Estimation of soil organic carbon stocks of two cities, New York City and Paris

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−2) 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 (Db) 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, Db 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−2 in NYC; 9.9 ± 3.9 kgC m−2 in Paris). Nevertheless, SOCS was lower in sealed soils (2.9 ± 2.6 kgC m−2 in NYC and 3.4 ± 1.2 kgC m−2 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.