Can 15N leaf-labelling reliably quantify rhizodeposited nitrogen remaining after a nodulated legume crop?

Soybean is Brazil´s most important grain crop and accumulates over 250 kg N ha −1 , principally from biological N 2 fixation. The residual N benefit depends heavily on the quantity of the belowground N at harvest, much of which cannot be directly recovered in roots. The purpose of this study was to investigate different aspects of the 15 N shoot-labelling technique to quantify non-recoverable N in rhizodeposits. Three pot experiments were performed and the aerial tissue was labelled with highly enriched 15 N-labelled urea or glutamine at between 27 and 39 days after planting. In all experiments sequential harvests were taken until late grain-filling phase. After only 2 or 3 days between 5.8 and 21.3% of enriched N was found in the soil but the excess 15 N deposited until the final harvest was in all cases less than twice this amount, respectively. Evidence obtained suggested that this early deposition of labelled N was an artefact of the labelling technique. Discounting this initial tracer N decreased estimates of rhizodeposited N by between 51 and 66%. Nodules were much lower in 15 N enrichment than roots. Nodule N constituted 39 to 76% of belowground N, such that the inclusion of none or all of this N to calculate the 15 N enrichment of the roots increased the estimates of rhizodeposited N by between 34 and 58%. We conclude that even if the immediate post-labelling deposition of enriched N is discounted, estimates of rhizodeposited N of nodulated legumes will not be reliable.