Global climate change continues to impact our ecosystems and communities. Lakes, rivers and reservoirs are shrinking, sea surface temperatures are rising, and forests are ablaze. At the turn of this new decade, the planet continues to face a surge of local, national and global environmental challenges – a climate emergency – which is acknowledged by millions of citizens around the world. On their climate campaign marches, unprecedented numbers of demonstrators and activists have crowded around the gates of government buildings, calling for swift, meaningful action. Few of them realise that part of the solution lies underneath their feet.
For many, soils are simply dirt – something to keep out of sight and out of mind – and yet soils play a substantial role in our everyday lives. Each year, soils support food production, purify our water, and provide raw materials for buildings and infrastructure. As a result, our soils are a critical resource for tackling many of the 17 identified UN Sustainable Development Goals, including food security, clean water and sanitation, and good health and wellbeing.
Soils also serve as the habitat for billions of organisms. A teaspoon of soil contains more microorganisms than there are people on the planet! Part of the reason why there is so much life in soils is because of soil organic carbon. This carbon helps to provide energy for the growth of microorganisms, and to promote the formation of new cells.
“a teaspoon of soil contains more microorganisms than there are people on the planet”
But the significance of soil carbon extends beyond energy supply. It can also help to abate climate change
Our soils contain 1,500 billion tons of carbon, representing more than that found in the atmosphere and plants combined. In fact, plants have taken up approximately a quarter of the carbon dioxide emissions that humans have released into the atmosphere. When plants absorb carbon dioxide, it is initially converted into a food resource. However, when these plants die, they decompose into organic matter, transferring this carbon into the soil. An ambitious aspiration to increase soil carbon in the uppermost 30-40 cm of soil by 0.4% year-on-year was declared at the COP21 Paris meeting in 2015. Policies that would facilitate this ambition include reducing the rate of deforestation, as well as small-scale agro-ecological practices implemented at the farm level, such as nourishing soils with manure and compost.
Soil is on the move
Soils face burgeoning demands. By 2050, the global population is estimated to soar to more than nine billion, potentially reaching eleven billion by the close of the century. Soils will need to provide more food, clean water and raw materials. These challenges are exacerbated by the fact that over a third of global soil resources are currently either moderately or highly degraded. Each year, about 60 billion tons of topsoil are eroded globally. Much of this is human-induced, or ‘accelerated’: the use of conventional, intensive agricultural techniques reduces the soil’s resistance to erosion.
One of the key issues here is where does this eroded soil end up? Often soils are washed beyond field boundaries into rivers, lakes and other water-bodies, with more than 25 billion tons of soil being flushed into oceans annually. Over time, a plethora of off-site impacts starts to accrue: forcing rivers to change their course, damaging fish-spawning gravels in rivers, disturbing marine fauna, shrinking lakes and reservoirs, and intensifying flood risk. In addition, soil can also transport large volumes of fertilizers, agricultural chemicals, and other contaminants. For example, when phosphorus is washed into rivers, it can cause algae to grow which in turn can deplete the rivers of their oxygen. This can have serious impacts on aquatic animals and plants. Thus, soil erosion leaves a large environmental footprint, with the off-site effects potentially stretching for tens, if not hundreds, of miles. Back at the site from where the soil was originally eroded, the impacts are no less grave.
Left unchecked, erosion causes soils to thin. This is because accelerated rates of soil erosion outpace those of soil formation, which are comparatively very slow. To gauge some idea as to how slowly soils form, the depth of soil amassed in a year approximately equals the
width of a human hair (about 0.04 mm). In some regions of the world, rates of soil erosion are more than one hundred times greater than those of soil formation. We may relate this to our own body mass: if we burn off more calories than our calorie intake, our body mass will steadily fall. Likewise, if soil erosion exceeds formation, the soil gets thinner. This has implications for many soil ecosystem services.
Many soil functions and processes are fundamentally affected by soil thickness. The thickness of a soil has been shown to influence hillslope stability, and over shorter periods, the runoff and residence times of water (i.e.: how long water remains in the soil, before evaporating or flowing out into water-ways). This has consequences for the storage of plant-available water, and as a result, plant productivity. In addition, soil thickness significantly determines the size and stability of the soil organic carbon reservoir, because deeper soils are able to store carbon for longer time periods.
A thinning soil profile is part of a vicious cycle of soil degradation. Subsoils tend to exhibit poorer structure than topsoils and, in consequence, can be more erodible. Without effective amelioration, the trajectory of any thinning soil is one that ultimately leads to the exposure of the underlying bedrock. In the absence of soil, it is palpably clear that the long list of soil ecosystem services cannot be provided. For some regions in the world, where soils have been stripped from their rocky foundations, attempts have been made to fashion a cultivable soil out of the regolith. Tillage equipment, otherwise made redundant, has been employed to crush mudstone and similar soft-rock into shallow, unconsolidated debris. However, without sufficient aggregate stability, a supply of organic carbon and, most crucially, nutrients, it is unlikely that cultivation can be sustained over multiple seasons in this way. This prompts the question: for the soils which are thinning, how many years might they have left?
“our work represents, to the best of our knowledge, one of the first in the world to make a clear scientific estimate of soil lifespans”
How many harvests left?
Recently, there has been much interest and coverage in the media about the number of harvests that we may have left. Some reports suggest that there may be as little as 60 years of topsoil remaining. Others repeatedly insist that there are just 100 harvests left. However, there is very little, if any, scientific basis for these claims.
Forecasting the future sustainability of our soil resources is a complex process. One of the reasons for this is because of their heterogeneity. In the World Reference Base for Soil Resources (an international soil classification system) there are over 30 different types of soil, and a further 185 taxonomic descriptors that can be used to differentiate them. Given this variety in the physical, chemical and biological make-up of soils, making statements about their sustainability at the global scale is a challenging task. Moreover, soils sit at the interface with multiple environmental systems, each of which may be uniquely influenced by both short- and long-term perturbation. Thus, forecasting how our soils may respond to future environmental change requires a more holistic, and often multidisciplinary, understanding of how wider ecosystems may react, and how this may indirectly affect the functionality of soils.
That notwithstanding, expressing the sustainability of the soil in units of time has been attempted in the past. The ‘soil lifespan’ approach aims to calculate the length of time until the complete removal of the soil cover and the exposure of the underlying bedrock. These estimations are based on the net erosion rate (in other words, rates that take both gross soil erosion and soil formation into account) and soil depth. One of the advantages of this relatively simplistic model is that this depth term can be adjusted to focus on the lifespan of a particular soil horizon, or the minimum depth for crop production, if such information is known. After all, crop yield has been shown to diminish long before the soil is completely removed. Moreover, the depth term can also be adjusted to account for any compacted, plough- or iron-pan layers that may be present down the soil profile, through which roots are unable to penetrate. However, supporting cultivation is not the soil’s only jurisdiction. Some ecosystem functioning may continue below the minimum depth for crop production.
Since the early 1980s, a number of soil lifespan estimates have been made. However, these estimates are fundamentally flawed because rates of soil formation were rarely employed, and if they were, they were seldom measured in parallel with those of soil erosion at the site under study. One would not forecast a bank balance using just expenditures, yet past efforts have applied such flawed logic to soils. This may be because our knowledge of soil formation rates is extremely slim when compared with the global data we have for soil erosion. As a result, there have been few parallel measurements of how fast soil is being formed and eroded, especially on land currently supporting agriculture. Without both soil erosion and soil formation being accounted for, the magnitude of the threat that erosion places on soil sustainability is unknown.
Calculating soil lifespans in the UK
It is this gap that our research has sought to fill. Our work represents, to the best of our knowledge, one of the first in the world to make a clear scientific estimate of soil lifespans, using measured rates of soil formation and erosion. This gives farmers and scientists a more accurate idea of how sustainable the world’s soil resources may be. It is also one of the first to measure rates of soil formation for sites currently supporting arable agriculture. Undertaking this work on arable soils is essential. Not only do these soils sit at the heart of our food industry, the ‘cropping year’ subjects them to frequent and often intense cultivation management regimes. As a result, they can be more prone to erosion than a soil which is kept in grass, or one that has been afforested. A better understanding of the timescales over which arable soils remain productive is the first step in helping to inform policy makers about the land management decisions necessary to sustain these valuable resources for future generations.
Our research was based on an arable farm in Nottinghamshire, UK. You could stare at the field for hours from the roadside, and the soil would appear motionless. But soil is on the move here. We selected this hillslope because it has been subject to a number of soil erosion studies throughout the last thirty years. Consequently, we have a relatively strong grasp of how, and at what rates, soil is redistributed downslope. Long-term data suggests that soil erosion occurs at around 1.2 mm per year, and at 2.2 mm per year in a worst-case scenario. Our principal task was to measure the rates of soil formation at this site, and compare them with those of soil erosion.
We measured soil formation using a technique called cosmogenic radionuclide analysis. It starts as far away from the soil as you can imagine: in space. During the death of a star, cosmic rays are discharged from the supernova and they bombard the Earth. You can neither see them nor feel them, but stand outside and you, too, will be bombarded by these cosmic rays. When they interact with minerals in the uppermost metres of the Earth’s surface (in this case, the bedrock), a chemical reaction occurs, and cosmogenic radionuclides are produced.
At our site in Nottinghamshire, the bedrock (Triassic sandstone) is not at the surface, but approximately two metres below the soil. It’s two metres down because over time the uppermost parts of the sandstone have weathered into soil. Critically, we wish to know the rate at which this occurs. Luckily, cosmic rays are still able to penetrate through this soil and bombard the quartz minerals in the sandstone, producing a radionuclide called Beryllium-10.
By studying stable landscapes where soil erosion and soil formation are in balance, we know the rates at which Beryllium-10 is produced in the soil and rock. In such conditions, the concentration declines from the surface but Beryllium-10 can still be measured up to approximately three metres below the surface. One of the factors that control the concentration of Beryllium-10 is the long-term rate at which soil is formed. If we can quantify the relationship between the concentration of Beryllium-10 and the rate of soil formation, then we can measure and use the Beryllium-10 concentration to calculate these long term rates.
Rates of soil formation at our site in Nottinghamshire ranged from 0.026 to 0.084 mm per year. These rates are similar to those that have been measured using the same technique on other sandstone-based soils in temperate regions, around the world. Using these data, we then set out to estimate the soil lifespans for this field. Our calculations showed that, in a worst-case scenario, the uppermost 30 centimetres of soil could be eroded in as little as 138 years, with the underlying sandstone bedrock emerging in 212 years. Given that these soils have been functioning for the last 10,000 years, these projections could represent the final 1% of their lifespan.
Although these estimates are some of the first to have been calculated using evidence ‘from the field’, it is likely that soils in other regions of the world, particularly those that have been subject to centuries of intensive agriculture, and have already thinned significantly, have shorter lifespans. For instance, in parts of sub-Saharan Africa, regions of Asia and areas of Latin America, soil erosion poses a great threat to longterm soil functioning. Furthermore, if precipitation intensifies (as it is expected to do in countries like the UK) and soil erosion rates increase, these soil lifespans may shorten. Similarly, if agricultural equipment and cultivation regimes change, and soils become less protected from wind, water, and tillage erosion, the lifespans of these soils may be further curtailed. As the demands on soil intensify with global population growth, land management regimes will need to adapt to reverse the trajectory of soil thinning and, instead, find ways of thickening soils and extending their lifespans.
Extending soil lifespans: what’s in our toolkit?
Luckily, there’s a well-tested toolkit of soil conservation techniques that we can deploy to save our soils. These include relatively minor changes to our cultivation techniques. For example, shifting towards reduced or even zero tillage has also been shown to reduce soil erosion by water, as it preserves soil structure. This not only makes soil particles more resistant to erosion, but it can enhance the infiltration of water, meaning less water runs off the slope to entrain soil particles in the first place. Reduced or zero tillage also reduces and prevents tillage erosion, which occurs due to the downslope movement of soil during cultivation and, in rolling agricultural landscapes, this can be the major process inducing soil thinning. Other methods include sowing cover crops to avoid leaving expanses of soil bare, exposed, and vulnerable to soil erosion. Some cover crops have also been shown to have additional benefits, such as suppressing fungal diseases and weeds, and helping to establish greater wildlife diversity.
In some regions, the most effective strategy may be a larger scale solution. Hillslope terracing has been adopted by many civilisations for over a thousand years, and if they are implemented in the right context and regularly maintained, it can prove a valuable method for ameliorating soil erosion. Similarly, afforesting arable land has also been shown to be a successful soil conservation strategy. Notwithstanding the fact that a forest can take a relatively long time to become fully established, afforestation often involves reducing the acreage available for cultivation. However, some argue that trees do not necessarily need to replace farmland everywhere. Instead, trees can be integrated into arable farmland, not only as a means to prevent soil erosion, but also to reduce the run-off of agricultural chemicals and pollutants, offer shade for livestock, and provide forage for pollinators.
This toolkit of soil conservation strategies offers us a real hope of sustaining these valuable resources and their services for future generations. This must now become a global priority over the next decade. Although at the outset it may seem like an unattainable goal, we should be buoyed in the knowledge that we have been at this juncture before. Throughout the last century, medical advancements, improvements in living conditions, and changes to individual lifestyles have significantly extended the lifespans of our own species. Human beings now live longer and healthier lives. We must now employ the same energy and endeavour to extend the lifespans and health of our soils, before it is too late.