Nitrogen deposition accelerates soil carbon sequestration in tropical forests

Area of Science:

• Ecology
• Climate Change Science
• Biogeochemistry

Background:

• Terrestrial ecosystems, particularly tropical forests, are critical for global carbon cycling and climate change mitigation.
• The impact of increasing nitrogen (N) deposition on below-ground soil carbon stocks in tropical forests remains uncertain, as N is not always a limiting factor for growth.

Purpose of the Study:

• To quantify soil carbon sequestration in response to a decade of continuous nitrogen addition in a nitrogen-rich primary tropical forest.
• To determine the mechanisms driving changes in soil carbon stocks under elevated nitrogen deposition.
• To assess the generalizability of N-induced soil carbon sequestration in tropical forests through meta-analysis.

Main Methods:

• Conducted a long-term (over a decade) experimental N addition study in a primary tropical forest.
• Quantified changes in soil carbon stocks and associated output fluxes.
• Performed a meta-analysis of existing studies on N addition effects in tropical forests.

Main Results:

• Long-term N additions significantly increased soil carbon stocks by 7-21%.
• Increased carbon sequestration was primarily due to reduced carbon output and enhanced physical protection of organic matter, not altered chemical composition.
• Meta-analysis confirmed that N addition generally stimulates soil carbon sequestration in tropical forests, with soil nitrogen sequestration keeping pace with carbon.

Conclusions:

• Mature tropical forests can enhance below-ground carbon sequestration under chronic excess nitrogen deposition.
• Findings have significant implications for predicting future terrestrial carbon sinks under rising atmospheric CO2 and N deposition.
• A conceptual model was developed to improve terrestrial carbon cycle models by incorporating N deposition and N cycling dynamics.