Assessment of fertilizer nitrogen accumulation in Pinus sylvestris trees and retention in soil by ¹⁵N recovery technique

Abstract

The distribution and quantitative recovery of fertilizer N were determined in three 29‐ to 43‐year‐old stands of Scots pine, located in western Uppland, Central Sweden. The experimental technique involved was based on the use of ¹⁵N‐labelled fertilizer materials and non‐trenched microplots of special design. The standard dose of nitrogen applied was 160 kg N ha^‐1. The primary topics examined were (1) fertilizer nitrogen accumulation in trees and in the soil system as influenced by nitrogen source, nitrogen application rate, time of application during the growing season, granule size of the nitrogen material, and method of placement, (2) distribution of labelled N within the tree (different organs of the tree), and (3) redistribution of accumulated N in the tree over an observation period of two to three growing seasons. When quantified at the end of the second growing season, the labelled N accumulation in trees, concerning plots treated with calcium nitrate or ammonium nitrate, averaged 36% (SD=7). The accumulation resulting from split doses did not differ significantly from that resulting from a single‐dose application, nor did granule size of the nitrogen materials have any significant effect on accumulation. For urea source of nitrogen, the recovery figure was markedly lower, averaging 28% (SD=6). When expressed as a percentage, the accumulation in trees was not significantly different for 40 and 160 kg nitrogen application rates. In one of the experiments application timing during the growing season proved to be an important factor determining the extent of fertilizer nitrogen accumulation in trees. The figures for total recovery of labelled nitrogen in above‐ground parts of trees and in soil, when restricting measurements to the forest floor and the 0–32 cm mineral soil, ranged from 46 to 84%, with an average of 60%. The recovery was lowest for a treatment with calcium nitrate, when applied relatively late in the growing season, and highest for a split application of urea. Treatments with urea source of nitrogen were characterized by a high accumulation of immobilized N in the organic LFH layer of the soils. Recovery figures exceeded 82 % for a 40‐kg N dose of nitrogen, regardless of the nitrogen source.

Key words:

• Pinus sylvestris
• forest fertilization
• ¹⁵ N‐tracer technique
• fertilizer nitrogen uptake
• immobilization in soil
• redistribution.