Fall nitrogen fertilization and the biology of Pinus taeda seedling development
In mid-September when stems and roots of nursery-grown loblolly pine (Pinus taeda L.) seedlings are actively accumulating dry weight (DW), an extra 10, 20, or 40 kg NH4NO3 ha-1 (10N, 20N, 4ON) was applied. Seedlings receiving no extra N (0N) were the controls. The temporal patterns of seedling growth, nutrient concentrations, and sugar-metabolizing enzyme activities were determined during that fall and winter to assess the dynamics of seedling vigor. The 40N-treated seedlings had significantly fewer culls and greater first-order lateral root numbers, root collar diameter, DW of needle, stem, and root, and N concentration (percentage of DW) and content (milligrams per seedling) than controls and the 10N-treated seedlings. However, the temporal patterns of DW allocation, sugar metabolism, or the concentrations of P, K, Mg, and Ca were not affected by fall N fertilization. These results fit the hypothesis that basic plant morphological and biochemical processes, e.g., periodicity in stem and root DW growth or in stem and root sucrolysis and glycolysis, were not altered by human-made changes such as fall N fertilization. Fall N fertilization near 40 kg N-ha-1 is a beneficial treatment because it decreased the number of culls and increased seedling N concentrations and DW without causing nutrient imbalance or detectably disturbing seedling development.