A paper that is nearly 60 years old shows us just how intrinsically important CO2 is to life.
An acre of corn is a living machine drawing CO2 from the air around it. In windless conditions, CO2 concentrations over a cornfield build up each night as CO2 diffuses from higher air and the organic matter and bacteria create CO2 from the soil. A paper by Chapman et al from 1954[1], shows that as soon as the sun comes up, to power-up those dormant photosynthetic cells, the plants rapidly draw down as much CO2 as possible, and when the CO2 levels fall too low, plant growth surely slows.
On a windless day CO2 values rose to 410ppm overnight and fell to 210ppm during the morning.
This graph shows CO2 content of the air over a cornfield on a still day (no wind). Sunrise occurs at 5am and CO2 levels plummet til 8am, reaching their lowest by 1pm, which is nearly half the CO2 concentration of the peak reached overnight. The corn is affecting CO2 levels in air even as high as 150m or 500ft above. These level out by around 8am and only start to increase again, a couple of hours after sunset.
No wonder some farmers use greenhouses and pump in CO2 to boost their yields. The afternoon sun goes a-wasted as plants growth slows because CO2 levels are not high enough.
The message to gardeners is that this is why plants that get morning sun have an advantage.
Fig. 1. Variations in the C02 content of air in a corn field and 152 m above it on a still day. A C02 deficit of more than 100 lbs an acre was developed within 3 hrs after sunrise, to remain nearly constant until late afternoon.
Chapman point out that there is 40,000 lbs of CO2 normally present in the air over each acre.
This graph shows that on a windier day the effect is muted because CO2 is being brought in by the wind from areas around the field. CO2 falls ’til about 12 noon before leveling off as plant growth slows. Again, even air 150m above the field shows the effects of the plants underneath.
Fig. 2. Variations in the C02 content in a corn field and 152 m above it on a day with winds of 3 to 8 mph. Changes were less abrupt than those shown in figure 1, but very considerable deficits were still present.
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Figure 3B below shows what happens after a frost on a mature corn crop. Growth is noticeably slowed and delayed.
Fig. 3. A. CO2 variation on a windy morning — similar to figure 2. B. CO2 variation after frost. Note higher concentrations and erratic changes in the absence of important amounts of photosynthesis.
Discussion and Summary
The daytime decrease in the C02 content of the air among plants carrying on active photosynthesis has been confirmed and shown to extend, with some lag, to a height of 152 m or 500 ft. A typical fluctuation in the C02 content at 152 m was 0.03 % at night, decreasing to 0.027 % during the day. With winds of 5 mph or more the C02 content of the air in a corn field sampled at a height of 1 m, was nearly the same as that at the 152 m level, but on still days and nights a maximum variation of 0.02 % has been observed; from 0.041 % at night to 0.021 % during active photosynthesis. Since the equilibrium C02 level at which net photosynthesis becomes zero is in the neighborhood of 0.01 % (21) such marked drops in the 02 content of the air may be expected to limit photosynthesis and crop yields. The estimated annual per acre production of C02 in Iowa is of the same order as its use in photosynthesis, with decay of organic matter in the soil, or respiration of soil organisms, the dominant factor in C02 production. Although daily production may be considerably more or less than use, the 40,000 lb reserve of the air, plus exchange with other regions by air currents, serves to bring the C02 supply in the air a few meters above the soil to near “normal” each night. A deficit of 10% in C02 at 150 m during rapid photosynthesis may represent the gradient across which excess use by plants can be replaced from higher levels of the atmosphere. The rapid fluctuations in the C02 content of field air indicate that attempts to measure variations in different growing seasons in the average C02 content of the air (14) are subject to large errors and the results difficult to interpret. Similar problems beset attempts to measure long-time changes in average C02 concentration (9).
REFERENCES
[1^]Chapman H. W .,Gleason L. S., Loomis W. E. (1954): The carbon dioxide content of field air. Plant Physiology 29,6, pp 500-503 [PDF freely available]