Natural ¹⁵N abundance in two nitrogen saturated forest ecosystems

Area of Science:

• Ecology
• Biogeochemistry
• Isotope Science

Background:

• Coniferous forests receiving high nitrogen loads may become nitrogen-saturated.
• Natural ¹⁵N abundance is a potential indicator of ecosystem nitrogen status.

Purpose of the Study:

• To investigate natural ¹⁵N abundance in vegetation and soil compartments of nitrogen-saturated Douglas fir and Scots pine stands.
• To assess the influence of nitrogen deposition and ecosystem processes on ¹⁵N values.
• To evaluate the utility of ¹⁵N abundance as an indicator of nitrogen saturation.

Main Methods:

• Measurement of natural ¹⁵N abundance (δ¹⁵N) in needles, twigs, wood, soil, precipitation, throughfall, and soil water.
• Analysis of inorganic nitrogen (NH₄⁺ and NO₃^-) δ¹⁵N in precipitation and soil water.
• Application of a dynamic nitrogen cycling model to simulate ecosystem responses to varying N deposition and isotope fractionation.

Main Results:

• Vegetation δ¹⁵N values were depleted in ¹⁵N (Douglas fir: -5.7 to -4.2‰; Scots pine: -3.3 to -1.2‰).
• δ¹⁵N values increased with soil depth at both sites.
• NH₄⁺-N in soil water was enriched and NO₃^--N depleted in ¹⁵N, suggesting high nitrification rates.
• Simulated N-saturated ecosystems were sensitive to atmospheric N inputs and fractionation during nitrification/mineralization.

Conclusions:

• Nitrogen-saturated coniferous forests are not characterized by elevated natural ¹⁵N abundance.
• Natural ¹⁵N abundance can indicate nitrogen saturation only when considering deposited N's δ¹⁵N and isotope fractionation factors.
• Combining dynamic isotope models with natural ¹⁵N abundance measurements offers a promising approach for ecosystem interpretation.