Excess nitrogen fertilization and the structure of Scots‐pine needles

Abstract

SummaryThe effects of excess nitrogen fertilization on the structure of Scots‐pine needles (Pinus sylvestris) were examined in Finnish Lapland. The trees were fertilized with calcium ammonium‐nitrate, and the levels of nitrogen applied were 0, 75, 150, 250, 500 and 1000 kg N/ha, respectively. The thickness and width of the needle and thicknesses of the tissues were measured, and the conditions of various celltypes were classified at the light‐microscopy level. In addition, the areas of the cell organelles were measured and the conditions of the chlomplasts and other cell compartments observed at the ultrastructural level. Larger needles and a thicker adaxial mesophyll were observed following excess N application (250–1000 kg N/ha). The natural xeromorphic structure of the conifer needles became more mesomorphic, possibly influencing the protection afforded against environmental stresses. A high level of nitrogen fertilization caused changes in the central cylinder, especially in the vascular bundle and the sclerenchyma. The needle nitrogen concentration was high in the highly fertilized trees and the boron concentration was simultaneously extremely low. As a consequence of the changes in the central cylinder, photoassimilate transport from the mesophyll to the phloem may be disturbed at high fertilization levels. The changes at the ultrastructural level, i.e. injuries to the chloroplast thylakoids and the plasma membrane, and the formation of lipid accumulations, were indicators of the adverse effects of excess nitrogen. With an increasing atmospheric‐nitrogen load, lower nitrogen fertilization levels in pine forests naturally adapted to low soil N should be considered in forest management.