A meta-analysis of the impacts of nitrogen addition on plant multiple-element contents in natural ecosystems

Numerous studies have exhibited that nitrogen (N) addition significantly influenced terrestrial plant N and phosphorus (P) concentration. However, to date, the impacts of N addition on global plant macronutrients (K, Ca, Mg, S) and micronutrients (Na, Al, Fe, Cu, Mn, Zn,), especially microelement, have not been clearly understood. Here, we conducted a meta-analysis to explore the responses of ten variables associated with terrestrial plant nutrients to N addition. Results indicated that N enrichment produced a significant increase in plant Mg, Mn, Cu and Zn concentrations by 3.44, 33.39, 5.01 and 10.24%, respectively, and a notable decrease in plant Ca and Fe concentrations by 2.67 and 4.95%. Among different ecosystems, plant K, Ca, Mg, S, Mn, Cu and Zn exhibited significant increases in grassland, plant K and Ca showed greater decreases in subtropical and boreal forests, and plant Mn and Al notably increased in subtropical forests after N addition. Similarly, N addition significantly enhanced K, Mg, S, Mn, Cu and Zn by 8.61, 12.98, 22.09, 46.02, 6.40 and 11.83% in herbaceous, but not in woody plants. Besides, our results exhibited that environmental factors (ambient N deposition, mean annual temperate, mean annual precipitation, elevation and soil pH values) and experimental factors (N addition rate and form) affected plant nutrients responses to N enrichment. In summary, our study exhibited that N enrichment altered terrestrial plant nutrient contents, with the magnitude and direction of the responses differing among ecosystem type, plant functional group and N addition rate, which have great significance for understanding plant growth, survival strategies and nutrient cycling under increasing ambient N deposition.