Root uptake of N-containing and N-free low molecular weight organic substances by maize: A 14C/15N tracer study

Most studies showing potential organic nitrogen uptake were conducted with amino acids. They conclude that, in some ecosystems, amino acids significantly contribute to the N demand of plants and that roots have special transporters to re-uptake amino acids released into the rhizosphere. However, the relevance of the uptake of organic N compounds can only be evaluated by comparing the uptake of N-containing and N-free organic substances. We compared the uptake of alanine, glucose and acetate labelled with 14C by maize. Additionally, the N uptake was estimated by 15N labelled alanine and KNO3. We found a similar uptake of 14C from alanine, glucose and acetate, amounting for the whole plant less than 1% of 14C input. These results show that maize did not prefer N-containing to N-free organic substances. The uptake of 15N by maize exceeded that of 14C (10- to 50-fold), irrespective of the 15N source. However, plant uptake of nitrate (23.6–35.2% of 15N input) always exceeded the uptake of N from alanine (9.6–28.8%). The uptake of organically bound N by maize growing in soil occurred mainly by transpiration flow – as dissolved organics. The contribution of specific amino acid transporters was minor.