13C pulse labeling to assess C partitioning in dogwood – from foliage to fungi

Background/Question/Methods

The rates, timing and ultimate fate of plant carbon (C) partitioning of photosynthate remains a key uncertainty in mechanistic ecosystem models. In the Community Land Model (CLM), C is partitioned between storage, growth and respiration of primary plant components (foliage, wood, root), but does not consider other C sinks, such as reproductive structures or mycorrhizal fungi. Our objective was to measure seasonal patterns of C flux within individual dogwood trees following pulse-labeling with ¹³CO₂. Shading treatments were added to manipulate the magnitude of available photosynthate for partitioning to assess relative sink strength response. Spatially isolated mature dogwood trees were heavily instrumented to monitor sap flow, stem growth, root growth, soil moisture, soil respiration, and soil and air temperature. Trees were further isolated by trenching and herbaceous control, and root or root+fungal exclusion chambers were installed into each plot. A brief ¹³CO₂ label was applied to each of four replicate trees seasonally (spring, summer, fall), then a shade treatment was applied to two of the trees. The progression of ¹³C was tracked through foliage, phloem, roots, and soil, root and fungal respiratory soil CO₂ efflux.

Results/Conclusions

Foliage was heavily labeled with ¹³C following exposure to air enriched with ¹³CO₂. The label rapidly progressed from foliage through phloem and into roots within several days.  There were substantial seasonal patterns of C flow into emerging second flush leaves and into the developing fruits. ¹³CO₂ surface efflux from fungal ingrowth chambers (i.e., 61μm aperture mesh) was similar in timing (~3 days) to control chambers and enrichment (0 to +20‰) was similar to control (0 to +75‰) and well above exclusion chambers (-15 to -10‰). Fungal hyphae extracted from bulk soil were also labeled with ¹³C, further supporting rapid transfer of labeled photosynthate from roots into the fungi. Two glomeromycetes (arbuscular mycorrhizal fungi) were found in the fungal ingrowth chambers: Glomus etunicatum (abundance= 80%) and G. intraradices (abundance= 20%). Results will be used to assess partitioning routines in a point version of CLM4.5 that has been modified to include a parallel ¹³C pathway. Results provide data of mechanistic processes not well-refined in the models and will lead to improvements in model representation of C partitioning.