Comparison of natural abundance and enriched ¹⁵N methods to quantify nitrogen fertilizer recovery in maize under field conditions

This study compared the ability of natural abundance (NA) and ¹⁵N-enrichment (EN) methods to quantify contribution of different N sources (fertilizer and legume N derived from fixation) to crop N uptake by maize crop grown in No-Till cropping system under field conditions. The quantitative estimates of different N contributions were then compared between methods.

A field experiment was established on a Vertosol by sowing legume (Vicia faba) or non-legume (Triticum aestivum) grain crops. The following maize crop was fertilized using either urea or ¹⁵N-enriched (5 atom%) urea, at five N rates in spatially separated subplots in each field plot.

The proportional recovery of fertilizer N in grain (%Ndff) showed that EN method provided higher estimates than that determined by NA method, with differences greatest in wheat–maize rotation. The NA method was better able to quantify residual benefits of fixed N from faba beans in a following maize crop. Unfortunately, different biomass and grain sampling times necessitated by the size of fertilised plots used for each isotopic method confounded comparisons of Ndff in crop biomass and grain yields.

While larger plot sizes able to be used in cheaper NA method provided better estimates of crop and grain biomass and N content, and enabled quantification of recovery of atmospheric N₂-fixed legume N (%Ndfa) in a following maize crop, EN method was better able to estimate fertilizer N recovery by maize in the field. The reasons for differences in %Ndff between methods and crop histories require further investigation.