2.2 Experimental design
A randomized block design with complete factorials for N and P addition was used in this experiment, which was set up in early April 2012. Three levels of N addition were crossed with three levels of P addition, producing nine different nutrient addition treatments. For the N addition, the three levels were ambient N (no N added, N m-2 yr-1), low N addition ( N m-2 yr-1), and high N addition ( N m-2 yr-1); likewise for P addition, its three levels were ambient P (no P added, P m-2yr-1), low P addition ( P m-2yr-1), and high P addition ( P m-2yr-1). The field experiment consisted of four blocks, with each having nine plots randomly assigned to the nine nutrient addition combinations. Each plot was × in size and the distance between any two adjacent plots within each block was . Nutrient addition was applied to plots annually, in July. Ammonium nitrate (NH4NO3) and calcium superphosphate (CaH4P2O8) were used in the N and P addition plots, respectively.