> 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.