Dude, Where's My Carbon?
Now that researchers have detected CO2-induced nutrient deficiencies, they are seeking to understand why they happen. And they think they have found some relatively simple underlying causes -- simple to scientists, that is, although perhaps not to those of us who glazed over in high-school biology.
We live in a carbon world, scientists explain: All life on earth, from oranges to orangutans, is carbon-based. Most of this carbon comes from our atmosphere, which is absorbed by plants, which pass it on to grazing animals, which in turn pass it on to their predators. Change the levels of atmospheric carbon, and all plants and animals along the chain may be affected.
Here's how: Plants create much of their biomass out of thin air, from a steady diet of CO2 sucked through small leaf openings called stomata. Then, via the miraculous sleight-of-hand known as photosynthesis, the plants combine CO2 and water in the presence of chlorophyll and sunlight to make carbohydrates, simple sugars, and complex starches, which provide energy for plant growth. Much of the remainder of what plants need -- nitrogen and trace elements -- doesn't come from the air, but is pulled up through the root system from the soil.
Scientists have isolated two mechanisms that potentially explain how elevated CO2 levels reduce plant nutrients. The first is a "biomass dilution" effect. As plants absorb more airborne carbon, they produce higher-than-normal levels of carbohydrates but are unable to boost their relative intake of soil nutrients. The result of this dilution effect is increased yields of carbohydrate-rich fruits, vegetables, and grains that contain lower levels of macro- and micronutrients. Put simply, a bite of bread in our current CO2 atmosphere ends up being more nutritious than one in the CO2-enriched atmosphere of the future.
A second problem: Plants exposed to increased CO2 levels start to narrow the stomata through which they inhale CO2 and exhale water vapor via transpiration. This benefits plants by making them more drought resistant, but it also means that fewer waterborne nutrients flow into the roots. According to Loladze, if carbon-dioxide levels are doubled, transpiration decreases by about 23 percent.
A particularly disturbing study suggests that the mechanisms of CO2 nutrient depletion may already be causing a decline in the quality of our food supply. Josep Penuelas of the Center for Ecological Research and Forestry Applications in Barcelona, Spain, compared historical plant samples grown at preindustrial levels of atmospheric CO2 with modern equivalents. He found that today's plants had the lowest levels of calcium, copper, iron, potassium, magnesium, sodium, sulfur, and zinc than at any time in the last three centuries.
