Elevated CO2 promotes long-term nitrogen accumulation only in combination with nitrogen addition.

Area of Science:

• Ecology
• Biogeochemistry
• Environmental Science

Background:

• Nitrogen (N) availability is predicted to control ecosystem carbon uptake under rising carbon dioxide (CO2).
• Some models suggest elevated CO2 could enhance ecosystem N accumulation, potentially mitigating N limitation and pollution.
• Tidal marshes, with open N cycles and rapid organic matter accrual, are ideal for studying decadal N dynamics.

Purpose of the Study:

• To investigate the impact of elevated CO2 on nitrogen dynamics in a tidal marsh ecosystem.
• To test the hypothesis that elevated CO2 promotes ecosystem N accumulation over a nine-year period.
• To quantify the fates of nitrogen: plant/soil retention, denitrification, and tidal export.

Main Methods:

• A nine-year experiment manipulating CO2 and N levels in a tidal marsh.
• Assessment of changes in plant and soil nitrogen pools.
• Use of a 15N tracer to quantify nitrogen retention and estimate lateral N movement.

Main Results:

• Elevated CO2 alone did not increase plant N mass, soil N mass, or 15N tracer retention.
• A significant interaction between CO2 and N was observed.
• The combined N+CO2 treatment led to increased ecosystem N accumulation, despite evidence of increased N losses (reduced 15N retention).

Conclusions:

• Elevated CO2 alone is unlikely to cause long-term N accumulation in N-limited ecosystems.
• Additional nitrogen inputs are likely required for elevated CO2 to overcome N limitation and increase plant N uptake.
• The interaction suggests complex feedbacks between CO2, N availability, and N cycling processes in tidal marshes.