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.