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\begin{document}

\title{The size and the age of the metabolically active carbon in tree roots}



\author[1]{Boaz Hilman}%
\author[2]{Jan Muhr}%
\author[1]{Juliane Helm}%
\author[1]{Iris Kuhlmann}%
\author[1]{Erst-Detlef Schulze}%
\author[1]{Susan Trumbore}%
\affil[1]{Max Planck Institute for Biogeochemistry}%
\affil[2]{University of Goettingen}%


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  \date{May 24, 2024}


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\begin{abstract}
Little is known about the sources and age of C respired from tree roots.
Previous research in tree stems has identified two functional pools of
non-structural carbohydrates (NSC): an `active' pool supplied directly
from canopy photo-assimilates that supports metabolism and a `stored'
pool used when fresh C supplies are limited. We compared the C isotope
composition of water soluble NSC and respired CO\textsubscript{2} for
aspen roots (\emph{Populus tremula} hybrids) that were cut off fresh C
supply via stem-girdling and prolonged incubation of excised roots. We
used bomb radiocarbon to estimate the time elapsed since C fixation for
respired CO\textsubscript{2}, water-soluble C, and structural
\selectlanguage{greek}α-\selectlanguage{english}cellulose. While freshly excised roots respired CO\textsubscript{2}
with mean age \textless{}1 yr, within a week the age increased to
1.6-2.9 yr. Freshly excised roots from trees girdled \textasciitilde{}3
months previously had similar respiration rates and NSC stocks as
un-girdled trees, but respired older C (\textasciitilde{}1.2 yr). We
estimate the NSC in girdled roots must be replaced 5-7 times by reserves
remobilized from root-external sources. Using a mixing model and
observed correlations between \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{14}C of water-soluble C
and \selectlanguage{greek}α-\selectlanguage{english}cellulose, we estimate \textasciitilde{}30\% of C is `active'
(\textasciitilde{}5 mg C g\textsuperscript{-1}).%
\end{abstract}%



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