Evaluation of the wick method for in situ 13C and 15N labelling of annual plants using sugar-urea mixtures

Abstract

To investigate the amount and fate of root-derived C and N, often tracer techniques are used, where plants are labelled with isotopes. In the present study, we evaluated the suitability of the cotton wick method for in situ labelling of peas (Pisum sativum L.) and oats (Avena sativa L.) with 13C and 15N simultaneously. With two greenhouse experiments we investigated how the wick method and aqueous urea and sugar solutions at a variety of concentrations affected plant development. In addition, we investigated the distribution of 13C and 15N in plants from column experiments under outdoor conditions. Solution was taken up by the plant from a small vial connected to the stem by a cotton wick which was passed through a hole in the stem of the plants. Generally, solution uptake varied between individual plants and decreased with increasing sugar concentrations. Below-ground, above-ground and total plant dry matter, were not significantly affected by the wick method and the applied solutions. Mixtures of aqueous glucose solutions at 2 to 4% and aqueous urea solutions at 1% are useful carriers of 13C and 15N. However, in the investigated plants isotopes were not homogeneously distributed among plant parts. Above-ground plant biomass was preferentially enriched with 13C and 15N, whereas below-ground plant biomass was generally lower enriched. Moreover, isotope distribution ratio of individual plants varied considerably, independent of plant part or timing of labelling. This must be taken into account when estimating root-derived C and N. Future studies comparing labelling methods need to present the isotope distribution ratios among plant parts to allow a true comparison of the methods and the evaluation of their suitability for estimating rhizodeposition.