Epsilon Archive for Student Projects

Abstract

In a large-scale field experiment, single trees in a Scots pine stand in Northern Sweden were subjected to different amounts of nitrate fertilizer containing the tracer isotope 15N. This was done on two plots, one control plot and one that had been fertilized during the previous 6 seasons, all trees were about 90 years old. Measurements of needle nitrogen (N) content and delta 15N abundance and analyses of N-forms in the xylem sap were conducted. Also, sap flux density was measured with Granier-type sensors, enabling assessment of when added N may have reached the canopy. The hypotheses were 1) that the ectomycorrhizal N transfer efficiency to the plant partner is enhanced by conditions of high soil N availability; and 2) that N transport through the trees’ xylem is shifted to compounds with lower C/N ratio when the N availability is increased. The results supported the first hypothesis and gave some support to the second. Sap flux measurements indicated a surprisingly slow water transport in the xylem, suggesting that the added N reached the upper canopy four weeks after fertilization. Xylem sap nitrate concentrations exhibited a peak four weeks after fertilizer addition, corroborating the slow xylem transport rates assessed with the Granier elements.