PS 43-69 - N-microbial properties are key indicators of fertility in maturating soils built for urban greening.

Background/Question/Methods

Soils constructed from urban planning wastes provide a sustainable solution for urban planners to design urban green spaces 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.

Results/Conclusions

After eighteen months maturation, while topsoil addition did not affect stabilization nor N-availability, total organic nitrogen became gradually available, especially in mixtures that included intermediate to large amounts of compost.

This was explained by a significant increase in nitrification, and a drastic reduction and stabilization at low rate of denitrification, which limited gaseous N-losses. The increases of NEA and organic nitrogen were positively correlated to ammonia oxidizing bacteria abundances. Although compost input higher abundances of nirK denitrifiers, these populations did not vary with seasons, nor with maturation. Similarly, and despite seasonal variations associated to vegetation, free-living N-fixation was highest at intermediate proportions of compost, highlighting its sensitivity to high organic matter inputs.

Plant species richness and diversity monitored during 2021 showed no significant difference associated to soil mixture. However in July, mixtures harboring large amounts of compost and topsoil significantly promoted the development of a more diverse plant community, as compared to the mixture including only deep soil.

Our results suggest microbial indicators of soil functioning are truly relevant to monitor biological fertility of constructed soils in urban ecosystems.