Carbon in the time of Covid

Mark Roberts

This actually has nothing to do with Covid-19 or cholera (as in the book title which the headline is very loosely based on). If you cast your mind back a few months, the world was fixated on carbon sequestration.

It seemed like every few weeks another tree planting programme was announced: a million trees here, 10 million trees there – trees will save us. Every programme was different, yet in essence every programme was exactly the same: plant trees so they can trap and store atmospheric carbon to either mitigate or defer global warming and slow climate change.

But not everyone was convinced; tree haters, flat-earthers and pandemic deniers had a different view. Tree planting will not save us: ‘It’ll take a 100 years’, ‘There is not enough land’, ‘When trees die the carbon is released’, ‘Who will look after them?’, ‘We’ll be planting the wrong sort of trees’ etc. And if the opinions of the vocal minority weren’t bad enough, those with money offered another view: they produced leading scientific authorities and experts from unknown places reporting things like ‘trees emit reactive volatile gases that contribute to air pollution and are hazardous to human health’, ‘if all of these trees grow there will be so much shading species diversity will stop’ (we will go extinct).

But there was also a third group, this group of people focused on carbon sequestration through different mechanisms: ‘We don’t need to plant trees because we can trap carbon in the soil and in the ocean. In fact, we don’t need to do anything differently because we’ve got dirt and there are oceans of water in the sea.’ Like all good theories, conspiracy and otherwise, there is something to this – but how much something, and is that something actually something?

Soil and carbon

Soil carbon sequestration, as the name suggests, is where carbon dioxide is removed from the atmosphere and stored in the soil. This process is mainly undertaken by plants through photosynthesis, and carbon is stored below ground as soil organic matter (SOM). SOM can improve soil structure, increase the retention of water and nutrients, and reduce erosion. SOM can increase food security and decrease the negative impacts on natural environments and ecosystems. SOM is great, and as we are only using about 11% of the land available on the planet for crop production, we could double what we do and still not cover all of the potentially usable lands. So not only will soil carbon sequestration save the environment, defer global warming and slow climate change, but we can feed the hungry at the same time. Soil carbon sequestration is the answer … or is it?

Carbon is contained in the soil through a complex mixture of carbon compounds, consisting of decomposing plant and animal tissue, soil microbes (protozoa, nematodes, fungi, and bacteria), and soil minerals. In a healthy soil, many of these carbon compounds are in a continuous state of flux, cycling from one state to another. Throughout this process, carbon can remain stored in a soil for hundreds if not thousands of years, but when the soil is disturbed it can be quickly released back into the atmosphere. Minimising soil disturbance and maximising organic activity are key to retaining soil carbon.

In an agricultural context, soil productivity can be directly linked to SOM levels, a reduction in organic matter can reduce crop yields and SOM-depleted soils can be a precursor to large-scale ecosystem collapse. Common agricultural practices such as fertilising, ploughing and tilling can disrupt natural carbon cycling and increase erosion and nutrient leaching from soils; apart from effectively releasing carbon back to the atmosphere, these practices can contribute to eutrophication of inland aquatic and coastal ecosystems.

Luckily low-productivity soils can be remedied through the addition of fertiliser. But the addition of synthetic/inorganic agricultural fertilisers can temporarily inhibit microbial growth and reduce rhizobial bacteria and mycorrhizal fungi populations. With organic soil activity reduced, the soil productivity can continue to decline – so more fertiliser must be added. As more fertiliser is added, organic soil activity reduces, SOM levels decline and eventually the water-holding capacity of the soil also decreases. Reduced soil moisture equals a further reduction in organic soil activity and reduced nutrient availability for the crop. Awkward, but there is more. With the loss of soil moisture and SOM, soil texture changes; to improve water infiltration and assist root growth, soil conditioning in the form of ploughing and tilling is required – which effectively releases carbon back to the atmosphere …

This is, of course, the worst-case scenario and not all agricultural activities deplete SOM, release stored carbon and reduce soil productivity, but some do, especially largescale intensive cropping. Intuitive producers (the ones that see benefits in carbon sequestration, saving the environment, slowing climate change and feeding the world) undertake soil-regenerative techniques that include maintaining year-round crop cover, minimising soil disturbance, increasing soil organic content, increasing species diversification and … planting trees.

In short, the process of soil carbon sequestration can be expedited by planting trees.

Phytoplankton and carbon

So, what about ocean carbon sequestration? Carbon is mainly sequestered in the ocean by phytoplankton, floating microscopic marine plants (algae) that contain chlorophyll. Just like terrestrial plants, they consume carbon dioxide during photosynthesis and the carbon is incorporated into the plant’s structure. It turns out that phytoplankton consume almost as much carbon dioxide as terrestrial plants (about 50 billion tonnes of inorganic carbon per year) and are responsible for producing over 75% of the world’s oxygen. So maybe there is something to ocean carbon sequestration after all.

Phytoplankton are indeed important to the planet’s carbon cycle, but when it comes to storing the carbon they consume, we need to consider the life of our average marine algae. To keep the oceanic carbon cycle pumping, phytoplankton need to entirely reproduce themselves about every eight days (on average 45 times a year). Terrestrial plants, on the other hand, reproduce themselves entirely about once every 10 years. So, the issue with phytoplankton is biomass, or lack thereof. The combined biomass of all terrestrial plants is estimated to be about 500 billion tonnes (much of it in the form of wood), whereas our floating microscopic algae might be as little 45 billion tonnes. And of those 45 billion tonnes of living, dying and cycling algae, only a fraction of one percent actually makes it all that way down to the bottom of the ocean and becomes trapped as sediment.

So, the mechanism of ocean carbon sequestration is not all that efficient, although given enough time it obviously works as the majority of ‘fossils’ that our fossil fuel comes from were once plankton. And oceans remain the largest carbon pools on earth, containing about 50 times more carbon than in the atmosphere and 20 times more than in the soil. But as a way and means of addressing climate change in the short term phytoplankton are unlikely to save us.

Trees and carbon

So, in this time of Carbon and Covid, it actually seems that tree planting programmes are the best carbon-sequestration solution currently available. A million trees, a billion trees or just one tree – if you have survived Covid-19 you should probably go out and start planting trees.

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