CO2- Good or Bad For Plant Growth?

Back in the day when I used to grow pot, I made a simple carbon dioxide generator with empty 2-liter bottles filled with yeast. When the yeast was fed a little sugar, it would produce CO2 which plants use for photosynthesis. This is pretty common for growers to do, although there are much more efficient ways (albeit more expensive) to do it.

But I recently read that excess CO2 can actually lower plant growth! When plants are provided with more CO2, they use less water because they don't need to keep their stomata open as long to get enough CO2. But that means that they don't cycle as much nutrients (nitrogen, potassium, magnesium, trace minerals) from the soil that they need, because the water isn't flowing. The concentration of nitrogen decreases, thus decreasing the plant's protein levels and growth ability. So, their growth is stunted. And as they are nutrient deficient, when leaves drop, or they die, fewer nutrients are returned to the soil, and this creates a cycle of diminishing returns.

So why the discrepancy? It turns out some plants do better in elevated CO2 environments, while others do not. The difference in response to elevated CO2 has to do with different photosynthetic types. Most plant species (~90%) utilize a photosynthetic process known as C3 photosynthesis. Other species use either of two physiologically distinct processes known as C4 and CAM photosynthesis.