Maybe this is what you like:
\documentclass{article}\usepackage{tikz-cd}\begin{document} \begin{center} \begin{tikzcd}[every matrix/.append style={name=mymatr},row sep=1.5em,column sep=0.3em, ampersand replacement=\&, execute at end picture={\draw (mymatr-2-1.south west) -- (mymatr-2-7.south east);}] {1} \arrow{d} \& {} \& {} \& {} \& {} \& {} \& {} \\ {2} \& {+} \& {3} \& {+}\arrow{d} \& {4} \& {+} \& {5} \\ {} \& {} \& {} \& {6} \& {} \& {} \& {} \\ \end{tikzcd} \end{center}\end{document}