Also, this. Carbon dioxide-enhanced crops lose nutritional value
https://www.forbes.com/sites/fionamcmillan/2018/05/27/rising-co2-is-reducing-the-nutritional-value-of-our-food/#3dee05b75133
Interesting article, but puzzling. It's been known for years that modern agricultural practices tend to reduce the nutrient and vitamin content of food crops in general, compared with preindustrial times when farming was more natural and organic with less use of growth-enhancing fertilizers such as Nitrogen.
CO2 is like a hidden fertilizer. It's always there, but increases so slowly from year to year that its effect would be impossible to detect in a single growing season, outside of an artificial environment of significantly enhanced CO2 levels, such as in a greenhouse or a FACE experiment.
However, the nutritional content of any particular type of food crop can vary enormously depending on the location where it was grown, the farming practices used, the type of fertilizers used, the biodiversity and health of the soils, the mineral content of the soils, and so on.
Being concerned about a possible lack of Selenium in my diet a few years ago, I did some research into the Selenium content of Brazil nuts. I came across recommendations that as little as
one Brazil nut per day could meet the recommended daily dosage of 55 mcg. Other sites recommended as many as 5 or 6 Brazil nuts per day, which seemed rather odd, so I did some more searching.
I came across some scientific research that rigorously examined the Selenium content of Brazil nuts grown in many different locations around the world. I was amazed that the Selenium content varied by a factor of 10. In other words, if just one Brazil nut grown in ideal conditions could meet my daily needs for Selenium, it could take as many as 10 Brazil nuts grown in less ideal conditions to meet the same daily requirements for Selenium.
One major issue I have with experiments that show that enhanced CO2 levels reduce the protein, mineral and vitamin content of major food crops such as rice, is that these foods are already being stripped of much of their nutritional value through processing, particularly the processing of brown whole grain rice into nice, clean and attractive white rice, which most people eat, even in desperately poor countries.
From the following site
http://www.whfoods.com/genpage.php?tname=foodspice&dbid=128"The complete milling and polishing that converts brown rice into white rice destroys 67% of the vitamin B3, 80% of the vitamin B1, 90% of the vitamin B6, half of the manganese, half of the phosphorus, 60% of the iron, and all of the dietary fiber and essential fatty acids. Fully milled and polished white rice is required (by law in the US) to be "enriched" with vitamins B1, B3 and iron." In other words, poor people who are undernourished should be encouraged to change their diet from white rice to whole-grain brown rice. The solution is education, not reducing CO2 levels.
A doubling of CO2 levels results in approximately a 1/3rd increase in rice yields, all else remaining the same. If you were living in a poverty stricken community where people were starving and undernourished, and someone offered you a choice of 90 Kg of polished white rice grown in preindustrial CO2 levels of 280 ppm, or 133 Kg of brown rice grown in twice the levels of CO2 (560 ppm), which would you choose? (I've used the figure 90 instead of 100, for the white rice, on the assumption that about 10% of the mass is thrown away during polishing).