Experimental design
In December 2003, 12 experimental plots were established in a 6-ha section of the Chinese fir plantation. Each plot was 20 m × 20 m and had similar site conditions. In each plot, a central area of 15 m × 15 m was designated for treatments, arranging one of four levels of N. The treatment codes N0, N1, N2, and N3 indicated the loading of 0, 60, 120, and 240 kg N ha-1 yr-1, respectively. Each treatment was represented by three randomly assigned replicate plots. Urea [CO(NH2)2] was used as the N source, and the required quantity was dissolved in 20 L of water per plot. From January 2004 to January 2014 (when plots were sampled), 20 L of N solution was sprayed onto the soil surface in the central area (15 m ×15 m) of each plot once per month. Control plots (N0) were sprayed with the same volume of water without CO(NH2)2.