You're going to extrapolate from lower nitrogen levels in plants in higher CO2 conditions (without a citation, might i add) to higher CO2 levels causing overall lower nutrition? and more aggressive soil depletion? Keep in mind that obviously more plant growth means more plant decay which cycles back int othe ecosystem.
Are you trying to suggest that we should be decreasing agricultural yields so as to prevent soil depletion? There are better ways.
Odd card to play when your comment had no citations either, and you seem to be ignoring my request for them....
My claim that increased CO2 leads to more biodiversity is more tenuous but is more likely than a decrease in biodiversity, all other factors being controlled for.
https://www.mpg.de/6650626/carbon-dioxide-crop-yields
Sure doesn't look as simple as the order of colors in a rainbow to me.
So, all in all, the downsides outweigh the upsides -- faster growth, lower nutrition, more aggressive soil depletion, and increased water runoff. On the bright side, carbon sequestration increases up to 40%, so the efficacy of planting forests as a sequestration measure will increase.
As for biodiversity -- well, dieoffs are already outpacing speciation. Unless you believe in abiogenesis, the trendline is moving sharply downward.