Stable isotope variants as tracers for studying biosphere-atmosphere exchange

Rural air samples were all collected from the layer of air close to the ground . . . under circumstances where the metabolic activity of plants might be expected to influence the carbon dioxide composition of the air. This is so because plants exchange carbon dioxide with the atmosphere by means of respiration and assimilation and also because carbon dioxide is evolved from the ground through decay of organic material in the soil and respiration of plant roots . . . Thus the relationship between carbon isotope ratio and molar concentration observed for the carbon dioxide of rural air is explained if carbon dioxide is added to or subtracted from the atmosphere by plants or their decay products.Charles Keeling (1958), Scripp Institution of Oceanography