Allocation and residence time of photosynthetic products in a boreal forest using a low-level 14C pulse-chase labeling technique

Much of our understanding about how carbon (C) is allocated in plants comes from radiocarbon (C-14) pulse-chase labeling experiments. However, the large amounts of C-14 required for decay-counting mean that these studies have been restricted for the most part to mesocosm or controlled laboratory experiments. Using the enhanced sensitivity for C-14 detection available with accelerator mass spectrometry (AMS), we tested the utility of a low-level C-14 pulse-chase labeling technique for quantifying C allocation patterns and the contributions of different plant components to total ecosystem respiration in a black spruce forest stand in central Manitoba, Canada. All aspects of the field experiment used C-14 at levels well below regulated health standards, without significantly altering atmospheric CO2 concentrations. Over 30 days following the label application in late summer (August and September), we monitored the temporal and spatial allocation patterns of labeled photosynthetic products by measuring the amount and C-14 content of CO2 respired from different ecosystem components. The mean residence times (MRT) for labeled photosynthetic products to be respired in the understory (feather mosses), canopy (black spruce), and rhizosphere (black spruce roots and associated microbes) were < 1, 6, and 15 days, respectively. Respiration from the canopy and understory showed significantly greater influence of labeled photosynthates than excised root and intact rhizosphere respiration. After 30 days,similar to 65% of the label assimilated had been respired by the canopy,similar to 20% by the rhizosphere, and similar to 9% by the understory, with similar to 6% unaccounted for and perhaps remaining in tissues. Maximum C-14 values in root and rhizosphere respiration were reached 4 days after label application. The label was still detectable in root, rhizosphere and canopy respiration after 30 days; these levels of remaining label would not have been detectible had a C-13 label been applied. Our results support previous studies indicating that a substantial portion of the C fueling rhizosphere respiration in the growing season may be derived from stored C pools rather than recent photosynthetic products.