Reassessment of the Contribution of Belowground N from Soybean after Testing Different 15N Leaf-Labelling Strategies

Abstract

Purpose: Soybean is the most important grain crop in Brazil with a mean N accumulation of over 250 kg N ha-1, principally from biological N2 fixation. The residual N benefit depends heavily on the quantity of the belowground N at harvest, much which cannot be directly recovered in roots. The purpose of this study was to investigate different aspects of the 15N leaf-labelling technique to quantify non-recoverable root N (NRRN) derived from senescent roots and nodules (rhizodeposits). Methods: Soybean plants were grown in pots of soil and at 27 days after planting (vegetative stage V4) cut or whole leaves were exposed to highly enriched 15N-labelled urea or glutamine. Seven sequential harvests of the plants and soil were taken until the final grain harvest at 70 days after labelling.Results: After only 48 h, the plants labelled with 15N urea transferred approximately 5% to the soil, while only 1% was found in the roots. Leakage of 15N label was even more pronounced when the leaves were labelled with 15N glutamine. After this initial leakage, the excess 15N deposited in the soil only increased by a further 2.6% of applied label, which suggested that only part of this N represented senescence of roots or nodules.Conclusions At the final harvest, N in roots separated from the soil amounted to 6.4% of total plant N. Discounting the early rapid deposition of 15N-enriched N to the soil, our calculations indicated that at final harvest the total NRRN was 2.8% of total plant N.