Global terrestrial nitrogen fixation and its modification by agriculture

Area of Science:

• Ecology
• Biogeochemistry
• Environmental Science

Background:

• Biological nitrogen fixation (BNF) is a primary natural source of nitrogen, vital for terrestrial productivity.
• Current global estimates of terrestrial BNF are marked by significant uncertainty.
• Existing field measurements exhibit sampling bias, overrepresenting nitrogen-fixing organisms.

Purpose of the Study:

• To address uncertainties in global terrestrial BNF estimates by correcting for sampling bias.
• To develop revised estimates of global terrestrial BNF using spatially explicit data.
• To differentiate and quantify BNF contributions from natural and agricultural ecosystems.

Main Methods:

• Upscaling field BNF measurements using spatially explicit abundances of N-fixing niches.
• Integrating data on major biogeochemical N-fixing niches across diverse ecosystems.
• Statistical analysis to correct for sampling bias in field data.

Main Results:

• Natural biomes provide an estimated 65 (52-77) Tg N yr⁻¹ of BNF, lower than previous estimates, primarily in tropical forests and drylands.
• Agricultural systems contribute significantly with an estimated 56 (54-58) Tg N yr⁻¹ of BNF.
• Agricultural BNF has increased total terrestrial BNF by 64% and overall terrestrial N inputs by 60% since pre-industrial times.

Conclusions:

• Revised estimates suggest lower BNF in natural ecosystems than previously thought.
• Agricultural BNF is a substantial and growing component of the global nitrogen cycle.
• BNF may exert stronger controls on terrestrial carbon sinks and represents a larger agricultural nitrogen source than commonly recognized, with implications for nitrogen use limits.