Rising atmospheric CO₂ reduces nitrogen availability in boreal forests

Area of Science:

• Ecology
• Environmental Science
• Biogeochemistry

Background:

• Anthropogenic nitrogen pollution causes eutrophication globally.
• Some ecosystems show signs of oligotrophication, potentially linked to increased atmospheric carbon dioxide (CO2).
• Plant nitrogen isotope (δ15N) chronologies suggest oligotrophication, but the drivers (CO2 vs. N deposition) are debated.

Purpose of the Study:

• To investigate the drivers of declining nitrogen availability (oligotrophication) in Swedish forests.
• To construct and analyze nitrogen isotope (δ15N) tree-ring chronologies across a wide latitudinal gradient.
• To determine the relative influence of rising CO2 and nitrogen deposition on forest ecosystem changes.

Main Methods:

• Construction of δ15N tree-ring chronologies from archived Swedish forest samples (1961-2018).
• Analysis across a 1,500-km latitudinal range with uniform CO2 rise and variable N deposition.
• Application of linear mixed-effects models to identify key predictors of δ15N values.

Main Results:

• Consistent declining δ15N chronologies were observed throughout Sweden.
• Rising CO2 emerged as the strongest predictor of δ15N values.
• Nitrogen deposition, temperature, and forest basal area showed lower explanatory power.

Conclusions:

• Elevated atmospheric CO2 is the primary cause of oligotrophication in Swedish boreal forests.
• This finding has significant implications for understanding the future role of forests as global carbon sinks.
• The study highlights the pervasive impact of CO2 on ecosystem nutrient cycling.