Nodule Formation and Nitrogen Fixation in Acacia holosericea Plants Grown in Soil Admixed with Iron Ore Tailings

Legume plants such as Acacia spp. could form nodules and fix N 2 from the atmosphere to facilitate N acquisition, and has great potential for revegetation of iron (Fe) ore tailings. However, it is not clear if Acacia spp. and their association with N 2 -fixing bacteria could overcome adverse physicochemical conditions in the tailings. The present study aimed to investigate whether a native pioneer leguminous species, Acacia holosericea , could form nodules and acquire N through N 2 fixation when grown in soils with different levels of tailing stress. In the greenhouse, A. holosericea was grown in sterilized river sand for 4 months and then transplanted into the soil-tailing mixture with different soil:tailing ratios for additional 3 months’ cultivation. We assessed plant growth, nutrition, nodulation, and N 2 fixation (isotopic 15 N dilution method) in soils admixed with different amount of Fe ore tailings. The tailings addition significantly limited plant growth, nodule formation, and nitrogen acquisition, compared to control soil. The amount of nitrogen fixed decreased with tailings stress from 34.6 mg per plant in the soil treatment to 12.7 mg per plant in the tailing treatment. The tailings (pH ~ 9) elevated pH in the soil from 6.7 to above 8, when the soil was admixed with 25 to 75% w/w tailings. Apart from the stress caused by alkaline pH conditions, the K toxicity and the slightly enriched NO 3 − in tailings may also restrain nodulation and N 2 fixation. Even early eco-engineered tailings cannot facilitate the symbiotic association and N 2 fixation due to continuous alkaline and elevated saline conditions. These findings collectively demonstrated that the prevailing alkaline conditions and excess salt elements derived from tailings conditions limit plant growth, nodule formation, and N 2 fixation in A. holosericea . Further research is needed to improve tailing physicochemical conditions, and identify other leguminous species and associated N 2 -fixing bacteria that are tolerant of alkaline and saline conditions to facilitate revegetation in the tailings.