Effects of lanthanum and mixtures of rare earths on ammonium oxidation and mineralization of nitrogen in soil

Summary Rare earths are applied widely in Chinese agriculture to improve crop nutrition and incidentally in fertilizers, yet little is known of their effect on the biological functioning of the soil. We have studied the effects of lanthanum and of mixtures of rare earths on the potential ammonium oxidation and nitrogen mineralization in soil by incubation experiments in the laboratory. The no‐observed‐effect and median effective concentrations of the rare earths on these two processes are reported, and their corresponding mechanisms are discussed. For mixtures, the no‐observed‐effect concentrations relating to potential ammonium oxidation and N mineralization were at 393 and 373 mg rare earths kg −1 soil, respectively, and their median effective concentrations were at 1576 and 1108 mg kg −1 soil, respectively. When lanthanum was applied alone, the no‐observed‐effect concentrations relating to potential ammonium oxidation and N mineralization were at 432 and 443 mg La kg −1 soil, respectively, and their median effective concentrations were at 18 212 and 1237 mg kg −1 soil, respectively. Therefore, the influence of mixtures of rare earths on potential ammonium oxidation or on N mineralization was slightly stronger in comparison with that of lanthanum. Mineralization of nitrogen is apparently more sensitive to the stress caused by rare earths than ammonium oxidation. We conclude that the influence of individual rare earths in the mixtures on the above two processes can be additive and that the present dosage of mixed rare earths (< 230 g ha −1 year −1 or 0.15 mg kg −1 soil year −1 ) currently applied in China can hardly affect the potential ammonium oxidation and N mineralization in the soil even over a long period.