G. Cadisch, K. Hairiah, K.E. Giller
Measurements of N 2 fixation by Arachis hypogaea grown on an Ultisol (Grossarenic Kandiudult) in North Lampung, Sumatra were obtained by i) the 15 N dilution method by applying a small dose of 15 N in solution mixed with a carbon source and ii) by the 15 N natural abundance method (δ 15 N). For both methods non-nodulating groundnuts and maize were used as reference plants. While the 15 N dilution method led to a large spatial variation (both in depth and time) in plant available 15 N, spatial variations of the natural 15 N abundance with soil depth (6-9 ‰), time (9-12 ‰ over one year) and space were comparatively small. The δ 15 N of the mineralizable N pool was greater than that of the total soil N which was reflected in high δ 15 N values of the reference plants. Above ground plant parts of groundnuts grown in a N free media were negatively enriched in 15 N while nodules were not enriched (0 ‰). Isotopic discrimination occurred both during N 2 fixation (-1.8/-1.0 ‰ 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 Nha -1 ) between the natural 15 N abundance method and 15 N dilution method where an adequate reference plant was available. However the 15 N dilution method was much more sensitive to a matching planting time between the reference and fixing plant compared to the δ 15 N method. Although the 15 N natural abundance method was less prone to temporal and spatial alterations in δ 15 N it is nevertheless advocated to use the same precautions as for the 15 N dilution method with regard to a careful matching of the legume and the reference plant and accounting for 15 N variation within the plant. It is concluded that under the relatively high plant available 15 N conditions in this soil the 15 N natural abundance method is a viable alternative method to measure N 2 fixation of groundnut under field conditions.
Department of Biological Sciences, Wye College, University of London, Wye, Kent TN25 5AH, United Kingdom; Department of Soil Science, Faculty of Agriculture, Brawijaya University, Malang, Indonesia
