Differences in carbon flux patterns between an urban and a natural Mediterranean ecosystem

Terrestrial ecosystems play a fundamental role in the context of the global carbon budget acting as net sinks of CO2 through their photosynthesis processes. Urban environments are CO2 source hot spots resulting from the sum of biogenic and human contributions, with fossil fuel combustion being the main responsible of anthropogenic emissions. Moreover, cities center typically records higher temperatures than rural surroundings (urban heat island effect) due to a presence of buildings and complex surfaces with respect to trees and lawns. In this work, eddy covariance measurements of carbon dioxide exchange from two different nearby sites (a natural Mediterranean maquis ecosystem and an urban site in central Sassari, Italy) have been analysed and compared over the same multi-seasonal period aiming to characterize the daily and seasonal trend of carbon fluxes. Due to their proximity, the two sites are expected to experience similar weather conditions but differences in air temperature, humidity, and precipitation have been further investigated. CO2 exchanges are significantly different between the two sites: in the natural site, carbon fluxes show a seasonal and diurnal pattern linked to the physiological activity of the vegetation and soil microrganism (with peaks of fluxes occurring in spring and autumn) whereas during the summer months carbon uptake decrease due to water stress conditions. At the urban site, carbon fluxes are characterized by a different trend, especially at daily scale, where peaks emissions are clearly related to traffic. Similarities are observed during the summer weekends, due to urban vegetation uptake and a decrease in vehicular traffic (holiday period). Other emissions sources in the urban site have been analyzed, such as domestic heating during winter months, as well as the mitigation potential of increased vegetated areas.