In the answer to this question it is pointed out that while lunar soil may provide nutrients for plants, water and carbon dioxide are also needed. Water seems to be available, but how could there be carbon dioxide, or any carbonaceous material? Is carbon-containing matter known on the Moon?
I remember reading somewhere ages back that there is enough material in a cubic metre of lunar regolith to make a sandwich. I can't find the original source for that, but this Forbes article makes a similar statement. Keep in mind a sandwich worth of carbon per ~2 tonnes of regolith is still pretty scarce.
Digging around for ppm values, there seem to be between 90 ppm and 130 ppm carbon from the Apollo sources of Lunar Regolith. Link
For the milennials out there, carbon has about half the abundance of diamonds in minecraft.*
As to what form that carbon is in, I couldn't find any information, and suspect it's not really known. I'll quote part of the abstract of this paper.
Although the majority of the solar wind implanted carbon may be released and quantitated by pyrolysis there is little information to identify which elements were bonded to the carbon in the sample, if indeed any bonds were present at all.
The carbon content is analysed by pyrolysis, i.e. heating the sample up a lot and analysing the vapour that comes off. This process breaks the bonds to the other compounds it was originally attached to.
*There is an average of 3.7 diamond blocks per 16384 blocks of stone (1 chunk, or 16 x 16 x 64). This works out to about 220 ppm.