Soil organic carbon stock in different urban land uses: high stock evidence in urban parks

Urban areas are major producers of atmospheric CO2 emissions, but at the same time they can offset some of the associated C losses by retaining stable organic carbon in their soils (SOC). Despite the importance of urban SOC there are still many uncertainties associated, resulting in low accuracy of SOC stock estimations or in its neglection in the regional and national carbon budgets. We focused on one of the most extended urban areas of Italy (Milan) estimating SOC stock in the topsoil and comparing it for different urban land uses (park/non-park) and covers (woodland/grassland). We also compared urban SOC stocks with other typologies of natural and semi-natural land use in the region. We found generally high variability in urban soil properties. Average SOC of urban parks was higher (7.9 ± 2.4 kg m−2) than urban non-parks (5.3 ± 2.5 kg m−2); while SOC stock did not significantly differ for urban land cover types. Urban parks had SOC stock values comparable with forest, pasture and grasslands of the same region and higher than SOC of croplands. At the same time, urban non-parks had the lower SOC stocks values found in the region. The results highlighted the significant contribution of urban SOC stock to carbon budget estimations and differences based on urban land use types. Furthermore, we described the main pedological characteristics of investigated urban soils and we identified the factors that contribute to limiting our capacity to model urban SOC stock that can be partially overcome by the addressed strategies.