Soil and Plant Preparation:Prepare your soil and plants with a known initial N15 content. You can do this by adding a known amount of N15-labeled nitrate (NO3-) to the soil. Ensure that the plants are well-established and have access to sufficient water and other nutrients before starting the experiment.

Regarding the use of N15 with spectrophotometric measurements:

• No direct interference: N15 itself does not interfere with spectrophotometric measurements. The absorbance of the enzyme reaction products will be the same whether the nitrogen is N15 or N14.
• Indirect effects: However, there may be indirect effects if the N15 labeling affects the enzyme activity or the concentrations of substrates or products. Therefore, it is important to validate your methods and ensure that the N15 labeling does not introduce significant errors.

Additional considerations:

• Isotope dilution mass spectrometry (IDMS): For more precise measurements of N15 enrichment, consider using IDMS instead of spectrophotometry. IDMS can provide very accurate measurements of isotopic ratios.
• Control experiments: Conduct control experiments with unlabeled nitrate to assess the background levels of enzymes and to ensure that the N15 labeling is not affecting the results.
• Data analysis: Use appropriate statistical methods to analyze your data and assess the significance of the observed differences between treatments.

By following these guidelines and carefully considering the potential impacts of N15 labeling on your measurements, you can successfully use N15 as a tracer in your experiments on nitrogen uptake and metabolism in plants.

May I also add that you should also measure a complete suite of nutrient elements in the plants to verify their health

I agree with Raghad, the mass of a stable isotope does not impact on colorimetric measurements, Paul.