Soil… at first glance it’s not the sexiest subject out there, I agree. But there’s far more excitement beneath your feet than you’d ever think possible. Full of life, microbes, fungi and an intricate food web, just one teaspoon can contain up to a staggering one billion bacteria and thousands of other living organisms. Then there’s the small matter of it keeping us all alive by feeding us, growing materials, decomposing organic matter, and providing us with many more vital ecosystem services (Adhikari et al., 2016).
What’s more, soil can be utilised to become a heavyweight champion in the fight against climate change. With a little help from plants, soil has the amazing ability to draw carbon out of the atmosphere and store (sequester) it into its earthy goodness. Not so boring now, right?
Unfortunately however, soil has a dark side too. Degraded, polluted, eroded or otherwise poorly managed soils have the opposite effect; they release their stored carbon back into the atmosphere, adding to our escalating accumulation of carbon dioxide (CO2) (Smith, 2008).
So how do soils sequester carbon? It all starts above ground; plants draw in CO2 through photosynthesis, generating sugars and respiring oxygen and some CO2. Some of the sugars and carbon are released via the roots, feeding the microbes eagerly awaiting in the root zone (rhizosphere) in the surrounding soil, which then provide nutrients back to the plant – including carbon (as seen in Fig. 1). This is a mutually beneficial (symbiotic) relationship; the plant and organisms in the soil help each other to thrive, gradually building up soil organic matter – a mixture of plants and animals in varying stages of decomposition (Fenton et al., 2008). Additional carbon is also incorporated into soil once the plant dies; carbon in the plant is broken down by microbes and other members of the soil food web, with some remaining stored as ‘soil organic carbon’ (SOC) (Center for Food Safety, 2015).
Save our soils
Currently we’re not utilising this potentially incredible resource; in fact, we’re degrading it. Land use changes, deforestation, pesticide use, intensive farming, urbanisation, erosion and poor land management, to name but a few, are causing previously stored carbon to be released back into the atmosphere as CO2 – to the tune of 133 billion tonnes of carbon since agriculture began, according to a recent report by Sanderman et al. (2017). Shockingly, around one third of the world’s soils are degraded, and an enormous 25% are seen as “severely degraded” (FAO, 2015).
Not only does this add to the problems of climate change, it misses a key opportunity to actively mitigate such problems through using a variety of practises to sequester carbon and to create – or protect – ecosystems that store more carbon than they release: carbon sinks. While there are various estimations of just how much CO2 could potentially be sequestered, it’s estimated at 9 – 38 megatons (MtCO2) per year; the equivalent of 7% of the EU’s total agricultural emissions (Frank et al., 2015). However, there are concerns that the amount of carbon soils can actually sequester is less than first thought (Yujie et al., 2016), with some scientists expressing criticisms due to carbon sequestration being finite, reversible, and indirectly cause the release of nitrous oxide and methane (Powlson et al., 2011).
Down with the monoculture
Healthy soils are key to carbon sequestration, crop productivity, resilience to disease and pests and could possibly affect the micronutrients in the food itself (Johnston et al., 2009). Sadly, agricultural intensification has led to farming systems that rely upon intensive monocultures – the growing of vast quantities of one crop type – resulting in substantial soil degradation (Altieri, 2009).
Soil health can be greatly enhanced by using holistic, regenerative approaches to farming, such as permaculture, agroecology and carbon farming (Lal et al., 2007). Many of these have significant overlap, all sharing one core aim – enhancing soil. Regenerative approaches often include the use of polycultures (see Fig. 3), perennial plants that return yearly, no-dig approaches (minimal digging of the soil), agroforestry (food forests), mulches (a covering of organic matter to protect and enrich the soil), cover crops, companion planting, use of compost to enhance soil organic matter and biodiversity enhancement (Altieri et al., 2015). However, there can be limitations with such approaches; scaling them up may be difficult, they often require larger quantities of physical labour, and there may be a long wait before results are seen – something many farmers simply cannot do (Rosset and Altieri, 2017).
Utilising strategies for healthier soils have a wealth of additional benefits. It creates more resilient systems, better adapted to deal with the uncertainty brought about by climate change, such as drought, flooding or temperature extremes. Healthy soils suffer less from topsoil loss, erosion, disease, and can recover at a quicker rate than degraded soils in non-regenerative farming systems (FAO, 2017). They’re also far richer in nutrients needed for crop production – resulting in less susceptibility to pests and disease (Altieri et al., 2015).
Healthy soils can also lessen our reliance on fossil fuel-based inputs such as synthetic nitrogen fertilisers or pesticides, and see less usage of industrial machinery, resulting in the consumption of less fossil fuels (Quarles, 2018).
Utilising soil as a climate change mitigation resource – as well as a tool to build resilience – is definitely a worthwhile endeavour, although importantly, not the only mitigation strategy we should be taking.
Evidently, we still need further research to establish the rate and efficacy of soil carbon sequestration, and to acknowledge barriers we may face when implementing improved farming practises. Still, making efforts to avoid further soil degradation and to actively enrich the soil are positive strategies regardless. Shifting agricultural practises away from pesticide-heavy monocultures to regenerative systems is arguably a ‘no-regrets’ approach due to the wealth of positive outcomes gained.
So, if you’re not sure where to start in dealing with climate change, the first step could be right beneath your feet.
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