Cadmium in New Zealand pasture soils: toxicity to <i>Rhizobia</i> and white clover

<p>A survey of 26 pastures was conducted and soil and plant material analysed for total cadmium (Cd) and phosphorus (P), and soils further analysed for a variety of edaphic properties. Background Cd in pasture soils ranged from 0.06–0.85 mg Cd kg⁻¹ soil (mean of 0.23) and correlated closely with total P (<i>R</i>² = 65.5; <i>P</i> < 0.001), while weaker relationships with Olsen P, nitrate-N and calcium were evident. Cadmium in the pasture dry matter had no correlation to underlying soil total Cd (<i>P</i> = 0.113) or soil P (<i>P</i> = 0.75). Strains of <i>Rhizobium</i> spp. were isolated from a reference soil high in Cd (Morrinsville; 1.34 mg Cd kg⁻¹ soil) and a mid-range Cd soil (Whataroa; 0.32 mg Cd kg⁻¹ soil). The IC₅₀ of these strains, along with the commercial inoculant for white clover (<i>Rhizobium leguminosarum</i> bv. <i>trifolii</i> TA1), to Cd were tested using a respiration sensitive assay (MicroResp). Rhizobia from the Whataroa soil had significantly greater tolerance to Cd than those from Morrinsville soil (<i>P</i> = 0.008). We propose a general stress-tolerance mechanism has evolved in the rhizobia from Whataroa, conferring metal co-tolerance. The Cd IC₅₀ of <i>Rhizobium</i> TA1 was 3.06 ppm, but levels of respiration inhibition were evident at lower concentrations. Given the importance of the legume:rhizobia symbiosis to New Zealand's pastoral sector, determination of ‘no observable effect concentrations' is required for <i>Rhizobium</i> spp., pasture legumes and, most importantly, the establishment and function of symbiosis (nodulation and N₂ fixation, respectively).