Methodology for Field Trials with Nitrogen‐15‐Depleted Nitrogen

Following nitrogen (N) transformations and movement in natural systems against a background of large amounts of this element requires use of isotopically labeled material which can be distinguished from that which is indigenous to the system. Methods are described for sampling and analyzing for labeled N in various components of the soil‐plant‐water system, and estimates of variability presented. The procedures described permitted construction of a balance sheet for long‐term field trials with corn ( Zea mays L.) in two locations in California. Unaccounted for losses ranged from 1% on Yolo fine sandy loam at 90 kg N/ha to > 50% in the coarse‐textured Hanford sandy loam with high rates of N application. It was found possible to identify labeled N from 15 N‐depleted fertilizer in the soil organic fraction even after extensive dilution by using the isotopic composition of N in the corresponding layer of soil from unfertilized plots as the reference value rather than an overall mean.