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 13 C‐ 15 N double‐labeled with glucose and urea using a stem‐feeding method (wick method). However, it is unclear how the 13 C applied as glucose is released into the soil as rhizorespiration in comparison with the 13 C applied as CO 2 using a natural uptake pathway. In the present study, we therefore compared the short‐term fate of 14 C and 15 N 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 14 CO 2 ‐enriched atmosphere and 15 N urea was applied to the roots (atmosphere–soil) following the natural assimilation pathways, or plants were simultaneously labeled with 14 C and 15 N by applying a 14 C glucose– 15 N 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 15 N : N ratio in plant parts was significantly ( p < 0.05) affected by the labeling method, indicating more homogeneous 15 N enrichment of plants labeled via root uptake. After 14 CO 2 atmosphere labeling of plants, the cumulated 14 CO 2 release from roots and soil showed the common saturation dynamics. In contrast, after 14 C‐glucose labeling by the wick method, the cumulated 14 CO 2 release increased linearly. These results show that 14 C applied as glucose using the wick method is not rapidly transferred to the roots as compared to a short‐term 14 CO 2 pulse. This is partly due to a slower 14 C uptake and partly due to slow distribution within the plant. Consequently, 14 C‐glucose application by the wick method is no pulse‐labeling approach. However, the advantages of the wick method for 13 C‐ 15 N double labeling for estimating rhizodeposition especially under field conditions requires further methodological research.