Elevated nitrogen (N) deposition has been show to have a variety of effects on to forest ecosystems, including nutrient imbalance, growth disturbance, and foliar accumulation of free amino acids in individual plants. The purpose of this study is to determine the effects of simulated N deposition on free amino acids and soluble protein of three dominant understory plants (Randia canthioides, Cryptocarya concinna, and Cryptocarya chinensis) in a monsoon evergreen broad\|leaved forest of subtropical China. The experimental design was a randomized complete block with three replications (sets of plots) and four N treatments. The treatments consisted of Control (0 kg hm-2 a-1), Low N (50 kg hm-2 a-1), Medium N (100 kg hm-2 a-1) and High N (150 kg hm-2 a-1). Each plot covered an area of 10×20 m2 surrounded by a 10-m-wide buffer zone. From July 2003, NH4NO3 solution, with concentrations of about 0, 0.3, 0.6 and 0.9 mol N•L-1 respectively for Control, Low N, Medium N and High N treatments, was sprayed onto the forest floor of these plots once every month. Here, we reported the results after 21 months of treatment.
Results indicated that nitrogen addition at all levels affected the free amino acid concentrations and soluble protein concentrations of the three tree species. The composition of the free amino acid pool and the dominant amino acids varied with species. The dominant amino acids by species were: aspartic acid, serine, glutamic acid, arginine and γ-aminobutyric acid (γ-GABA) for Randia canthioides; aspartic acid, serine and γ-aminobutyric acid for Cryptocarya concinna; and aspartic acid, serine, glutamic acid and γ-aminobutyric acid for Cryptocarya chinensis. For Randia canthioides and Cryptocarya chinensis, the responses of the total free amino acid concentrations to simulated N deposition were similar, with the concentrations in trees of N-treated plots higher than those of the Control. For Cryptocarya concinna, the total free amino acid concentrations showed a decreasing trend with increasing N additions, but the differences between treatments were not significant. The soluble protein contents of the three trees increased by N treatments, and the soluble protein contents were significantly higher in high and medium N treatments than in control plots for Cryptocarya concinna and Cryptocarya chinensis (p<0.05). The study also revealed some indicators for detecting the (impacts) of N deposition on forest trees. Good indications of enhanced N deposition were concentrations of: serine, γ-aminobutyric acid, and especially arginine for Randia canthioides; γ-aminobutyric acid for Cryptocarya concinna; and serine and γ-aminobutyric acid for Cryptocarya chinensis. In addition, the possible effects of the γ-aminobutyric acid contents on insect herbivores are discussed.