A case study of nitrogen saturation in western U.S. forests

Area of Science:

• Forest Ecology
• Environmental Chemistry
• Soil Science

Background:

• High nitrification rates in San Bernardino Mountain forests lead to nitrogen (N) saturation and nitrate dominance in soil solutions.
• Elevated nitric oxide (NO) emissions from soil are linked to high nitrification activity.
• Unlike eastern U.S. forests, high base cation saturation in these soils mitigates immediate risks of calcium depletion or soil acidification.

Purpose of the Study:

• To investigate the factors predisposing California forests to nitrogen saturation.
• To assess the impacts of nitrogen deposition and ozone on forest health and sustainability.
• To understand the response of forest growth to nitrogen fertilization in N-saturated ecosystems.

Main Methods:

• Monitoring nitrification rates and nitrogen (N) dynamics in forest soils.
• Assessing physiological disturbances in ponderosa pine trees related to ozone and N deposition.
• Conducting N fertilization experiments to evaluate bole growth responses.

Main Results:

• Nitrogen (N) saturation is driven by high nitrification, resulting in nitrate-rich soil solutions and elevated nitric oxide (NO) emissions.
• Physiological disturbances in ozone-sensitive ponderosa pines are a greater threat than soil acidification.
• Nitrogen deposition appears to counteract ozone-induced growth depression, and N fertilization unexpectedly increased bole growth in N-saturated sites.

Conclusions:

• Western ecosystems with high nitrification are susceptible to nitrogen saturation, leading to significant hydrologic and gaseous nitrogen losses.
• Mediterranean climate forests face increased nitrate leachate risks due to high winter runoff.
• Reduced belowground carbon allocation, potentially due to pollutant stress, may enhance bole growth in response to nitrogen fertilization.