| Biological nitrogen fixation (BNF) is strongly associated with global nitrogen cycling and global changes due to its effects on release of greenhouse gas. However, quantifying BNF is very difficult because of the inherent defect of the study methods such as N-15 dilution method and ethylene reduction method. N-15 natural abundance method is adopted more and more widespreddly in quantitative study of BNF, which exploits naturally occurring differences of N-15 between the available N in the soil and the N2 in the atmosphere. The principles and technical details of this method are reviewed in the paper, and the contributing factors of its accuracy, such as determination of B value, δ15N in reference plants and sampling tissues of nitrogen-fixation plants, are also discussed. The paper suggests that N-15 natural abundance may be a promising method for quantification of BNF in man-made ecosystems, such as agriculture and agroforesrty system, by designation of reasonable sampling strategies. |