No abstract

In cities, the strong heterogeneity of soils, added to the lack of standardized assessment methods, serves as a barrier to the estimation of their soil organic carbon content (SOC), soil organic carbon stocks (SOCS; kgC m) and soil organic carbon citywide totals (SOCCT; kgC). Are urban soils, even the subsoils and sealed soils, contributing to the global stock of C? To address this question, the SOCS and SOCCT of two cities, New York City (NYC) and Paris, were compared. In NYC, soil samples were collected with a pedological standardized method to 1 m depth. The bulk density (D) was measured; SOC and SOCS were calculated for 0-30 cm and 30-100 cm depths in open (unsealed) soils and sealed soils. In Paris, the samples were collected for 0-30 cm depth in open soils and sealed soils by different sampling methods. If SOC was measured, D had to be estimated using pedotransfer functions (PTFs) refitted from the literature on NYC data; hence, SOCS was estimated. Globally, SOCS for open soils were not significantly different between both cities (11.3 ± 11.5 kgC m in NYC; 9.9 ± 3.9 kgC m in Paris). Nevertheless, SOCS was lower in sealed soils (2.9 ± 2.6 kgC m in NYC and 3.4 ± 1.2 kgC m in Paris). The SOCCT was similar between both cities for 0-30 cm (3.8 TgC in NYC and 3.5 TgC in Paris) and was also significant for the 30-100 cm layer in NYC (5.8 TgC). A comparison with estimated SOCCT in agricultural and forest soils demonstrated that the city's open soils represent important pools of organic carbon (respectively 110.4% and 44.5% more C in NYC and Paris than in agricultural soils, for 0-30 cm depth). That was mainly observable for the 1 m depth (146.6% more C in NYC than in agricultural soils). The methodology to assess urban SOCS was also discussed.

Within the context of a worldwide emergence of various forms of urban agriculture, there is a growing awareness concerning the health risks associated to the presence of different pollutants influencing the urban products safety. Among the most common pollutants found in soils and vegetables grown in the city, Trace Metals (TM's) are of major concern. This paper deals with risks assessment associated with the presence of TM's in soil, via two main exposure path ways: soil and vegetables ingestions. Risks assessments were conducted for various types of real scenarios encountered in three forms of urban farms near Paris (Ile-de-France Region). The farms have soil TM's levels in abnormally high concentrations (Pb (Lead), Cd (Cadmium), Hg (Mercury), Cu (Copper) and Zn (Zinc) contents higher than geochemical backgrounds and threshold values for sludge spreading, often used as reference values in France). The results of the Hazard Quotient (HQ)-based risk assessment approach (HQ defined as the ratio of estimated daily intake/tolerable daily intake) show that the most risky scenarios concern urban farmers (HQ tot = 1.02, because of the on-site working on a daily basis all year round), children gardeners (HQ tot = 1.29) and regular children consumers (HQ tot = 1.6 in maximalist scenario, where the consumer would exclusively consume the vegetables of the farm). Next would be the adult gardener scenario (HQ tot = 0.9), while the least risky are adult consumer scenarios (HQ tot = 0.62) and the farm workers (HQ tot = 0.45). For the highest risk scenarios (urban farmers and children), specific and drastic measures may be considered, such as reducing the site frequentation by sensitive populations (child and

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Cities and towns exert significant pressures on both urban and surrounding environments (Chambers et al., 2016;Kaushal, McDowell, & Wollheim, 2014). Urban soils are particularly impacted, in that they are heavily managed, with whole soil profiles often being anthropogenic. For example, fertile agricultural soils from rural areas are often imported for use in constructing urban green spaces (Yang & Zhang, 2015). Furthermore, they are subjected to the addition of waste materials, pollutants from numerous point and diffuse sources, atmospheric deposition of carbon and nitrogen, or heat island effects. Town planning and landscaping decisions can also result in rapid changes in land-use, which modify soil properties in a complex manner, for example capacity to store carbon or water (Wang, Adhikari, Zhuang, Gu, & Jin, 2020). In most cases, they do not result solely from slow pedogenic processes from a parent materiel under the influence of climate, vegetation, topography and time (Effland & Pouyat, 1997;Vialle & Giampieri, 2020).Urban soils have a number of common features. They generally have high organic carbon content due to an