Cadmium in New Zealand pasture soils: toxicity to Rhizobia and white clover

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 (R-2 = 65.5; P < 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 (P = 0.113) or soil P (P = 0.75). Strains of Rhizobium 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 (Rhizobium leguminosarum bv. trifolii TA1), to Cd were tested using a respiration sensitive assay (MicroResp(TM)). Rhizobia from the Whataroa soil had significantly greater tolerance to Cd than those from Morrinsville soil (P = 0.008). We propose a general stress-tolerance mechanism has evolved in the rhizobia from Whataroa, conferring metal co-tolerance. The Cd IC50 of Rhizobium 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 Rhizobium spp., pasture legumes and, most importantly, the establishment and function of symbiosis (nodulation and N-2 fixation, respectively).