Stable-isotope labeling and probing of recent photosynthates into respired CO2, soil microbes and soil mesofauna using a xylem and phloem stem-injection technique on Sitka spruce (Picea sitchensis)

Rationale: Here we report on the successful application of a novel stem-injection stable-isotope-labeling and probing technique in mature trees to trace the spatial and temporal distribution of rhizosphere carbon belowground. Methods: Three 22-year-old Sitka spruce trees were injected with 6.66 g of 13C-labeled aspartic acid. Over the succeeding 30 days, soil CO2 efflux, phospholipid fatty-acid (PLFA) microbial biomarkers and soil invertebrates (mites, collembolans and enchytraeids) were analyzed along a 50 m transect from each tree to determine the temporal and spatial patterns in the translocation of recently fixed photosynthates belowground. Results: Soil δ13CO2 values peaked 13–23 days after injection, up to 5 m from the base of the injected tree and was, on average, 3.5‰ enriched in 13C relative to the baseline. Fungal PLFA biomarkers peaked 2–4 days after stem-injection, up to 20 m from the base of the injected tree and were 13C-enriched by up to 50‰. Significant 13C enrichment in mites and enchytraeids occurred 4–6 days after injection (by, on average, 1.5‰). Conclusions: Stem injection of large trees with 13C-enriched compounds is a successful tool to trace C-translocation belowground. In particular, the significant 13C enrichment of CO2 and enchytraeids near the base of the tree and the significant 13C enrichment of PLFAs up to 20 m away indicate that mature Sitka spruce (Picea sitchensis) have the capacity to support soil communities over large distances.