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

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Abstract
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 C2H2 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.