Contrasting carbon and nitrogen rhizodeposition patterns of soya bean (Glycine max L.) and oat (Avena nuda L.)

Legumes can use photosynthetic energy to fix atmospheric nitrogen (N) by rhizobium-legume symbiosis and this might result in contrasting patterns of carbon (C) and N rhizodeposition compared to those of cereals. Accurate quantification of C and N derived from rhizodeposition (CdfR and NdfR) is crucial to estimate the C and N input by legumes and cereals. We quantified the amounts of CdfR and NdfR from soya bean (Glycine max L.) and oat (Avena nuda L.) with the cotton-wick labelling method. The plants were labelled twice with a pulse of a solution containing both C-13-glucose and N-15-urea at the fifth and sixth week after planting. The amount of CdfR and NdfR in soya bean was two to six times larger than that in oat. More than 70% of belowground C and N was contributed by rhizodeposition and 61-85% of this was present at a 0-25-cm (top) depth. The 0-25 to 25-50-cm (top-to-sub) ratio of rhizodeposition was larger than that of root biomass in soya bean, indicating stronger root activity and deposition capacity in topsoil than in subsoil. In contrast, a similar top-to-sub ratio of rhizodeposition and root biomass was observed in oat. The C:N ratio of rhizodeposition from soya bean was three times larger than that of roots, whereas the C:N ratios of roots and rhizodeposition were similar in oat. The larger N rhizodeposition from soya bean represents an important N pool, which increases the N-availability for co-existing or subsequent crops.