season turfgrass (CT) and shade-enduring turfgrass (ST), both dominated by C₃ plant species (in photosynthesis, CO₂ is fixed as three-carbon compounds)) at the Urban Ecosystem National Observation and Research Station, Beijing. The measurements were performed by a static opaque chamber method combined with gas chromatography analysis. Our results showed that across the experimental period, soil CO₂ emissions and soil CH₄ uptakes from all urban turfgrasses exhibited comparable seasonal patterns. Soil CO₂ emissions from urban turfgrasses were positively correlated with soil temperature and soil water content, and their combined effects could explain approximately 77%–87% of the variations in soil CO₂ emissions. In contrast, the variations of soil CH₄ uptake were mainly regulated by soil water content. The soil CH₄ uptake was negatively correlated with soil water content. Over the annual scale, the cumulative soil CO₂ emissions for all urban turfgrasses were 12.1–15.2 t C ha⁻¹ a⁻¹, and annual CH₄ uptakes were 3.71–4.27 kg C ha⁻¹ a⁻¹. Generally, low temperatures during the nongrowing season usually reduce soil CO₂ emissions and CH₄ uptakes by inhibiting the related microbial activities. However, our results revealed that total soil CO₂ emissions and CH₄ uptakes across the nongrowing season contributed to 10%–18% and 39%–51% of the annual budgets, respectively, highlighting the importance of measurements spanning the full year. Among the three urban turfgrasses, WT exhibited significantly higher annual soil CO₂ emissions but lower annual soil CH₄ uptakes compared with CT and ST. This difference was mainly related to the differences in vegetation characteristics and soil properties among the turfgrasses. Overall, our findings suggest that in the context of substantially increasing use of various urban turfgrasses accompanying rapid urbanization, the conscious planning and design of C₃-related urban turfgrasses may help reduce the soil–atmosphere exchanges of CO₂ and CH₄, thereby contributing to mitigating climate change.