Use of the natural enrichment of 15N in plant available soil N for the measurement of symbiotic N2 fixation

In most N2 fixation investigations utilizing 15N, the concentration of 15N in the soil is raised with 15N enriched compounds. However, the very small differences in natural 15N abundance between air and soil which are found in many soils can also be utilized for N2 fixation measurements. A suitably precise mass spectrometer is needed, and there must be no change in isotopic composition of samples during preparation for analysis. The natural abundance of 15N in plant available soil N appears to be influenced by the patterns of land use, but tends to be relatively uniform down a soil profile and generally does not change rapidly during a growing season. The authors have used the natural abundance method to estimate N2 fixation in crop, pasture and tree legume studies in temperate and tropical environments. (author). 22 refs, 2 figs, 4 tabs