A pulse-chase method to 13carbon-label Douglas-fir seedlings for decomposition studies

Tracking litter decomposition and root carbon (C) flow via stable isotopes, such as C-13, is gaining popularity because of reliable and affordable mass spectrometry. Although model C-13-labeled compounds such as glucose or acetate are available commercially, there is a need for large quantities of C-13-labeled plant materials to realistically study decomposition, C cycling, and biogeochemical processes in situ or in the laboratory. The objective of this research was to determine whether a pulse-labeling technique to label Douglas-fir seedlings with (CO2)-C-13 would result in significant quantities of C-13-labeled needles, stems, and roots that would be sufficiently enriched for in situ tracking of plant C into soil organic matter fractions. Once a week, for up to 9 weeks, 670 Douglas-fir seedlings were exposed to (CO2)-C-13 in a closed chamber. The (CO2)-C-13 (99 at.%) was injected into the chamber in quantities proportional to the photosynthetic rate, providing about 3 L of (CO2)-C-13 in total. More than 1.5 kg of seedling dry matter was produced, and the C-13 distribution was relatively similar among plant parts, with 1.57 in needles, 1.45 in stems, and 1.36 atom% C-13 in roots compared with control litter that had less than 1.084 at.% 13 C. Adding an additional pulse to one half the seedlings at 9 weeks resulted in needles and stems being further enriched over seedlings that had received (CO2)-C-13 pulses over 8 weeks. A 2-month field incubation study showed that for microcosms amended with labeled needle or root materials, C-13 could be tracked into whole soil, free particulate organic matter, or the humin fraction (recalcitrant C pool that is insoluble in strong base or acid). The pulse-labeling technique and chamber design provide large quantities of C-13-labeled plant material that can be used to trace residue decomposition rates in field or laboratory studies.