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.