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Threshold and multiple indicators for nitrogen saturation in subtropical forests.

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  • 1State Key Laboratory of Environmental Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, 100084, China.

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Subtropical forests show high nitrogen (N) saturation thresholds, requiring revised global models. Foliar nitrogen content and δ15N effectively indicate regional N status and deposition impacts.

Keywords:
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Area of Science:

  • Forest Ecology
  • Environmental Chemistry
  • Biogeochemistry

Background:

  • Nitrogen (N) deposition significantly impacts forest ecosystems, with N status being a critical determinant.
  • Subtropical forests experience extremely high N deposition, necessitating practical indicators for N saturation.
  • Assessing N saturation aids forest management and global N balance/carbon sequestration evaluations.

Purpose of the Study:

  • To quantify the N mass balance in South China's subtropical forests.
  • To assess current N status using N content, C/N ratio, and 15N natural abundance (δ15N).
  • To develop practical indicators for N saturation in these high-deposition ecosystems.

Main Methods:

  • Quantified N mass balance across subtropical forest sites.
  • Measured foliar N content, soil C/N ratio, and δ15N.
  • Analyzed throughfall N deposition, ranging from 13.8 to 113 kg N ha-1 yr-1, dominated by ammonium (NH4+).

Main Results:

  • Identified a novel N leaching threshold of 26-36 kg N ha-1 yr-1 for subtropical forests, 160% higher than temperate forests.
  • Established a critical C/N ratio of 20 for the O/A soil horizon as an indicator of N saturation.
  • Found positive correlations between foliar N content, δ15N, and N deposition, indicating regional N status.

Conclusions:

  • Subtropical forest N leaching thresholds are higher than previously estimated, necessitating revisions in global N deposition models.
  • Foliar N content and δ15N are reliable indicators for assessing regional N status and the influence of N deposition.
  • The δ15N enrichment factor (Ɛfoli/So2) shows consistent trends with increasing N deposition, offering a versatile tool for studying N deposition impacts across diverse forest ecosystems.