Toxicity thresholds for oat (Avena sativa L.) grown in Ni-impacted agricultural soils near Port Colborne, Ontario, Canada

This study established Ni phytotoxicity thresholds for oat (Avena sativa L.) in four soil types, each created by blending a low and a high Ni soil, to generate a range of concentrations. The first quartile effective concentration (EC25) for soil and shoot tissue Ni concentration and reduction in shoot dry weight (DW) was determined using a Weibull function. The EC25 (for soil Ni concentration) was 1350, 1950, 1880 and > 2400 mg Ni kg(-1) soil, for sand, till clay, heavy clay and organic muck, respectively. The EC25 (for shoot Ni concentration) was 71, 21, 52 and >35 mg Ni kg(-1). shoot DW, for sand, till clay, heavy clay and organic muck, respectively. Total soil Ni concentration, soil pH and soil cation exchange capacity (CEC) accounted for 70% of the variation of Ni accumulation in tissue when the data for all four of the soils were combined; this was similar to the amount of variation accounted for by fitting Ni concentration in tissue to ammonium oxalate extractable soil Ni. Manganese deficiency may have impaired plant growth at higher soil Ni concentrations in the clay soils. Speciation of Ni was similar in all soils studied, and the relationship between Ni concentrations in soil and in tissue was less closely related to chemically extracted soil Ni than it was to a combination of total soil Ni, soil pH and CEC. These are the soil characteristics known to influence both equilibrium among metal species in soil solution, and uptake of cations by plants.