Has COVID-19 Lockdown Affected C and N Level and Isotope Composition in Urban Soils and Plant Leaves?

This study aimed to evaluate how seasonal variations in environmental conditions and atmospheric NO2 impact C and N cycle in an urban environment by determining their elemental concentration and isotopic composition (613C, 615N) at spatial scale (urban and peri-urban sites) and species level (evergreen plants). Leaves and soil across the medium-sized city of Pisa were collected over 1 year including COVID-19 lockdown, taking advantage of the unprecedented containment measures causing a substantial NO2 drop. The enrichment in heavier isotopes of organic matter in urban soil was most likely due to the longterm contribution of both 615N-enriched depositions and greater C and N cycling rates in comparison with peri-urban soil. Leaf 615N represented a valuable proxy of the urbanization degree depending on microclimate and N inputs from atmospheric NO2. Leaf 613C showed a seasonal trend linked to plant functional types, with significant differences between sites and species. Differently from Nerium oleander and Pittosporum tobira, Quercus ilex showed a positive correlation between 613C and NO2, highlighting the positive effect of N deposition on its intrinsic water-use efficiency. Moreover, the lockdown-induced NO2 reduction was reflected in a decreasing trend of leaf N concentration and change in intrinsic water-use efficiency depending on the plant species and urbanization degree. Q. ilex showed the best adaptability to the more NO2-polluted site, being able to uptake and immobilize high amounts of 15N-enriched atmospheric depositions into its leaves without toxic effects. Overall, these results must be considered in urban greening programs to improve air quality in NO2-polluted areas.