Carbon Sequestration and Emission of Urban Turfs in Hong Kong and Shenzhen

The rapid pace of urbanization heightens our interest to understand the role of urban areas in mediating climate changes at local, regional and global levels. The increasing amount of carbon (C) released by growing cities may influence the surrounding climates and lead to a rise in global ambient temperature. Fortunately, urban greenery may mitigate the risk of rising C by storing it in vegetation and soils. On the other hand, urban greenery may become a net CO2 or other greenhouse gas emitter due to heavy maintenances. Thus, the exact impact of urban greenery on carbon balance in major metropolitan areas remains controversial.;We first investigated C footprints of urban turf operation and maintenance by conducting a research questionnaire on different Hong Kong turfs, and showed that turf maintenance contributed 0.17 to 0.63 kg Ce m-2 y-1 to C emissions. We then determined C sequestration capacity by urban turfgrasses and soils through both field study and laboratory-based investigation. Our data from field study showed that the C stored in turfgrass systems at 0.05 to 0.21 kg C m-2 for aboveground grass biomass and 1.3 to 4.9 kg C m-2 for soils (to 15 cm depth). We estimated that the C sink capacity of turfs could be offset by C emissions in 5--24 years under current management patterns, shifting from C sink to C source.;We further showed that dissolved organic C (DOC) export also contributed to C release from urban soils, under the influence of the maintenance practices of turfgrasses. Both irrigation and nitrogen (N) fertilizer inputs could enhance DOC export. We also observed that soil enzyme activity was sensitive to turfgrass maintenances practices, and played a key role in soil organic C (SOC) decomposition. In particular, cellulase was found to be a major player in regulating DOC and dissolved organic N (DON) export. On the other hand, urease activity was shown to be dependent on grass species, fertilizer inputs and substrates.;Finally, we then determined the CO2 fluxes of turfgrass systems by chamber-based measurement. Our data demonstrated that grass species and photosynthetically active radiation (PAR) played a dominant role in CO 2 fluxes in greenhouse study. Maintenance practices also contributed to CO2 fluxes of turfgrass systems. Both fertilizer and irrigation showed species-specific effect on CO2 fluxes. We then showed in field studies that CO2 fluxes and respiration rates of urban turfs varied with grass species, and were higher in wet season than dry season.;Together, our studies suggested that maintenance practices for urban greenery played a key role in SOC decomposition through regulating DOC export and C cycle associated enzymes, therefore may determine the C balance of urban greenery. Our studies further suggest that we can improve maintenance practices to reduce C footprint and enhance the environmental benefits of urban turfgrass systems. We propose that the environmental impact of turfgrass systems should be optimized by the rational design of maintenance schedules based on C sink and emission principles.;Keywords: C balance, net ecosystem exchange (NEE) of CO2, DOC, soil enzyme, turfgrass, urban greenery, maintenance.