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

Abstract

Measurements of N2 fixation by groundnuts grown on an Ultisol (Grossarenic Kandiudult) in North Lampung, Sumatra (Indonesia) were obtained by (i) the 15N dilution method by applying a small dose of 15N in solution mixed with a carbon source and (ii) by the 15N natural abundance method (delta 15N). For both methods non-nodulating groundnuts and maize were used as reference plants. While the 15N dilution method led to a large spatial variation (both in depth and time) in plant available 15N, spatial variations of the natural 15N abundance with soil depth (6-9 per mille), time (9-12 per mille over one year) and space were comparatively small. The delta 15N of the mineralizable N pool was greater than that of the total soil N which was reflected in high delta 15N values of the reference plants. Above-ground plant parts of groundnuts grown in a N-free media were negatively enriched in 15N while nodules were not enriched (0 per mille). Isotopic discrimination occurred both during N2 fixation (-1.8/-1.0 per mille for soil inoculum and Bradyrhizobium WYE 899 respectively) and transport of fixed N into different plant tissues. The proportion of N derived from N2 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 per mille). There was a good agreement in the amount of N2 fixed on average of the two years (21-24 kg N ha-1) between the natural 15N abundance method and 15N dilution method where an adequate reference plant was available. However the 15N dilution method was much more sensitive to a matching planting time between the reference and fixing plant compared to the delta 15N method. Although the 15N natural abundance method was less prone to temporal and spatial alterations in delta 15N it is nevertheless advocated to use the same precautions as for the 15N dilution method with regard to a careful matching of the legume and the reference plant and accounting for 15N variation within the plant. It is concluded that under the relatively high plant available 15N conditions in this soil the 15N natural abundance method is a viable alternative method to measure N2 fixation of groundnut under field conditions.