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Cambridge University Science Magazine
Met in almost every step we take, soils, despite being so familiar, are often overlooked in discussions of natural systems. This is evident, for example, in making climate models and predictions and even more so when thinking about humanity’s future outside Earth.

The entirety of soils and the space where their formation takes place is called the pedosphere. It is a dynamic interface and junction point for many other systems: the lithosphere - the top 100 km of the Earth’s rocks - erodes with the help of atmospheric, hydrological and biological processes to form this complex mixture of solids (loose rock particles and sediment, animal and plant debris), liquids (mostly aqueous - water based - solutions) and gases (both in pores and dissolved).

To really understand what soil is, we need to look at the roles it plays in global processes. While there are formal definitions and descriptions, it is often the case that function precedes the name of soil. In overview, it is:
  • a modifier of Earth's atmosphere - it stores and cycles carbon, making it important in understanding climate change
  • a medium for plant growth – currently a fundamental part of food production
  • a means of water storage, supply and purification
  • a diverse ecological host

Soil is not a mere tool or catalyst for the processes above, it is central to them. Soil in turn is modified by all of them and keeps evolving with time. While not all soils have all the mentioned functions, the common denominator is hosting and supporting life.

It is now evident that biology is not only affected by, but also influences atmosphere and in turn climate. A lot of that effect comes from feedback loops in the global carbon cycle, where the role of soils has only recently been realised. There the carbon is stored in a few forms - organically as living biomaterial or undecomposed remains of organisms and inorganically in minerals, carbon dioxide, methane and their hydrates (complexes with water). The majority of soils can be assumed to be more or less in equilibrium with the atmosphere, at least with respect to carbon. But what about permafrost? Permafrost is the soil, rock or sediment remaining at or below the freezing point of water for at least two years. With global temperatures rising, high latitude countries such as Canada, Iceland, Greenland and much of Russia are already seeing thawing of the permafrost and more soils being uncovered by retreating ice. Recent studies estimate that in northern permafrost soil carbon content equals more than double the amount currently in the atmosphere.

Horrified by the predicted rates by which climate change could occur if a lot of this carbon leaves the soil suddenly (in a few tens of years), the scientific community are researching the consequences of soils getting warmer. While similar, even if slower, climatic changes have occurred in the past, the geological record associated with past warmings does not show a soil-carbon signal. This suggests that retreating permafrost and warming soils do not just dump the stored carbon to the atmosphere.

A recent study by Dr Robert Sparkes from Manchester Metropolitan University, looking at terrestrial permafrost sediments being redeposited to the Arctic Ocean, found that at least 80% of the carbon is either reused or redeposited.

While that is a bit of a relief, more context is needed - Siberia and Canada have mature and old soils, but geologically young Iceland has immature andesols (volcanic ash and recently eroded volcanic rocks) and sands. To an extent, even this substrate - material in which growth takes place - fulfils the aforementioned functions, however, some would hesitate in calling Iceland’s cover a soil. In truth, it does behave a bit differently with changing temperatures to what we see in Siberia's permafrost, as has been found by Dr Utra Mankasingh and other scientists at the University of Iceland, by looking at released carbon in carbon dioxide and methane forms. While the warmer the soil, the more carbon - especially methane - is released, biological activity also increases, more carbon is then cycled, and a positive feedback loop is therefore partly prevented. Including such data and processes in climate modelling permits not only making more accurate predictions, but also allows us to better control the soil aspect of global changes.

One such change concerning everyone on the planet is in land that has been or can be used for growing food. In this case, while soil-stored carbon is also used, plants take-up atmospheric carbon dioxide. A key limiting factor for the growth and health of crops are the nutrient levels of the substrate. Current ‘quick-fix’ approaches, such as utilising extremely fertile Amazon soil, have clear negative effects on biodiversity, ecology and climate. Everywhere, natural nutrient resources are exhausted in just a few subsequent harvests, depending on the soil, climate and crop. This has mostly been overcome with mineral fertilisers. However, their continued use, especially together with commonly used pesticides, jeopardise other roles soils play in nature, especially water purification and supply. Thus, to feed and ensure health for the growing multi-billion population in the long term, the process has to become fully sustainable and economically viable. Using fertilizers and huge funds, even sandy and arid Arabian Peninsula lands were turned into a productive food source, despite irrigational as well as nutritional challenge.

Such a success story is hopeful not just for us here on Earth, but also for future generations exploring more of the solar system.

The Moon and Mars are the two likely next extra-terrestrial human habitats, but Mars is expected to both be able, and needed, to support larger colonies in the future. Therefore, rather than bringing food or soil, we need to understand how materials that are there already could be used to produce sufficient fertile soil cover.

In truth, ‘Martian soil’ exists already, mostly in form of dust and physically weathered minerals such as olivine. Chemical weathering - on Earth mostly conducted by water - hence is required to break these minerals, liberate nutrients and neutralise toxic compounds, such as possibly present perchlorate salts, before any farming can take place. As biology is important in soil development, introducing some bacterial life might be a viable option in the case of Mars. This could help us in creation of an actual Martian soil.

Kasparas Vasiliauskas is a fourth year Earth Scientist at Churchill College.