Response of diameter growth, biomass allocation and N uptake to N fertigation in a triploid Populus tomentosa plantation in the North China Plain: Ontogenetic shift does not exclude plasticity

A field experiment was conducted in 2010–2012 to investigate the effects of different N fertigation regimes on diameter growth (measured as individual tree basal area increment), N uptake and biomass allocation in a triploid Populus tomentosa plantation in the North China Plain. The experiment included six N fertigation treatments (combination of three N application rates [NR:115 (N115), 230 (N230) or 345 (N345) kg ha-1 year-1] and two application frequencies [NF: two (F2) or four times (F4) per year] and a single irrigation control treatment. Diameter at breast height, biomass and N uptake were monitored. N fertigation significantly increased aboveground diameter and N uptake compared with the control. Use of NR above 115 kg N ha-1 year-1 did not result in growth and N uptake benefits. NF showed almost no effect on tree growth, but F4 resulted in significantly higher N uptake. N fertigation resulted in a shift in biomass allocation from roots to leaves, but had no effect on relative allocation to wood biomass, which was significantly influenced by ontogeny (size effect). Biomass allocation of triploid P. tomentosa can be altered by N fertigation to some extent, but this was also under ontogenic control. Higher NR (N230 or N345) and NF (F4) are not necessary to achieve optimum growth, but higher NF is required for optimal N uptake. N115F4 is recommended as the N fertigation regime for triploid P. tomentosa as this results in higher N uptake and optimal growth