Serpentine soils are characterized by high nickel (Ni) contents, attributed to their particular geological background. This provides an ecological niche for a specific group of metal-accumulating plants, called hyperaccumulators, which have been investigated for a couple of decades to identify the underlying mechanisms of trace metal solubilisation, accumulation and tolerance. The influence of soil characteristics on the Ni accumulation potential is still insufficiently known. Therefore, a study based on field-collected soil and plant samples was carried out for assessing the relationship between soil characteristics and plant Ni concentration in the hyperaccumulator Noccaea goesingensis.
In the east of Austria two gradients of Ni concentration and soil formation were evaluated, each consisting of three sampling locations. On each location 6 plant and rhizosphere samples and one composite bulk soil sample were collected and analyzed for their element concentrations as well as physicochemical and microbiological characteristics. Standard soil parameters (C, N, pH, CEC, etc.) as well as concentrations of operationally defined Ni fractions were recorded (extracted by H2O, Sr(NO3)2, DTPA, and assessed by the diffusive gradients in thin films technique (DGT)). Furthermore, the bacterial density and diversity were determined for bulk and rhizosphere soil samples as well as root samples. Functional bacterial diversity was assessed by cultivation in Biolog EcoPlatesTM.
Along the first gradient, a decrease of total Ni as well as pH was found, whereas organic carbon contents and CEC increased. Changes along the second gradient were less pronounced. Characteristics of plant-free bulk soil samples were correlated with shoot Ni concentrations. Among the assessed soil Ni fractions in bulk soil samples from each sampling spot, the highest correlations with shoot Ni concentrations were found for water-extractable Ni (r2 = 0.85) and total Ni (r2 = 0.82). Non-significant (p < 0.05) correlations were found for DTPA (r2 = 0.59) and Sr(NO3)2-extractable Ni (r2 = 0.56). In rhizosphere soils of all except one sampling location DTPA-extractable Ni concentrations were higher compared to bulk soil values. No clear trend was found for Sr(NO3)2- and water-extractable Ni as well as pH and Ni resupply (measured by the DGT technique).
Bacterial density and diversity was higher in rhizosphere soils compared to bulk soils. The diversity of root endophytes was lower than in the corresponding rhizosphere soils. The density of Ni-tolerant bacteria was significantly correlated with total Ni concentration (r2 = 0.9, p < 0.05).
Our results suggest that total Nickel concentration in soil is the most significant factor influencing plant Ni accumulation and rhizosphere bacterial community characteristics.