Nitrogen fixation in legumes and actinorhizal plants in natural ecosystems:values obtained using N-15 natural abundance

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 N-2 fixation in natural ecosystems.Aims: To assess the proportion of total plant N derived from the atmosphere via the process of N-2 fixation (% Ndfa) by actinorhizal and legume plants in natural ecosystems and their N input into these ecosystems as indicated by their N-15 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 N-15 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 N-2 fixation. N input into ecosystems from N-2 fixation was very variable for actinorhizal and legume plants and greatly dependent on their biomass within the system.Conclusions: Measurement of N-15 natural abundance has given greater understanding of where legume and actinorhizal plant N-2 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.