Field Comparison of Dinitrogen Fixation Determined with Nitrogen‐15‐Depleted and Nitrogen‐15‐Enriched Ammonium Sulfate in Selected Inbred Backcross Lines of Common Bean

Abstract

The field determination of N 2 fixation by 15 N‐isotopic methods provides a direct and precise measure of N 2 fixation. Since 15 N‐enriched fertilizers are expensive, 15 N‐depleted (NH 4 ) 2 SO 4 was used as a more economical, readily available alternative to determine N 2 fixation in common bean, Phaseolus vulgaris L. The objectives of this study were to determine whether 15 N‐depleted (NH 4 ) 2 SO 4 provided reliable estimates of relative differences in N 2 fixation among bean lines comparable to those obtained using 15 N‐enriched (NH 4 ) 2 SO 4 , and whether nine previously selected progeny lines differed from their respective recurrent parents for N 2 fixation ability. The nine progeny lines and three parents were grown in two randomized complete block field experiments that were treated identically except that each experiment received only one of the two 15 N fertilizers. Lines were harvested at an early reproductive stage (R3) and maturity (R9), partitioned into plant parts, and the 15 N and total N yield of the tissue determined. The two 15 N fertilizers produced comparable differences among lines at the R9 stage based on significant genotypic rank correlations between the two experiments, and similar fertilizer use efficiency values (% FUE). Three of the five selected progeny lines from population 24 fixed significantly more N 2 than their recurrent parent ‘Sanilac’. The four progeny lines from population 21 were not significantly different from or less effective in N 2 fixation than their recurrent parent ‘Porrillo Sintetico’, therefore, either previous selection based on indirect measures of N 2 fixation was not effective, or the two parents did not differ genetically for N 2 fixation. In all progeny and parental lines, seed N 2 fixed was positively rank correlated with plant N 2 fixed, and the highest N 2 ‐ fixing lines in both populations had more fixed N 2 in the seeds than did low N 2 ‐fixing lines.