Soil organic matter quality along an urbanization gradient in Paris region

Studying carbon stocks and fluxes in urban soils is becoming increasingly relevant, considering the continuous urbanization. Although they represent a relatively small area compared to forest or arable soils, urban soils could be considered as hot spots of anthropogenic carbon accumulation as it is estimated that they contain 3-5 times as much C per ha as natural soils (Vasenev & Kuzyakov, 2018). To date, few studies have investigated urban soil organic matter (SOM) quality and stability, i.e. its resistance to microbial decomposition. In this study, we evaluated the ability of thermogravimetry to predict SOM mineralization kinetic parameters at a regional scale. In order to achieve this, 180 soil samples were collected from two different land uses (lawns and woodlands) along a gradient of urban pressure (rural, suburban and urban areas) in the Paris region (France). We determined SOM mineralization kinetic parameters by measuring CO₂ emissions in long-term incubations and the thermal stability of SOM by thermogravimetry combined with differential scanning calorimetry and evolved gas analyzes (TG-DSC-EGA).  The SOM quality was also characterized by using Mid Infra-Red Spectrometry (MIRS). Overall, SOM thermal stability increased from the rural to the urban areas in both land-use types. Urban woodland soils had greater SOM thermal stability than urban lawns, probably because the woodlands are much older than the lawns and because of historical soil management legacy in Paris region. Significant and strong relationships were found between SOM thermal analysis indices (CO₂-T50 and energy density) and mineralization kinetics parameters (mineralizable C and turnover) measured in the laboratory. MIRS analyses revealed different chemical compositions depending on land use (higher aromaticity and condensation indices in lawns) and the urban gradient (lower polysaccharides content and aromaticity index in cities). This regional scale study suggests that 1) the thermal analysis of SOM, together with MIRS and soil physico-chemical measurements can be used to predict soil C mineralization potential and 2) SOM thermal stability and resistance to microbial mineralization are higher in urban soils, especially in woodlands.