14. Soils: Carbon Source or Sink?

We know burning fossil fuels is bad because it emits CO₂ (a greenhouse gas)… but did you know that soils emit CO₂ too?! Some of these ‘emissions’ are natural, but poor soil management has significantly increased them, accelerating climate change. HOWEVER, soils can also absorb CO₂, which could help mitigate climate change. The balance between emission and absorption determines whether soils are a carbon source (bad) or sink (good). Let’s investigate …

How much carbon is stored in the soil?
Around 2700Pg (Pg = petagram = 10¹⁵g = 1 billion tonnes) of carbon is stored in the soil globally. This consists of ~40% inorganic carbon (SIC), mainly carbonates and ~60% organic carbon (SOC). SOC is more dynamic (more easily lost and restored), so this post will focus on SOC. All SOC values are estimates that have varied widely over recent decades, as shown in the graph below. Total SOC is currently estimated at ~1500Pg.

Fig. 1. Estimates of global soil organic carbon stocks from the literature through time. Median across all estimates 1460.5 Pg C, range 504–3000 Pg C, n = 27 studies, based on spatially explicit (red; median 1437 Pg C, range 504–2469.5 Pg C, n = 7) and nonspatially explicit methods (blue; median 1388.5 Pg C, range 710–3000 Pg C, n = 20). Lines connect minimum and maximum estimates of soil organic carbon reported by the same study. (Source: Scharlemann et al., 2014) 

Is 2700Pg a lot or a little? To give some sense of scale I’ve placed these values in the context of the 5 main global carbon pools in the diagram below. (The circles aren’t drawn to scale because the oceanic pool is comparatively so large).

Fig. 2 The five major global carbon pools along with the % of total carbon that they contain (diagram drawn by me but based on data from Lal, 2008 and Scharlemann et al., 2014)

  
WOW – the oceans vastly overshadow the other four pools! However soils are clearly still significant. They store over 3 times as much as the atmosphere and around 4 times as much as all living organisms. In fact, soils are responsible for most of the biotic storage because ~80-90% of this is land plants.

The SOC budget:
The mass of SOC depends on the balance between inputs and outputs (Fig.3), just as the water level in a bathtub is determined by the flow rate in through the tap and out through the drain. The diagram below shows the main inputs and outputs to the ‘SOC bath’.

Fig. 3 The balance between carbon inputs and outputs determines the quantity and flux of the soil organic carbon pool

Fig. 4 Soil organic carbon budget represented as water in a bath along with the main carbon inputs and outputs (diagram made by me).

Distribution of SOC:
SOC levels vary globally. The first map below shows large-scale SOC trends that result from differences in climate. The most carbon rich soils are found in cold, often waterlogged, northern regions where oxidation of organic matter is slow. The second map shows carbon stored in the terrestrial biome. Dr Ed Tanner, co-author of these maps, explained that, “if we’re interested in conserving carbon, which we ought to be, we ought to conserve soils in temperate regions, and plants in tropical regions”.

Fig. 5 (A) global distribution of soil organic carbon (SOC) storage (tons of C ha^-1) to 1m depth based on Harmonized World Soil Database version 1.1; (B) global distribution of biotic carbon storage (tons of C ha^-1) (Source: Scharlemann et al., 2014)

    

Are we losing SOC?
Yes! Cultivated (although not all) soils have lost between 50-75% of their original SOC pool. But remember that only chemical SOC loss increases atmospheric CO₂. Depletion of SOC has so far contributed about 78±12 Pg C to the atmosphere. Land-use change has contributed a further 136±55 Pg C. This compares to an estimated 270±30 PgC contributed by fossil fuel combustion.

Fig. 6 The historic CO₂ emissions from the pedologic, biotic and geologic carbon pools into the atmosphere (based on data from Zomer et al., 2017)

Why are we losing SOC?

(1) Land use change from forests to agriculture has reduced organic inputs because the crops that replaced the trees are harvested, not left as litter when they die.
(2) Conventional ploughing increases the exposure of soil to air, which increases the rate of oxidation. (I had no idea that ploughing was such an issue!).

(3) You may imagine the carbon rich soils of the Northern hemisphere are safe because these areas are too cold for agriculture... but unfortunately not. Increased temperatures due to climate change are predicted to increase rates of decomposition and therefore increase the amount of CO₂ they release, triggering a positive feedback loop. 

Fig. 7 A farmer tilling the soil in preparation to plant strawberries in Watsonville, CA (Source: Kip Evans – National Geographic Society)

Can't nature counterbalance this?

Increased CO₂ = more photosynthesis = more terrestrial carbon storage… this makes sense but:

(1) there still needs to be land available for this extra plant growth
(2) a recent paper found that increased atmospheric CO₂ levels may also increase soil microbial activity and thus the rate of SOC decomposition. The authors estimate that this would largely neutralise the positive effect of increased photosynthesis, suggesting that ‘nature may not be as efficient at slowing global warming as we previously thought’.

Fig. 8 Increased atmospheric CO₂ may increase photosynthesis (which absorbs CO₂) but it may also increase the rate of microbial decomposition of soil organic matter (which releases CO₂). These two processes are predicted to balance each other out.

How can we increase soil carbon?
(1) Reforestation/afforestation
(2) Soil management techniques such as retention of crop residues, cover cropping and no till planting (see video below). A recent paper estimates that altering land management practices on cropland could sequester up to 1.85Pg of carbon per year. This is around 25% of fossil fuel CO₂ emissions (~7.5-8Pg yr^-1), equal to the transportation sector’s current carbon emissions!!!

(3) There is a limit to sequestering carbon using the above techniques and this is approximately equal to the historic C loss (~80Pg). Adding biochar (i.e. charcoal used for soil amendment) to soils has been proposed as a method of surpassing this limit. Biochar is stable, resistant to microbial degradation and can remain in the soil for millennia. Using data from the World Bank, I calculated that adding ~70 t/ha of biochar to all cultivated soils would sequester enough carbon to return atmospheric CO₂ to pre-industrial levels.

Fig. 9 A fine textured biochar that could be added to the soil to help sequester carbon (Source: NOVA – PBS, 2015)

(4) Let’s not entirely forget SIC. When silicate rocks are exposed to air and water they react with CO₂ to form innocuous bicarbonates. These are then stored in the soil or washed into the oceans. For the magnesium-silicate olivine, the reaction is as follows:

If silicates were mined, milled and spread over the soil surface it would increase the rate of weathering, remove CO2 from the atmosphere and increase long-term SIC stocks. 

Fig. 10 An olivine (magnesium-silicate) rock (Source: University of Arizona, 2011) 

In conclusion…

Current agricultural practices are causing soils to emit more CO₂ than normal, accelerating climate change. Rising temperatures may further increase these emissions (particularly on unfarmed northern soils), creating a positive feedback loop. Changing agricultural practices could restore much of the SOC that has been lost. Addition of biochar and accelerated silicate weathering could then increase soil carbon storage beyond natural levels. Converting soil from a carbon source to a carbon sink could be a key strategy to mitigating climate change.