In this global-climate-change-obsessed world we frequently hear the term carbon sink, which is a ‘natural or artificial reservoir that accumulates and stores some carbon-containing chemical compound for an indefinite period’. The emphasis on carbon is understandable since carbon dioxide is one of the most important greenhouse gases (GHGs) whose accumulation in the atmosphere contributes to an increase in global temperatures – the greenhouse effect. Intuitively, one way to reduce that aerial CO2 burden is to reduce the amount added to the air, and/or increase the rate at which it is removed from the atmosphere. And the botanically inclined amongst you will surely suggest that maintaining, or even increasing, areas of vegetation – which by dint of that marvellous photobiochemical process known as photosynthesis consume CO2 – will be a desirable thing. Which is why traditionally we have presumed large forest areas to be such important carbon sinks – not only do they remove CO2 in photosynthesis, but also much of that carbon is fixed within their trunks and stored for as long as the tree continues to grow. Hence there are major concerns over the rate at which forests are being removed in such areas as the Amazon. Well, it seems that in concentrating on the big things we have ignored the very small things that are also major players in this drama and which help to make construction of that arboreal sink possible. For example, it is easy to forget that building such massive structures as trees requires not only carbon, but also nitrogen and all of the other essential plant nutrients. So, regardless of how much carbon is available, the value of trees as a carbon sink will be limited by the factors that limit plant growth. That is why you should not ignore the contribution of the photoautotrophic cryptogams – cyanobacteria, algae, fungi, lichens and bryophytes. Being so small they are easily overlooked, yet can cover large areas of such surfaces as soil, rocks and even other plants, and according to Wolfgang Elbert et al., globally they may account for almost 50% of terrestrial biological nitrogen fixation. Which, in turn, is necessary to help make atmospheric nitrogen biologically available to other organisms, such as trees, whose role as carbon sinks is so important in ameliorating atmospheric carbon levels… And we shouldn’t underestimate the importance of these land- and oceanic-based carbon sinks (in which regard, look out for my next post), whose global carbon uptake has almost doubled in the past 50 years according to Ashley Ballantyne et al.
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