Research: Soils Under-Appreciated Role in Global Atmospheric CO2

An enzyme, carbonic anhydrase (CA), switches carbon dioxide between its dissoluble and soluble forms, and is found in both plants and animals. It is an incredibly important enzyme for both respiration and photosynthesis. If CO2 were ionizing on its own, without an enzyme, it would take far longer, and the systems would be far less efficient. However, it leaves a mark: a heavy isotope of oxygen.

The amount of [heavy isotopic] δ18O in the soil has traditionally been used as a measurement of plant photosynthesis. The possibility of CA activity in soil has been disregarded bcause of the high levels of δ18O in just the top few centimeters of soil, indicating that it is due just from decomposing leaves. ...

The 18 authors of this paper decided that this assumption wasn’t good enough. What if microorganisms are creating δ18O in the soil due to their own CA activity? What would this mean for the overall oxygen budget? First of all, it would mean that plant photosynthesis would have a lesser role. It could also change the estimations of photosynthesis vs. respiration in our atmosphere, since the microbes could be either photosynthetic algae or cyanobacteria, or respiratory little buggers. ...

... data showed that naturally measured δ18O levels were greater than the control levels without CA: up to 300x in the more productive ecosystems! This provided clear evidence that δ18O is being created by soil microorganisms through CA enzymatic activity on their own. ...

So, really, what is this paper saying? First of all, don’t ignore the dirt! Soil microbes may be small, but they are vast in number and can really have an impact on our element cycling. More than anything, this paper helps to adjust previous models. It suggests that the soil may be more of a carbon dioxide sink that we previously thought, because we now have evidence that respiration is taking place due to this increase in δ18O from CA use.

To me, what this paper really shows is how little we know. We’re trying to model oxygen and carbon in the atmosphere and earth, and there’s so little way of knowing. If it weren’t for this enzyme, carbonic anhydrase, that happens to incorporate a heavy oxygen isotope, where would we be? Modelling is important, don’t get me wrong. But it is also incredibly frustrating because we really don’t know enough to create very accurate models. This paper is a little slice, sure; but we could be missing huge impacts just because they are untraceable.