Use of 15N in evaluating symbiotic N2 fixation of field-grown soybeans

Various methods have been used to estimate N2 fixation by legumes (i.e. Kjeldahl N and the acetylene-ethylene assay). Recently 'Asub(N)' values by the legume and a non-nodulating crop using 15N-labelled N fertilizer were used to quantitatively estimate the amount of N2 fixed by legume crops growing under field conditions. The objective of this research was to evaluate Kjeldahl N procedures, the acetylene-ethylene assay and the 'Asub(N)' technique as estimators of N2 fixation by field-grown soybeans. The 'Asub(N)' value concept provided a reliable estimate of N2 fixation by soybeans which agreed with acetylene-ethylene measurements made weekly and the values compared favourably with Kjeldahl N measurements. (author)