Measuring Micronutrient Fertilizer Efficiency Under Field Conditions Using Enriched Stable Isotopic Labeling.

Millions of hectares of arable land worldwide, particularly in arid and semi-arid regions, are deficient in plant available micronutrients e.g. copper (Cu), zinc (Zn).  To overcome micronutrient deficiencies and associated crop yield loss, farmers have applied micronutrient fertilizers for over 100 years. However, the benefits of micronutrient fertilization have been limited by their low agronomic efficiency. Improving this agronomic efficiency requires the ability to accurately measure the efficiency of fertilizer under field conditions.

Radioisotopic tracing has been widely used to assess the plant uptake and efficiency of fertilizer applied to soil, but recent developments in stable isotopic labeling techniques potentially enable assessment under field conditions without the safety limitations of using radioisotopes. Fertilizer granules were coated with enriched sources of stable isotopes of Zn (⁶⁷Zn) and Cu (⁶⁵Cu). Durum wheat (Triticum durum L.) was planted in open-bottomed cores containing the labeled fertilizer and placed in-situ in the field under prevailing climatic conditions in southern Australia. Inductively coupled-mass spectrometry was used to measure the Cu and Zn isotopic ratio in plant shoot and grain digests to determine the uptake efficiency of the micronutrient fertilizer. For Zn, the stable isotopic tracing methodology was compared to measurements of fertilizer Zn efficiency using radioisotope dilution (using ⁶⁵Zn) in four soils in a separate glasshouse experiment.

Measurements of the isotopic ratio of ^67/68Zn and ^65/63Cu in digested plant parts, the labeled fertilizer and the natural isotopic ratio in the soil enabled calculation of plant Zn and Cu uptake derived from the micronutrient fertilizer. Plant uptake from the fertilizer was a significant proportion of total plant Zn and Cu uptake on this soil. For Zn, this result concurred with measurements of fertilizer efficiency using stable isotope tracing and radioisotope dilution assays using the same cultivar grown under glasshouse conditions in the same soil.