Global Marine N₂ Fixation Estimates: From Observations to Models.

Area of Science:

• Marine biogeochemistry
• Oceanic productivity
• Nitrogen cycle

Background:

• Fixed nitrogen (N) is a critical nutrient limiting primary productivity in low-latitude oceans.
• The oceanic nitrogen inventory is governed by the balance between nitrogen input and loss.
• Nitrogen loss primarily occurs via denitrification and anammox in low-oxygen environments.

Purpose of the Study:

• To compile and critically assess existing estimates of marine biological nitrogen fixation (N2 fixation).
• To identify limitations and uncertainties in current N2 fixation measurements and modeling approaches.
• To highlight the need for improved understanding of marine N2 fixation for accurate oceanic N-balance assessments.

Main Methods:

• Compilation of N2 fixation estimates from experimental measurements.
• Review of tracer-based geochemical approaches for N2 fixation quantification.
• Analysis of modeling approaches used to estimate N2 fixation rates.
• Discussion of limitations and uncertainties associated with various methodologies.

Main Results:

• Existing estimates of marine N2 fixation exhibit significant uncertainties.
• Inadequate experimental data coverage hinders accurate rate determination.
• Insufficient understanding of the ecological and environmental controls on N2 fixation persists.

Conclusions:

• Robust assessment of the oceanic nitrogen balance is challenging due to uncertainties in N2 fixation rates.
• Advancing the field requires a more comprehensive understanding of marine N2 fixers and their interactions.
• Future research should focus on improving data acquisition and ecological understanding to refine N2 fixation estimates and predictions.