Improvement of Ni phytoextraction by Alyssum murale and its rhizosphere microbial activities by applying nitrogen fertilizer

Phytoextraction represents an innovative approach in the management of nickel (Ni) rich soils whether natural (ultramafic) or anthropogenic (contaminated sites). However, its success depends both on the production of a high plant biomass and the ability of plants to accumulate metals. The application of nitrogen (N) fertilizer can improve the biological and chemical soil fertility and thus agricultural yields. Moreover, soil microorganisms play a key role by influencing nutrient flows, which are the main limiting factors of plant growth in degraded soils. In this work, we investigated the effects of two levels of both Ni and mineral N soil applications on the microbial activities and Ni phytoextraction efficiency by Alyssum murale growing in a pot experiment during 5 months. Plant growth, nutrients and Ni uptake, soil microbial populations and their enzymatic activities involved in the biogeochemical cycles of nitrogen, phosphorus, carbon and sulfur (urease, alkaline phosphatase, β-glucosidase and arylsulfatase, respectively) were determined. The results showed that plant dry mass was unsurprisingly not affected when the soil Ni concentration was increased. However, it led to an increase of the amount of Ni extracted per pot. A negative effect of Ni addition was observed on both total bacteria and urease activity, without any effect on other enzymes. On the contrary, N fertilizer played a significant positive role by promoting both plant growth and Ni phytoextraction, partly as a result of the stimulation and flourishing of bacterial populations.