Estimates of nitrogen fixation by trees on an aridity gradient in Namibia

Area of Science:

• Ecology
• Plant Physiology
• Biogeochemistry

Background:

• Nitrogen (N₂) fixation is a crucial process for plant nutrition, particularly in arid ecosystems.
• The Mimosaceae family is known for its symbiotic relationship with nitrogen-fixing bacteria.
• Understanding N₂ fixation in woody species is vital for arid land ecosystem functioning.

Purpose of the Study:

• To quantify the contribution of N₂ fixation to leaf nitrogen in 11 Mimosaceae woody species across an aridity gradient in Namibia.
• To compare nitrogen isotope signatures (δ¹⁵N) between Mimosaceae and non-Mimosaceae species.
• To investigate the relationship between N₂ fixation, water use efficiency, and carbon isotope ratios (δ¹³C).

Main Methods:

• Utilized natural abundance of stable isotopes (¹⁵N and ¹³C) to estimate N₂ fixation.
• Compared δ¹⁵N values in 11 Mimosaceae species with 11 non-Mimosaceae species.
• Analyzed variations in N₂ fixation along an aridity gradient in Namibian savannas.

Main Results:

• N₂ fixation contributed an average of 30% to leaf nitrogen in Mimosaceae, with significant species-specific variation (2% to 71%).
• Higher δ¹⁵N values in Mimosaceae were associated with lower δ¹³C values and reduced intrinsic water use efficiency.
• Background δ¹⁵N values and the difference between Mimosaceae and non-Mimosaceae δ¹⁵N were highest in lowland savannas.

Conclusions:

• N₂ fixation is a significant nitrogen source for Mimosaceae in Namibian arid environments, exhibiting substantial variability.
• N₂ fixation influences water use efficiency and carbon isotope discrimination in these woody species.
• The study highlights the ecological importance of N₂ fixation in arid land plant communities and its physiological implications.