Stem labeling results in different patterns of 14C rhizorespiration and 15N distribution in plants compared to natural assimilation pathways

To investigate C and N rhizodeposition, plants can be C-13-N-15 double-labeled with glucose and urea using a stem-feeding method (wick method). However, it is unclear how the C-13 applied as glucose is released into the soil as rhizorespiration in comparison with the C-13 applied as CO2 using a natural uptake pathway. In the present study, we therefore compared the short-term fate of C-14 and N-15 in white lupine and pea plants applied either by the wick method or the natural pathways of C and N assimilation. Plants were pulse-labeled in (CO2)-C-14-enriched atmosphere and N-15 urea was applied to the roots (atmosphere-soil) following the natural assimilation pathways, or plants were simultaneously labeled with C-14 and N-15 by applying a C-14 glucose-N-15 urea solution into the stem using the wick method. Plant development, soil microbial biomass, total rhizorespiration, and distribution of N in plants were not affected by the labeling method used but by plant species. However, the N-15 : N ratio in plant parts was significantly (p < 0.05) affected by the labeling method, indicating more homogeneous N-15 enrichment of plants labeled via root uptake. After (CO2)-C-14 atmosphere labeling of plants, the cumulated (CO2)-C-14 release from roots and soil showed the common saturation dynamics. In contrast, after C-14-glucose labeling by the wick method, the cumulated (CO2)-C-14 release increased linearly. These results show that C-14 applied as glucose using the wick method is not rapidly transferred to the roots as compared to a short-term (CO2)-C-14 pulse. This is partly due to a slower C-14 uptake and partly due to slow distribution within the plant. Consequently, C-14-glucose application by the wick method is no pulse-labeling approach. However, the advantages of the wick method for C-13-N-15 double labeling for estimating rhizodeposition especially under field conditions requires further methodological research.