I think you might be missing the point too. Yes, the carbon in the food we eat is where the carbon in our breath comes from. But the carbon that we used to get the ingredients in that food didn't certainly come from the atmosphere (e.g. half of the nitrogen used in agriculture comes from fossil fuels). You can't be a perfectly optimal salad eating machine. One of your fellow humans will ruin the equation the moment they buy produce from the modern supply chain.
No, all the carbon in food plants comes from CO2 from the atmosphere. (Ditto nitrogen by the way: the natural gas used in making nitrogen fertilizers is a source of hydrogen and possibly reaction energy, but not a significant source of N.)
> No, all the carbon in food plants comes from CO2 from the atmosphere.
Yes, this is what I said.
> the natural gas used in making fertilizer is a source of hydrogen and energy, but not a significant source of N.
There is no nitrogen at all in natural gas.
Plants cannot use atmospheric nitrogen on their own. They depend on either bacteria or humans to create some usable form of nitrogen. Any carbon captured in a plant that depended on a fossil fuel source of nitrogen cannot be considered carbon neutral, unless you draw a useless system boundary.
> Any carbon captured in a plant that depended on a fossil fuel source of nitrogen cannot be considered carbon neutral, unless you draw a useless system boundary.
What? That sounds really confused.
"Carbon neutral" in this context means a process that shuttles an atom of carbon around in a closed loop between the atmosphere and living organic matter.
To paraphrase what you're saying, human agriculture is not carbon neutral, so human breathing contributes to climate change, because humans require agriculture.
It's the kind of statement that is maybe technically correct if you look at it from the right perspective, but totally unhelpful to understanding ecological flows of atoms.
