Quantitative evidence of underestimated nitrogen rhizodeposition using 15N split-root labeling during spring wheat developmental period

Accurate quantification of nitrogen (N) derived from rhizodeposition (NdfR) is crucial to assess N stored in plants and the total N balance in plant-soil systems. However, there remains uncertainty in the estimates of NdfR for N-15-labeled plants with the split-root technique using the isotope ratio approach and the isotope mass balance approach. Here, we examined the two calculation approaches that estimate NdfR from N-15-urea (99 atom%) labeling of spring wheat (Triticum aestivum L.) using the split-root technique in a pot experiment over plant growth, and assessed the causative factors of their potential bias. The results showed that independent of the approach, the total amount of NdfR was 5.3-9.8 mg plant(-1) and contributed up to 19.9-26.3% of total plant N throughout the growth period. At the heading and filling stages after labeling, the amounts of NdfR using the isotope ratio approach (5.3 and 6.6 mg plant(-1), respectively) were significantly lower than those (7.1 and 7.8 mg plant(-1), respectively) using the isotope mass balance approach, due to soil N losses and an uneven distribution of N-15. At the ripening stage after labeling, however, the differences in the amount of NdfR between the two approaches disappeared (9.8 and 9.6 mg plant(-1), respectively). This indicated that the isotope ratio approach underestimates the amount of NdfR during the plant developmental period, while both approaches can be used to quantify the amount of NdfR at plant maturity.