Effect of herbicide residues in a sandy loam on the growth, nodulation and nitrogenase activity (C₂H₂/C₂H₄) of Trifolium subterraneum

The herbicides 2,4-D, amitrole, atrazine, diclofop-methyl, diquat, paraquat and trifiluralin were applied at rates of 0, 2, 5 and 10 μg ai. g^-1 to a sandy loam soil and allowed to degrade for 120 days. After this period, subterranean clover seedlings were transplanted into treated soil and the effect of herbicide residues on plant growth, number of nodules formed and nitrogenase activity was investigated. At all rates of atrazine and chlorsulfuron, and at all rates of amitrole in excess of 2 mg ai g^-1 of soil, sufficient herbicide remained to be lethal to the seedlings. When amitrole was applied at the rate of 2 mg ai g^-1 of soil, plant growth, nodulation and nitrogenase activity of plants were reduced. Residues of diquat reduced all plant parameters studied while, residues of 2,4-D reduced plant growth and nodule formation, but plant nitrogenase activity was unaffected. Residues of trifluralin had no effect on plant growth parameters but the number of nodules formed per plant was reduced. Residues of paraquat and diclofop-methyl had no effect on any of the plant parameters studied.