Respiration and C dynamics in Poplar roots

Large amounts of C are allocated to tree roots, but little is known about the age and dynamics of their non-structural C (NSC). We measured bomb-radiocarbon (¹⁴C) in respired CO₂, non-structural (mainly sugars), and structural (cellulose) C in roots. The steady decline of Δ¹⁴C in atmospheric CO₂ since the 1960s indicates the mean time elapsed since the C in these pools was fixed. We measured coarse (>2 mm, mean 2.91 mm) and fine (<2 mm) roots from 12 German poplar trees sampled before and after girdling of 6 of the trees. All samples were taken in 2018, an exceptionally dry summer in Europe. The mean Δ¹⁴C ±SD of root-respired CO₂ (4.1 ± 3.6 ‰) in June-July was equal to current atmospheric Δ¹⁴CO₂ (1.2 ‰), irrespective of the mean age of root cellulose. During extended incubations, the Δ¹⁴C of root-respired CO₂ increased to ~10 ‰ 8 days after harvesting and up to 42 ‰ 17 days after harvesting. The mean Δ¹⁴C of soluble sugars in the roots was ~21 ‰. In September-October, almost three months after girdling, roots from girdled trees respired CO₂ with Δ¹⁴C of 7.9 ± 6.6 ‰ vs. 2.3 ± 6.1 ‰ in the ungirdled control trees. However, in both groups the respired CO_2­-Δ¹⁴C correlated with cellulose-Δ¹⁴C (R² = 0.37, 0.26 for girdled and control trees, respectively), suggesting that roots respired more stored C in the later growing season in this drought year, independent of treatment. The Δ¹⁴C values of soluble sugars were correlated with the Δ¹⁴C values of the cellulose (R²=0.83). On average, C in sugars was fixed more recently than cellulose, suggesting mixing of young C from other parts of the tree into the roots. Stem girdling did not affect the Δ¹⁴C of soluble sugars. Average total sugar concentrations (sucrose+ glucose+ fructose) were ~42 mg g^-1 and did not vary with sampling date, root class or treatment. Starch, measured only in September-October, was higher in coarse than in fine roots (12 vs. 3.8 mg g^-1). Respiratory loss of C was higher in the fine roots (~4 mgC g^-1 day^-1) than coarse roots (~2.4 mgC g^-1 day^-1), with no effect of girdling or sampling month. When normalize (expressed per gram dry root material), the NSC reservoirs and C loss rates suggest C turnover rates are 2-fold higher in fine roots than in coarse roots. The extended incubations indicate that detached roots are able to quickly utilize stored NSC, as indicated by the sharp Δ¹⁴CO₂ increase. In comparison, stem girdling had no measurable effect on respired CO₂-Δ¹⁴C, suggesting internal re-allocation of C from the lower stem base or large roots to smaller roots, and/or lower than expected metabolic consumption of C in reaction to girdling or because of the exceptional drought.