Salix×jiangsuensis 'J172' enhanced lead tolerance under flooding stress by regulating the rhizosphere bacterial assembly and functions.

Lead (Pb) contamination threaten soil ecosystems, and the increased risk of flooding due to climate change may influence soil functions and metal bioavailability. However, the response of rhizosphere soil microbial communities to these combined stresses remains unclear. Here, fast-growing willow (Salix × jiangsuensis 'J172') was planted in soils with Pb contamination (control, 400 and 800 mg · kg-1) under non-flooded (NF) and flooded (IF: intermittent flooding and CF: continuous flooding) conditions for 60 d. At Pb800 contamination, IF and CF markedly decreased the soil available Pb by 17.7 % and 14.1 % (p < 0.05), respectively, compared to NF condition. Flooding (CF and IF) conditions increased bacterial phylogenetic diversity compared to NF, with IF exhibiting greater impact (10.6 %, p < 0.05). For assembly process, the bacterial community under CF condition exhibited a shift towards a more deterministic process compared to NF, although stochastic mechanisms were still dominant (normalized stochasticity ratio >0.5). Increased abundance of Pb tolerance genes and sulfate-reduction function under CF conditions promoted a decrease in soil Pb availability (R2 > 0.5, p < 0.05). Compared to NF, the bacterial functional redundancy index, which characterizes the stability of ecological functions, was reduced by 7.1-25.9 % under CF condition, regardless of Pb contaminations, while IF increased this index with Pb800. The findings provide valuable insight into the mechanisms underlying the response of soil bacterial communities for the studied Salix species to combined stresses of flooding and Pb, and enhance the comprehension of rhizosphere micro-ecological processes in remediation woody plants under complex multi-stress exposure scenarios.