Nitrogen fixation and growth of Lens culinaris as affected by nickel availability: A pre-requisite for optimization of agromining

• Legumes could be grown with hyperaccumulator plants to enhance soil quality and fertility in the context of agromining crops (nickel phytomining).
• Among the factors that create the serpentine syndrome, Ni toxicity is the most difficult to correct through soil amendments or practices.
• No previous study has tested whether lentils (Lens culinaris) are able to fix atmospheric N2 on nickel-rich soils.
• The ability of lentils to form functional nodules to fix nitrogen is negatively affected even at very low levels of available Ni; it is completely suppressed when ≈50 mg DTPA-Ni kg−1 is found in the soil.
• The use of lentils as a N-provider to the agromining crop must be limited to ultramafic soils with low or moderate Ni availability.

Low soil fertility in ultramafic soils limits the efficiency of nickel phytoextraction. Developing more efficient cropping systems for agromining can be achieved by the association of a hyperaccumulator with a legume by enhancing soil fertility. However, legume crops can result sensitive to ultramafic soil conditions, including nickel Ni availability. We assessed here whether Lens culinaris is adapted to ultramafic environments by growing on soils displaying a wide range of Ni concentrations and consequently producing functional nodules. The soil was enriched with different Ni concentrations ([Ni]) (0–90 mg Ni kg−1). Natural 15N abundance was used to assess N2 fixation (%Ndfa). Biotic parameters were investigated (nodule number, Ni, carbon and nitrogen concentrations, plant biomass…). Soil parameters were investigated (total [Ni], DTPA-extractable Ni, C and N concentrations…). Most of the physicochemical and biological parameters were significantly affected by the increased soil [Ni]. Nodule numbers per plant was lower under high [Ni] than control (soil without Ni). Nodules lost their capacities to fix N2 under high Ni addition (90 mg Ni kg−1). For many parameters, there were no significant differences between control and treatments up to 60 mg of Ni kg−1 added to the soil. Lentil is able to grow on a soil containing amounts of Ni-DTPA similar to those generally found in serpentine soils. It could be used in association with a hyperaccumulator plant as a nitrogen provider in order to optimize Ni agromining.