Effects of nitrogen addition on leaf nutrient stoichiometry in an old‐growth boreal forest

Abstract

AbstractBoreal forests have been evidenced to be highly sensitive to enhanced nitrogen (N) deposition due to prevailing N limitations, and external N inputs from atmospheric deposition are expected to alter plant nutrient stoichiometry. Previous studies have mostly focused on the dominant tree species while neglecting understory plants that often play important role in the nutrient cycles in forest ecosystems. By conducting a six‐year N‐addition experiment with four treatments of 0 (control), 20 (low N), 50 (medium N), and 100 (high N) kg N·ha−1·yr−1in a boreal forest in Northeast China, we assessed the responses of leaf nutrient stoichiometry (N, phosphorus [P], potassium [K], calcium [Ca], and magnesium [Mg]) for tree, shrubs, and grass. Although the responses of different species to N addition varied, six‐year N addition, especially the medium and high N treatments, generally increased the leaf N concentration and decreased the leaf P and Ca concentrations. As a result, the foliar N:P, N:K, N:Ca, and N:Mg ratios increased consistently across plant functional groups under the high N addition compared with the control, and the grassDeyeuxia angustifoliashowed a larger increase in its foliar N:P, N:Ca, and N:Mg than the shrubs. The leaf N concentration increased consistently with soil inorganic N in a nonlinear saturating form, while the other leaf nutrients either decreased (P, Ca) or were not affected (Mg, K) by the soil N availability. Consequently, the foliar N:P, N:K, N:Ca, and N:Mg ratios increased linearly with increasing soil N availability. In spite of this alteration of the leaf nutrient stoichiometry with increasing soil N availability, foliar nutrients and their stoichiometry were less affected under the low N treatment in both trees and understory plants, suggesting minor effects of current N deposition rates on the foliar nutrient balance in boreal forests.