N-microbial properties are key indicators of fertility in maturating technosols constructed for urban landscapes

Excavation of agricultural topsoils traditionally used in city landscaping results in strong environmental impacts and a rarefaction of peri-urban fertile soils. The construction of fertile technosols in a circular economy approach using urban wastes is an attractive alternative to such unsustainable practices. However, these soils have specific characteristics, such as rapid pedogenesis and strong influence of parental materials, that conventional agronomic indicators may overlook. Here we investigated 21 monitoring indicators during the 24 months maturation of nine stockpiles of constructed technosols, differing on initial compost proportions and presence of prairial cover. We compared standard agronomic measurements to pools of inorganic nitrogen (Ninorg) and to various microbial related parameters. Carbon Substrate-Induced Respiration was used as proxy for active microbial biomass. We focused on three N-cycling microbial activities associated to Ninorg available to plants: free-living bacteria nitrogen fixation, nitrification and denitrification enzyme activities. The relative abundances of microbial genes associated with all these activities were also measured to evaluate microbial total and N-specific populations. Conventional agronomic parameters disclosed the importance of the initial proportions of compost and of the prairial cover but failed to monitor the temporal dynamic of technosols fertility. The monitoring of microbial parameters complemented these assessments, revealed the differences in N-fertility among technosols and informed on the maturation 18 months after establishment. This work provides new insights into the understanding of the maturation of silt and compost mixtures. Furthermore, these results highlight the potential of microbial indicators to improve the management of technosols constructed for urban landscapes.