Adding invasive alien plant-derived biochar and stinging nettle powder in Populus nigra phytoremediation of arsenic- and lead-contaminated Technosol alters microbial community assembly and network stability

This study examined the effects of biochar and nettle Urtica dioica amendments on microbial diversity and community assembly in a historically contaminated Technosol collected from a former silver‑lead mine in Pontgibaud (Puy-de-Dôme, France). We investigated how two types of biochar obtained from the biomass pyrolysis of two invasive alien plant species (Ailanthus altissima and Solidago gigantea) at different doses (2 % vs 5 %), alone and in combination with stinging nettle (Urtica dioica) powder inputs that influence the soil's microbial community structure under controlled phytoremediation conditions using Populus nigra. We integrated niche theory and network analysis to examine how these amendments alter community-level cohesion and resilience under heavy metal stress, with implications for phytoremediation. We found that a 5 % biochar dose imposed stronger environmental pressure compared to a 2 % dose, resulting in a significant increase in soil alkalinity and electrical conductivity (EC). In amended soils, bacterial community assembly was primarily driven by stochastic processes, mainly due to dispersal. Fungal communities, in contrast, exhibited reduced stochasticity, particularly under the 5 % biochar–Solidago treatments. Furthermore, bacteria expanded their niche width, whereas fungi shifted toward specialist-dominated, narrower niches with greater overlap. The biochar co-application with stinging nettle increased network complexity for both microbial groups; however, the bacterial network responded with higher modularity and more negative links, while the fungal networks were non-modular and exhibited increased positive feedback loops. Network robustness analysis revealed that bacterial networks remained more stable under perturbation across all treatments, whereas fungal networks were more sensitive to hub node loss. Our findings suggest that both the concentration and type of biochar, as well as the presence of Urtica dioica amendment, distinctly affect microbial communities and should be carefully considered in designing optimal application regimes and strategies for soil remediation.