Fertilizer nitrogen distribution in a pinus sylvestris/ picea abies ecosystem, central Sweden

Abstract

The distribution and total recovery of ¹⁵N‐labelled fertilizer nitrogen was studied in a 50‐year‐old mixed coniferous stand, consisting of Scots pine and Norway spruce, located in Uppland, Central Sweden. The experimental plots, with an area of 113 m² each, were separated from the surrounding trees by trenching and emplacement of an aluminium sheet to a depth of 50 cm. The standard nitrogen application rate was 150 kg N ha⁻¹ and the nitrogen sources tested were calcium nitrate, ammonium nitrate and urea. Two growing seasons after the application of fertilizer, all the experimental plots were subjected to destructive sampling, including soil, field vegetation and trees, with a view to producing a nitrogen balance sheet for the experimental treatments involved.

The fertilizer nitrogen recovered in the part of the ecosystem under study varied between 76 and 92% of the fertilizer N applied (150 kg N ha⁻¹). The highest recovery figure applies to treatment with a split dose of ammonium nitrate and the lowest to calcium nitrate. For a low dose of ammonium nitrate (=50 kg N ha⁻¹), the recovery figure was close to 100%.

It was found rather characteristic that the nitrogen source indicating the highest accumulation figure for labelled N in the stand, i.e., calcium nitrate (44%), showed the lowest percentage recovery figure in the soil (28%). The urea source of nitrogen, on the other hand, showed a relatively low nitrogen accumulation figure in the stand (20%), but an extremely high figure for immobilization in the soil (62%). For ammonium nitrate, which is the dominating nitrogen material used for forest fertilization in Sweden, the distribution of recovered labelled N after two growing seasons was, on the average: trees, 33%; field layer vegetation, 1%; fine roots, 6%; and immobilized in soil, 48%. The split application of nitrogen decreased the labelled N recovery in the trees and increased the recovery in soil.

Concentrated placement of ammonium nitrate and urea (1 nest m⁻²), compared with that of broadcast nitrogen materials, did not measurably affect the amounts of fertilizer N taken up by the trees.

The accumulation of fertilizer nitrogen in the trees was closely related to the basal area of the stem. It was also found that the fertilizer‐N uptake was higher for Norway spruce than for Scots pine of the same basal area.

A certain measure of caution must be exercised in interpreting data on labelled‐N distribution in the ecosystem, particularly in view of the ongoing biological isotope exchange process in the soil media. An interference of this kind affects mainly the ammonium form of nitrogen. It results in an underestimation of the uptake of ammonium and urea form of fertilizer nitrogen by the trees, and in an overestimation of the processes leading to immobilization of ammonium N in the soil organic pool. By following the changes in the size and the isotope composition of the exchangeable ammonium pool during the first growing season, a rough value for the correction factor relating to the net nitrogen accumulation in the stand was estimated.

Key words:

• forest fertilization
• ¹⁵N tracer technique
• fertilizer nitrogen uptake
• fertilizer nitrogen balance sheet