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The composition of planetary solids and gases is largely rooted in theprocessing of volatile elements in protoplanetary disks. To shed light on thekey processes, we carry out a comparative analysis of the gas-phase carbonabundance in two systems with a similar age and disk mass, but differentcentral stars: HD 100546 and TW Hya. We combine our recent detections ofC$^{0}$ in these disks with observations of other carbon reservoirs (CO,C$^{+}$, C$_{2}$H) and gas mass and warm gas tracers (HD, O$^{0}$), as well asspatially resolved ALMA observations and the spectral energy distribution. Thedisks are modelled with the DALI 2D physical-chemical code. Stellar abundancesfor HD 100546 are derived from archival spectra. Upper limits on HD emissionfrom HD 100546 place an upper limit on the total disk mass of$\leq0.1\,M_{\odot}$. The gas-phase carbon abundance in the atmosphere of thiswarm Herbig disk is at most moderately depleted compared to the interstellarmedium, with [C]/[H]$_{\rm gas}=(0.1-1.5)\times 10^{-4}$. HD 100546 itself is a$\lambda\,$Bo\"{o}tis star, with solar abundances of C and O but a strongdepletion of rock-forming elements. In the gas of the T Tauri disk TW Hya, bothC and O are strongly underabundant, with [C]/[H]$_{\rm gas}=(0.2-5.0)\times10^{-6}$ and C/O${>}1$. We discuss evidence that the gas-phase C and Oabundances are high in the warm inner regions of both disks. Our analyticalmodel, including vertical mixing and a grain size distribution, reproduces theobserved [C]/[H]$_{\rm gas}$ in the outer disk of TW Hya and allows to makepredictions for other systems.

Volatile carbon locking and release in protoplanetary disks. A study of TW Hya and HD 100546