NPK fertilization at planting of three hybrid poplar clones in the boreal region of Alberta

Trees from three hybrid poplar clones (one Populus balsamifera × P. simonii clone and two P. deltoides × P. × petrowskyana clones), were grown for 3 years in an agricultural field and fertilized at planting with 27 combinations of nitrogen (N), phosphorus (P) and potassium (K) fertilizers. Fertilizers used were granular ammonium nitrate (34.5–0–0) at three levels (0, 8 and 16 g/tree), triple-super-phosphate (0–45–0) at three levels (0, 12.5 and 25 g/tree), and potassium sulfate (0–0–50) at three levels (0, 8 and 16 g /tree). Growth responses to N fertilization were negative during the first growing season: N fertilization reduced growth, net assimilation rate (A), stomatal conductance (Gs), and increased the abundance of 13C in the leaves. These results might be related to the soil pH at the site which ranged from 7.7 to 8.1, and to the dry conditions that prevailed during the first growing season. Leaf N concentrations were unaffected by the fertilization level. Conversely, N fertilization decreased leaf P concentrations, which were in turn positively correlated with growth and negatively correlated with carbon isotopic composition (δ13C). There were clonal differences in δ13C that varied between the first and second growing seasons in relation to the relative growth rates of the clones. Growth responses to fertilization at planting were nil or slightly positive during the second and third growing seasons. These results indicated that ammonium nitrate was unsuitable for fertilizing these hybrid poplar clones at planting, and that further study is required to test fertilization at planting with nitrate-N versus ammonium-N sources under these field conditions.