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 N-15 shoot-labelling technique to quantify non-recoverable N in rhizodeposits. Three pot experiments were performed and the aerial tissue was labelled with highly enriched N-15-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 N-15 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 N-15 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 N-15 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.

Automated summary

This study investigated the different aspects of the N-15 shoot-labelling technique to quantify non-recoverable N in rhizodeposits. The results showed that even if the immediate post-labelling deposition of enriched N is discounted, estimates of rhizodeposited N of nodulated legumes will not be reliable.