Application of ¹⁵N stable isotope techniques to biological nitrogen fixation in terrestrial ecosystems

Biological nitrogen fixation (BNF) is an important nitrogen source in terrestrial ecosystems. Accurate estimation of BNF rate is essential to accurately quantify atmospheric nitrogen input to natural ecosystems. ¹⁵N natural abundance is commonly used to measure the BNF in symbiotic and associative nitrogen fixing plants, but are highly dependent on the choice of the reference plants. In contrast, the ¹⁵N isotope labeling technique allows precise determination of BNF rates of symbiotic, free-living, and associative N-fixing types, and surpasses the previous methods in studying plant nitrogen fixation strategies, nitrogen transfer processes, and carbon-nitrogen trading between nodules and hosts. The ¹⁵N isotope dilution method is mainly used for plant nitrogen fixation research. Although the ¹⁵N stable isotope probe technique is technically challenging and expensive, it enables the detection and study of N-fixing microorganisms by labeling DNA or RNA, and provides an effective way for assessing asymbiotic microorganism nitrogen fixation rates. The development of ¹⁵N stable isotope technique provides a strong technical guarantee for biological nitrogen fixation research.