Cadmium uptake by ryegrass and ryegrass-clover mixtures under different liming rates

Cadmium accumulates in soils that receive repeated applications of Cd-rich superphosphate fertilizers. There is evidence that adding clovers to a crop solubilizes soil Cd, increasing the bioavailability of Cd. This can lead to high plant Cd concentrations. This research aimed to test whether liming-induced increases in pH in mixed crops of clovers and ryegrasses reduced forage Cd concentrations. A greenhouse pot trial applied lime at three rates (0, 1, and 2% of soil dry weight) to eight different plant treatments-four as monocultures (perennial ryegrass [ L.], Italian ryegrass [ Lam.], white clover [ L.], and red clover [ L.]) and four as ryegrass-clover mixtures (two plant types in each treatment)-in soil (initial soil pH = 5.1, initial soil Cd concentration = 1.31 mg kg) with added Cd (CdSO ∼ 1 mg kg). Adding lime increased soil pH in both mono- and mixed crops and, in most treatments, increased forage yields. However, the relationship between forage Cd and soil pH differed between plant treatments. In mono- and mixed crop treatments containing perennial ryegrass, adding lime increased the forage yield but did not increase the mass of Cd in the plants compared with the no-lime treatment. However, adding lime to treatments that included Italian ryegrass increased both the forage yield and the Cd compared with the no-lime treatment. The results show that a combination of certain plant species composition and lime rates can optimize forage yields without increasing forage Cd concentrations.