Applicability of the natural 15N abundance technique to measure N2 fixation in Arachis hypogaea grown on an Ultisol

Measurements of N-2 fixation by Arachis hypogaea grown on an Ultisol (Grossarenic Kandiudult) in North Lampung, Sumatra were obtained by i) the N-15 dilution method by applying a small dose of N-15 in solution mixed with a carbon source and ii) by the N-15 natural abundance method (delta(15)N). For both methods non-nodulating groundnuts and maize were used as reference plants. While the N-15 dilution method led to a large spatial variation (both in depth and time) in plant available N-15, Spatial variations of the natural N-15 abundance with soil depth (6-9 parts per thousand), time (9-12 parts per thousand over one year) and space were comparatively small. The delta(15)N of the mineralizable N pool was greater than that of the total soil N which was reflected in high delta(15)N values of the reference plants. Above ground plant parts of groundnuts grown in a N free media were negatively enriched in N-15 while nodules were not enriched (0 parts per thousand). Isotopic discrimination occurred both during N-2 fixation (-1.8 / -1.0 parts per thousand for soil inoculum and Bradyrhizobium WYE 899 respectively) and transport of fixed N into different plant tissues. The proportion of N derived from N-2 fixation varied from 45-54 % using the natural abundance method and non-nodulating groundnut and maize as references respectively in 1995 but fixation dropped significantly in the second year of evaluation (21-16 %). There was a good agreement in the amount of N-2 fixed on average of the two years (21-24 kg N ha(-1)) be tween the natural N-15 abundance method and N-15 dilution method where an adequate reference plant was available. However the N-15 dilution method was much more sensitive to a matching planting time between the reference and fixing plant compared to the delta(15)N method. Although the N-15 natural abundance method was less prone to temporal and spatial alterations in delta(15)N it is nevertheless advocated to use the same precautions as for the N-15 dilution method with regard to a cartful matching of the legume and the reference plant and accounting for N-15 variation within the plant. It is concluded that under the relatively high plant available N-15 conditions in this soil the N-15 natural abundance method is a viable alternative method to measure N-2 fixation of groundnut under field conditions.