**2. Materials and Methods**

### *2.1. The Experimental Site and Design*

Our experimental site is located at the Dongtai Forest Farm in Yancheng, Jiangsu Province, eastern China (120◦49 E, 32◦52 N). The farm is close to the Yellow Sea State Forest Park (on the coast of the Yellow Sea) and has a climate classified as Cfa (Humid subtropical climate) according to Köppen [40]. The mean annual temperature (MAT) is 13.7 ◦C and the mean annual precipitation (MAP) is 1051 mm. The soil of the forest farm is a desalting meadow and sandy soil with a pH value ~8 [41].

Our N addition experiment was established in May 2012. We chose 12-year-old pure poplar plantations (*Populus deltoids cv. 'I-35'*) with uniform site conditions and managemen<sup>t</sup> measures as our plots. We used a randomized block design with a gradient of five levels of N addition (0, 50, 100, 150, and 300 kg N ha−<sup>1</sup> year<sup>−</sup>1) in three replicate blocks (25 × 190 m). Each N treatment subplot was 25 × 30 m with a 10 m buffer zone between any two adjacent subplots. The distance between any two adjacent blocks was at least 500 m. We chose a range of N addition rates because the ambient N deposition rate is about 50 kg N ha−<sup>1</sup> year<sup>−</sup><sup>1</sup> for this area [42] and Liu et al. [18] estimated that the critical loads for N deposition in Jiangsu province could be more than 200 kg N ha−<sup>1</sup> year<sup>−</sup>1. In each month of the growing season (May through October), one-sixth of the yearly amount of NH4NO3 was dissolved in 20 L water and sprayed evenly under the canopy by backpack sprayer to simulate natural N deposition. Each control subplot received 20 L of water.
