Selection of soil health indicators for modelling soil functions to promote smart urban planning.

The contribution of soil health to global health receives a growing interest, especially in urban environment. Therefore, there is a true need to develop methods to evaluate ecological functions provided by urban soils in order to promote smart urban planning. This work aims first at identifying relevant soil indicators based either on in situ description, in situ measurement or lab analysis. Then, 9 soil functions and sub-functions were selected to meet the main expectations regarding soil health in urban contexts. A crucial step of the present research was then to select adequate indicators for each soil function and then to create adapted reference frameworks; they were in the form of 4 classes with scores ranging from 0 to 3. All the reference frameworks were developed to evaluate soil indicators in order to score soil functions, either by using existing scientific or technical standards or references or based on the expertise of the co-authors. Our model was later tested on an original database of 109 different urban soils located in 7 cities of Western Europe and under various land uses. The scores calculated for 8 soil functions of 109 soils followed a Gaussian distribution. The scoring successfully expressed the strong contrasts between the various soils; the lowest scores were calculated for sealed soils and soils located in urban brownfields, whereas the highest were found for soils located in city parks or urban agriculture. Despite requiring a soil expertise, the proposed approach is easy to implement and could help reveal the true potential of urban soils in order to promote smart urban planning and enhance their contribution to global health.

Home and community composts in Nantes city (France): quality and safety regarding trace metals and metalloids.

Home and community composting are key strategies for local organic waste management. The quality and safety of industrial composts are controlled, but those of home and community composts are not, and this could make them unsafe for use in kitchen gardens. Home (n = 20) and community (n = 41) composts, from urban and suburban areas including mildly Pb-contaminated allotment gardens, were analyzed for quality and safety regarding trace metals and metalloids (TMM) using mid-infrared Fourier transform spectrometry (FT-MIR) and portable X-ray fluorescence spectrometry, respectively. Home composts had a significantly higher Pb content (98 mg.kg-1 ± 10 mg.kg-1) than community composts (21 mg.kg-1 ± 2 mg.kg-1). Numerous home composts (85%) and a few community composts (17%) exceeded the organic farming thresholds for Pb (45 mg.kg-1) and Zn (100 mg.kg-1). The high mineral matter content and the relative abundance of chemical functions attributable to silicates (up to 35%) highly paralleled with TMM contents, mostly concentrated in the fine fraction. Co-inertia analysis highlighted strong and significant links between TMM contents and the whole chemical signature delivered by FT-MIR spectrometry. Pb-contaminated soil could be carried into home compost by green waste or by voluntary addition. Covariance analyses indicated that mineral matter and chemical functions only partly explained the variability in Pb content, suggesting a more complex combination of drivers. Community composting appears as a suitable local solution resulting in high-quality compost that complies with European organic farming regulations, while home composting from allotment gardens should be seriously evaluated to comply with such safety requirements.

Quantification of soil organic carbon stock in urban soils using visible and near infrared reflectance spectroscopy (VNIRS) in situ or in laboratory conditions.

Urban soils, like other soils, can be sink or source for atmospheric carbon dioxide, and due to urban expansion, are receiving increasing attention. Studying their highly variable attributes requires high-density sampling, which can hardly be achieved using conventional approaches. The objective of this work was to determine the ability of visible and near infrared reflectance spectroscopy (VNIRS) to quantify soil organic carbon (SOC) concentration (gC kg-1) and stock (gC dm-3, or MgC ha-1 for a given depth layer) in parks and sealed soils of two French cities, Marseille and Nantes, using spectra collected on pit walls or in laboratory conditions (air dried, 2 mm sieved samples). Better VNIRS predictions were achieved using laboratory than in situ spectra (R2 ≈ 0.8-0.9 vs. 0.7-0.8 in validation), and for sample SOC concentration than stock (R2val up to 0.83 in situ and 0.95 in the laboratory vs. 0.78 and 0.89, respectively). Stock was conventionally calculated according to four methods that variably account for coarse particles (>2 mm); and it was better predicted when coarse particles were not taken into account. This was logical using laboratory spectra, collected on 2 mm sieved samples; but concerning in situ spectra, this suggested the operator tended to put the spectrometer beside the coarsest particles during spectrum acquisition. This point is worth considering for urban soils, often rich in coarse particles. Stocks were then aggregated at the profile level: SOC stock prediction was more accurate at profile than sample level when using laboratory spectra (R2val = 0.94 vs. 0.89, respectively), probably due to uncertainty compensation; but this was not the case when using in situ spectra, possibly because samples collected for SOC analysis and corresponding VNIRS scans were not at the exact same location. This work demonstrates VNIRS usefulness for quantifying SOC stock time- and cost-effectively, in urban soils especially.

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 ±â€¯11.5 kgC m-2 in NYC; 9.9 ±â€¯3.9 kgC m-2 in Paris). Nevertheless, SOCS was lower in sealed soils (2.9 ±â€¯2.6 kgC m-2 in NYC and 3.4 ±â€¯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.