Nitrogen fixation by white clover ( Trifolium repens ) in grasslands on soils contaminated with cadmium, lead and zinc

Summary The nitrogenase activity (potential nitrogen fixation) of a mixed white clover‐grass sward growing on a range of soils containing up to 216 μg g −1 Cd, 30 000μg g −1 Pb and 20 000 μg g −1 Zn was determined using the acetylene‐reduction assay. The plants were incubated in situ using an intact soil‐core technique. Little change in the rate of C 2 H 2 reduction was observed during the daylight hours although a marked seasonal fluctuation was found, the maximum activity during the spring and declining to 20% of this by the autumn. Some reductions of nodule and plant size, and in nitrogenase activity, was observed in the most heavily contaminated sites. These effects were small; the plants and nodules otherwise appeared healthy. The potential for nitrogen fixation was equally high in all the sites at up to 80 g N ha −1 h −1 in the spring.