D2. [...] The CO2 we generate from fossil are not the same we breathe out. Trees don't absorb fossil fuel CO2 as well as what we breathe out.

I found a bit more information about plant preferences for carbon isotopes. It seems to be implying that plants absorb fossil fuel CO2 better than atmospheric CO2, since plants prefer the lighter carbon 12C isotope, plants have lower 13C/12C ratios than the atmosphere, and fossil fuel is made out of plants, hence fossil fuel has a similar carbon isotope ratio to plants.

This doesn't support or refute your comment about CO2 we breathe out being easier for plants to consume, but it does suggest that CO2 from burning fossil fuel is easier for plants to consume than general atmospheric CO2.

Carbon is composed of three different isotopes, 14C, 13C and 12C. 12C is the most common. 13C is about 1% of the total. 14C accounts for only about 1 in 1 trillion carbon atoms.

CO2 produced from burning fossil fuels or burning forests has quite a different isotopic composition from CO2 in the atmosphere. This is because plants have a preference for the lighter isotopes (12C vs. 13C); thus they have lower 13C/12C ratios. Since fossil fuels are ultimately derived from ancient plants, plants and fossil fuels all have roughly the same 13C/12C ratio – about 2% lower than that of the atmosphere. As CO2 from these materials is released into, and mixes with, the atmosphere, the average 13C/12C ratio of the atmosphere decreases.

Source: http://www.realclimate.org/index.php/archives/2004/12/how-do...