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

Rare earths are applied widely in Chinese agriculture to improve crop nutri tion and incidentally in fertilizers, yet little is known of their effect o n the biological functioning of the soil. We have studied the effects of la nthanum and of mixtures of rare earths on the potential ammonium oxidation and nitrogen mineralization in soil by incubation experiments in the labora tory. The no-observed-effect and median effective concentrations of the rar e earths on these two processes are reported, and their corresponding mecha nisms are discussed. For mixtures, the no-observed-effect concentrations re lating to potential ammonium oxidation and N mineralization were at 393 and 373 mg rare earths kg(-1) soil, respectively, and their median effective c oncentrations were at 1576 and 110 mg kg(-1) soil, respectively. When lanth anum was applied alone, the no-observed-effect concentrations relating to p otential ammonium oxidation and N mineralization were at 432 and 443 mg La kg(-1) soil, respectively, and their median effective concentrations were a t 18 212 and 1237 mg kg(-1) soil, respectively. Therefore, the influence of mixtures of rare earths on potential ammonium oxidation or on N mineraliza tion was slightly stronger in comparison with that of lanthanum. Mineraliza tion of nitrogen is apparently more sensitive to the stress caused by rare earths than ammonium oxidation. We conclude that the influence of individua l rare earths in the mixtures on the above two processes can be additive an d 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 t he potential ammonium oxidation and N mineralization in the soil even over a long period.