Ecosystem processes and nitrogen export in northern U.S. watersheds.

Area of Science:

• Forest Ecology
• Biogeochemistry
• Environmental Science

Background:

• Atmospheric nitrogen (N) deposition is a key environmental concern.
• Understanding nitrogen (N) inputs and outputs is crucial for assessing ecosystem health and potential nitrogen saturation.
• Ecosystem-level mass balance studies have yielded unclear relationships between N inputs and outputs.

Purpose of the Study:

• To investigate the relationship between atmospheric nitrogen (N) inputs and ecosystem N outputs in northern forested watersheds.
• To identify key ecosystem processes influencing nitrogen budgets and outputs.
• To evaluate the concept of nitrogen saturation in these ecosystems.

Main Methods:

• Long-term (up to 35 years) ecosystem-level mass balance studies in five northern forested watersheds.
• Analysis of wet-precipitation N inputs and streamwater N outputs.
• Measurement of nitrate (NO3-) concentrations in precipitation and headwater streams.
• Investigation of ecosystem processes including hydrology, soil N dynamics, and mineralization.

Main Results:

• Wet-precipitation N inputs varied widely (0.1 to 6 kg N ha(-1) year(-1)).
• Sites with lower N inputs exhibited higher output-to-input ratios, with some exceeding inputs by 70-250% (Alaska watersheds).
• Key processes influencing N outputs include hydrologic flowpath seasonality, soil freezing, forest-floor N pools, snowmelt percolation, and gross soil N mineralization.

Conclusions:

• The relationship between atmospheric N inputs and ecosystem N outputs is complex and not solely determined by input levels.
• Various ecosystem processes significantly modulate nitrogen retention and loss, challenging simple predictions of nitrogen saturation.
• Long-term watershed studies are essential for understanding the intricate dynamics of nitrogen cycling in forest ecosystems.