Contributions of current year photosynthate to fine roots estimated using a 13C-depleted CO2 source

The quantification of root turnover is necessary for a complete understanding of plant carbon (C) budgets, especially in terms of impacts of global climate change. To improve estimates of root turnover, we present a method to distinguish current- from prior-year allocation of carbon (C) to roots in global change experiments by using changes in C-13:C-12 ratios (delta(13)C values) resulting from application of C-13-depleted, tank-derived CO2. In a 4-year study examining effects of elevated CO2 and temperature on reconstructed Douglas-fir (Pseudotsuga mensiezii) ecosystems, delta(13)C patterns of fine roots and foliage were measured yearly. Native soil of low nitrogen (N) content was used, so plant N supply relied on natural soil N processes. Under these N-limited conditions, elevated CO2 or temperature did not affect the proportion of fine root C derived from current- year photosynthate. Regression analyses showed that 75% of fine root C originated from current- year photosynthate. The method is useful as an independent measure of the contribution of current- year photosynthate to root C and could be used to improve estimates of root C budgets with concurrent measurements of root C pools. We calculated a C-13 enrichment of root C relative to foliar C of 2%. This enrichment agrees with prior measurements of the enrichment of heterotrophic versus autotrophic plant tissues and must be accounted for when using shifts in foliar delta(13)C to calculate inputs of plant C into the soil. This enrichment probably contributes to the progressive enrichment in C-13 with increasing depth in soil profiles.