According to Tilman et al (5),

"LIHD plots had a total CO₂ sequestration rate of 4.4 Mg ha^-1 year^-1 in soil and roots during the decade of observation. Trends suggest that this rate might decline to about 3.3 Mg ha^-1 year^-1 during the second decade because of slower root mass accumulation."

The first decade total would be 44 Mg ha^-1, while the second decade total would be 33  Mg ha^-1, making a total of 77 Mg ha^-1 during the first two decades. But the proportion of carbon in CO₂ is 12/44 = 3/11. Therefore, 77 Mg CO₂ contains 21 Mg C, the figure used in the text.

Over the course of two decades, this comes to 21/20 = 1.05 Mg C, the quantity used further in the text.