Respiration and C dynamics in Poplar roots
Authors: Hilman Boaz , Muhr Jan , Helm Juliane , Kuhlmann Iris , Trumbore Susan 
Published: March 2020

&lt;p&gt;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 (&lt;sup&gt;14&lt;/sup&gt;C) in respired CO&lt;sub&gt;2&lt;/sub&gt;, non-structural (mainly sugars), and structural (cellulose) C in roots. The steady decline of &amp;#916;&lt;sup&gt;14&lt;/sup&gt;C in atmospheric CO&lt;sub&gt;2&lt;/sub&gt; since the 1960s indicates the mean time elapsed since the C in these pools was fixed. We measured coarse (&gt;2 mm, mean 2.91 mm) and fine (&lt;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 &amp;#916;&lt;sup&gt;14&lt;/sup&gt;C &amp;#177;SD of root-respired CO&lt;sub&gt;2&lt;/sub&gt; (4.1 &amp;#177; 3.6 &amp;#8240;) in June-July was equal to current atmospheric &amp;#916;&lt;sup&gt;14&lt;/sup&gt;CO&lt;sub&gt;2&lt;/sub&gt; (1.2 &amp;#8240;), irrespective of the mean age of root cellulose. During extended incubations, the &amp;#916;&lt;sup&gt;14&lt;/sup&gt;C of root-respired CO&lt;sub&gt;2&lt;/sub&gt; increased to ~10 &amp;#8240; 8 days after harvesting and up to 42 &amp;#8240; 17 days after harvesting. The mean &amp;#916;&lt;sup&gt;14&lt;/sup&gt;C of soluble sugars in the roots was ~21&amp;#160;&amp;#8240;. In September-October, almost three months after girdling, roots from girdled trees respired CO&lt;sub&gt;2&lt;/sub&gt; with &amp;#916;&lt;sup&gt;14&lt;/sup&gt;C of 7.9 &amp;#177;&amp;#160;6.6&amp;#160;&amp;#8240; vs. 2.3 &amp;#177;&amp;#160;6.1&amp;#160;&amp;#8240; in the ungirdled control trees. However, in both groups the respired CO&lt;sub&gt;2&amp;#173;&lt;/sub&gt;-&amp;#916;&lt;sup&gt;14&lt;/sup&gt;C correlated with cellulose-&amp;#916;&lt;sup&gt;14&lt;/sup&gt;C (R&lt;sup&gt;2&lt;/sup&gt; = 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 &amp;#916;&lt;sup&gt;14&lt;/sup&gt;C values of soluble sugars were correlated with the &amp;#916;&lt;sup&gt;14&lt;/sup&gt;C values of the cellulose (R&lt;sup&gt;2&lt;/sup&gt;=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 &amp;#916;&lt;sup&gt;14&lt;/sup&gt;C of soluble sugars. Average total sugar concentrations (sucrose+ glucose+ fructose) were ~42 mg g&lt;sup&gt;-1 &lt;/sup&gt;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&lt;sup&gt;-1&lt;/sup&gt;). Respiratory loss of C was higher in the fine roots (~4 mgC g&lt;sup&gt;-1&lt;/sup&gt; day&lt;sup&gt;-1&lt;/sup&gt;) than coarse roots (~2.4 mgC g&lt;sup&gt;-1&lt;/sup&gt; day&lt;sup&gt;-1&lt;/sup&gt;), 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 &amp;#916;&lt;sup&gt;14&lt;/sup&gt;CO&lt;sub&gt;2&lt;/sub&gt; increase. In comparison, stem girdling had no measurable effect on respired CO&lt;sub&gt;2&lt;/sub&gt;-&amp;#916;&lt;sup&gt;14&lt;/sup&gt;C, 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.&lt;/p&gt;