Frequently Asked Questions
Q: How is carbon sequestered in soil?
A: Carbon is sequestered in soil by plants through photosynthesis and can be stored as soil organic carbon (SOC). Agroecosystems can degrade and deplete the SOC levels but this carbon deficit opens up the opportunity to store carbon through new regenerative agricultural practices such as planting spineless opuntia.
Soil can also store carbon as carbonates. Such carbonates are created over thousands of years when carbon dioxide dissolves in water and percolates the soil, combining with calcium and magnesium minerals, forming “caliche” in desert and arid soil.
Q: What is unique about the way opuntia sequesters carbon dioxide?
A: There are at least three different pathways in which photosynthesis can occur:
C3 (normal conditions)
C4 (high temperature/high water/high light availability)
CAM (high temperature/low water availability)
CAM stands for crassulacean acid metabolism. It is essentially a means of isolating in time the carbon dioxide intake from sunlight-fueled photosynthesis. Opuntia are CAM photosynthesisers.
C4 and CAM photosynthesis are both adaptations to semi-arid conditions, because they are more efficient in the conservation of water. CAM plants are also able to “idle,” thus saving energy and water during periods of water scarcity and/or high temperatures. CAM plants include many succulents such as Cactaceae, Agavacea, Crassulaceae, Euphorbiaceae, Liliaceae, Vitaceae (grapes), Orchidaceae and bromeliads.
As a CAM plant, opuntia can preferentially take in carbon dioxide during the night hours, fixing it within the plant as an organic acid with the help of an enzyme. During the daylight hours, opuntia can have normal C3 metabolism, converting carbon dioxide directly into sugars or storing it for the next day’s metabolism for use in the evening. During the day, the stomata can remain closed, using the internally released carbon dioxide and thus sealing the plant off from the outside environment. This is a more efficient way to prevent water loss compared to normal plant respiration. This modified effect seems to more beneficial to the plant when there is a considerable difference between daytime and nighttime temperatures.