Decoupling of plant and soil metal nutrients as affected by nitrogen addition in a meadow steppe

Aims Determining base cation and trace element dynamics in plant-soil systems is important for sustainable utilization of grasslands affected by global ecosystem nitrogen (N) enrichment. Methods A 4-year field experiment was conducted with urea addition rates of 0, 2.5, 5, 7.5, 10, 15, 20, and 30 g N m(-2) yr.(-1) to investigate the dynamics of base cations (Ca, Mg, K) and trace elements (Fe, Mn, Cu and Zn) in soil and a dominant species Leymus chinensis in a semi-arid meadow grassland of northeast China. Results Soil showed a significant decrease in exchangeable Ca and increases in extractable Fe, Mn and Zn concentrations, while plant Ca and Zn remained unchanged and plant Fe decreased with N addition. For both plant and soil compartments, stoichiometric ratios of trace elements were more variable than base cations, mainly driven by plant physiological requirements, plant nutrient uptake intensity, and elemental interactions. Excessive soil Mn mobilization and plant uptake affected concentrations of Fe, Cu and Zn in the plant and even inhibited Fe absorption (antagonistic interactions), resulting in a plant Fe:Mn ratio lower than 1.5 with high N addition rates, indicating Mn phytotoxicity or Fe deficiency. As a result, plant elemental stoichiometry did not remain homeostatic and was even more variable than in soil. Conclusions Our results provided direct evidence of decoupled plant-soil elemental responses, stricter homeostatic control of macronutrients than trace elements in plants, and altered stoichiometric variability in the plant-soil system, and suggest that N addition enhances metal nutrient imbalances in the meadow steppe grassland.