Comparison of Methods of Enriching a Soil with Nitrogen‐15 to Estimate Dinitrogen Fixation by Isotope Dilution1

A 2‐yr field comparison of methods of labeling the soil with 15N was undertaken to increase the precision of estimating N2fixation by the 15N isotope dilution procedure. Dinitrogen fixation was estimated using ‘Aurora’ field bean (Phaseolus vulgarisL.) grown on a Chin loam (Typic Haploboroll) soil under irrigation in southern Alberta. Postemergence fluid injection of (15NH4)2SO4into the rooting zone of 1‐week‐old plants (with or without the nitrification inhibitor N‐Serve) resulted in excellent experimental precision and greater planning flexibility. It was superior to preemergence addition of liquid 15N in broadcast or banded form because of the high winds often encountered in the spring. The preemergence addition of low‐level 15N‐labeled legume or cereal organic residues compared favorably with mineral N addition and had the advantage that residues from previous 15N experiments could be utilized cost‐free. Isotope dilution of 15N at levels of natural isotopic abundance resulted in similar estimates of N2fixation with no consistent evidence of isotopic discrimination and appears promising as an inexpensive but qualitative screening procedure for field selection of N2fixation by legume breeders.