Nitrogen addition effects on tree growth and soil properties mediated by soil phosphorus availability and tree species identity

Abstract

Increasing evidence has suggested that nitrogen (N) deposition has great effects on forest productivity and nutrient cycling, but how these effects are mediated by soil phosphorus (P) availability and tree species identity remains unclear. Here, a two-year factorial experiment was conducted to examine the short-term effects of N addition (100 kg N ha−1 year−1), P addition (50 kg P ha−1 year−1) and their interactions on tree growth and soil nutrient cycling of a Mongolian pine (Pinus sylvestris var. mongolica) plantation and a neighboring Simon poplar (Populus simonii) plantation in Northeast China. Results showed that N addition promoted tree growth (stem increment), reduced soil microbial biomass carbon (C) and N in the Mongolian pine plantation, but did not affect those in the Simon poplar plantation, and these N addition effects were not interacted by P addition. P addition significantly increased microbial biomass C and N in the Simon poplar plantation. N addition elevated soil phosphatase activities and reduced net N mineralization rate, and these N addition effects were alleviated by simultaneous P addition in both plantations. N addition decreased soil labile inorganic P concentration in both plantations and elevated labile organic P in the Mongolian pine plantation, but only the decrease in labile inorganic P with N addition in the Mongolian pine plantation was counteracted by P addition. N addition significantly reduced soil pH, with a greater extent in the Mongolian pine plantation than in the Simon poplar plantation. Moreover, soil pH response explained most of the microbial and nutrient transformation response. Above results indicated that short-term N addition impacts on tree growth and soil microbial biomass were more strongly influenced by tree species identity than by P addition, while N addition effects on soil N and P transformations were strongly influenced by P addition. Drought resistance of trees and acid buffering capacity of soils are key traits influencing the short-term N addition impacts on the growth of trees and microorganisms in this semiarid region.