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Background: Nitrogen fixation has been quantified for a range of crop legumes and actinorhizal plants under different agricultural/agroforestry conditions, but much less is known of legume and actinorhizal plant N2 fixation in natural ecosystems. Aims: To assess the proportion of total plant N derived from the atmosphere via the process of N2 fixation (%Ndfa) by actinorhizal and legume plants in natural ecosystems and their N input into these ecosystems as indicated by their 15N natural abundance. Methods: A comprehensive collation of published values of %Ndfa for legumes and actinorhizal plants in natural ecosystems and their N input into these ecosystems as estimated by their 15N natural abundance was carried out by searching the ISI Web of Science database using relevant key words. Results: The %Ndfa was consistently large for actinorhizal plants but very variable for legumes in natural ecosystems, and the average value for %Ndfa was substantially greater for actinorhizal plants. High soil N, in particular, but also low soil P and water content were correlated with low legume N2 fixation. N input into ecosystems from N2 fixation was very variable for actinorhizal and legume plants and greatly dependent on their biomass within the system. Conclusions: Measurement of 15N natural abundance has given greater understanding of where legume and actinorhizal plant N2 fixation is important in natural ecosystems. Across studies, the average value for %Ndfa was substantially greater for actinorhizal plants than for legumes, and the relative abilities of the two groups of plants to utilise mineral N requires further study.