2.2 Experimental design
The simulated N deposition experiment commenced in 2014, the sample plots were treated with N application from June to mid-September. The required NH4NO3 for each treatment was dissolved in 20 L of water, and CK only adds 20 L of water. Each sample square 5 m × 30 m, bounded by a 10 m wide buffer strip, with a total of four gradients of CK (0 kg N ha−1year−1), LN (20 kg N ha−1 year−1), MN (40 kg N ha−1 year−1) and HN (80 kg N ha−1 year−1) ((Song, Zhang, Müller , & Jin, 2019). There were three replicates in each treatment with completely randomized design.