N-microbial properties are key indicators of fertility in maturating soils built for urban greening.

International audienceSoils constructed from urban planning wastes provide a sustainable solution for urban planners to design urban greenspaces that meet the growing demand for nature in the city while limiting artificialization of peri-urban agricultural areas. However, while the pedological and chemical characteristics of these constructed soils are easily assessed, their biological fertility, and especially their nitrogen (N) availability, remains essentially associated to their inhabiting microbial communities. Accordingly, the stability of microbial N- functioning and the prerequisite time to provide sufficient N to plant covers is an under-explored ecological question.To address this issue, nine soils were constructed from compost, topsoil, and deep soil mixed in different proportions. Half were hydroseeded once with a mixture of annual and perennial plants. We then monitored soil fertility for 24 months, to cover two growing and two wintering seasons, and assess N cycle seasonality in regards of plant community development and pedological and chemical characteristics. Free-living microbial N-fixation, nitrification and denitrification enzyme activities were measured in soil incubations following acetylene reduction assays, colorimetric quantifications of nitrite/nitrate accumulation, and N2O gas chromatography, respectively. Relative abundances of genes associated to these enzyme activities were assessed using targeted quantitative PCR.Our results suggest microbial indicators of soil functioning are truly relevant to monitor biological fertility of constructed soils in urban ecosystems