fate of 15N enriched throughfall in two coniferous forest stands at different nitrogen deposition levels

• Published in: Biogeochemistry Vol. 34; no. 1; pp. 19 - 44
• Main Authors: Koopmans, C.J, Tietema, A, Boxman, A.W
• Format: Journal Article
• Language: English
• Published: Heidelberg Kluwer Academic Publishers 01-07-1996; Springer
• Subjects: Acid soils; ammonia; Animal and plant ecology; Animal, plant and microbial ecology; biogeochemical cycles; Biological and medical sciences; Coniferous forests; Forest ecosystems; Forest soils; Fundamental and applied biological sciences. Psychology; Leaching; Mineral soils; Nitrogen; nitrogen metabolism; Organic soils; Pinus sylvestris; Pseudotsuga menziesii; Soil water; Synecology; Terrestrial ecosystems; Vegetation; Netherlands; Europe; Forests; Isotope labelling; Seeding induced precipitation; Biogeochemical cycle; Pteridophyta; Pinus sylvestris; Pseudotsuga menziesii; Soils; Pollution; Nitrogen cycle; Softwood forest tree; Gymnospermae; Coniferales; Spermatophyta; Ammonium Sulfates; Leachate; Deposition; Filicineae
• ISSN: 0168-2563; 1573-515X

The stable isotope 15N was added as (15NH4)2SO4 to throughfall water for one year, to study the fate of the deposited nitrogen at different levels of N deposition in two "N saturated" coniferous forests ecosystems in the Netherlands. The fate of the 15N was followed at high-N (44-55 kg N ha-1 yr-1) and low-N (4-6 kg N ha-1 yr-1) deposition in plots established under transparent roofs build under the canopy in a Douglas fir (Pseudotsuga menziesii (Mirb.) Franco.) and Scots pine (Pinus sylvestris L.) forest. The applied 15N was detectable in needles and twigs, the soil and soil water leaching below the rooting zone (90 cm depth). Total 15N recovery in major ecosystem compartments was 71-100% during two successive growing seasons after the start of a year-round 15N application to throughfall-N. Nine months after the year-round 15N application, the 15N assimilated into tree biomass was 29-33% of the 15N added in the Douglas fir stand and less than 17% in the Scots pine stand. At the same time total 15N retention in the soil (down to 70 cm) of the high-N plots was about 37% of the deposited 15NH4-N, whereas 46% and 65% of the 15N was found in the soil of the low-N deposition plots at the Douglas fir and Scots pine stand, respectively. The organic layers accounted for 60% of the 15N retained in the soil. The total N deposition exceeded the demand of the vegetation and microbial immobilization. Total 15N leaching losses within a year (below 90 cm) were 10-20% in the high-N deposition plots in comparison to 2-6% in the lowered nitrogen input plots. Relative retention in the soil and vegetation increased at lower N-input levels. Species differences in uptake and tree health seem to contribute to lower 15N recoveries in the Scots pine trees compared to the Douglas fir trees. The excessive N deposition and resulting "N saturation" lead to conditions were the health and functioning of biota were negatively influenced. At decreased N deposition, lower leaching losses together with increased soil and plant retention indicated a change in the fate of the 15N deposited. This may have resulted from changes in ecosystem processes, and thus a shift along the continuum of N saturation to N limitation.