Soil Inorganic Carbon Is a Major Hidden Part of Earth’s Carbon Cycle

Most people are familiar with the carbon in Earth’s atmosphere, and perhaps with the carbon stored in plants and animal bodies. Yet a sizeable share of the carbon within the planet’s land ecosystems sits in a highly visible place that can easily be missed: the soil.

When soil carbon does come to mind, it is generally the carbon in organic material we consider, including decaying plant matter, bacteria and animal waste. But soil’s inorganic, mineral fraction contains carbon too.

Our newly published study in Science reveals that soil inorganic carbon is far more abundant than previously recognised – and may have a remarkably significant role in Earth’s carbon cycle.

Using more than 200,000 soil observations collected worldwide, we calculated that the upper two metres of soils around the globe contain roughly 2.3 trillion tonnes of inorganic carbon. That is approximately five times the amount of carbon held in all terrestrial vegetation worldwide.

We calculate that about 23 billion tonnes (1%) of this carbon could be released during the next 30 years, potentially affecting Earth’s land, water and atmosphere in ways that remain poorly understood.

What is soil inorganic carbon?

Soils contain inorganic carbon in several forms. It may occur as trapped carbon dioxide gas, be dissolved in water or other liquids, or exist as solid carbonate minerals.

By weight, most soil inorganic carbon is made up of solid carbonates, frequently calcium carbonate – a common material in limestone, marble and chalk. These carbonates can give soils a pale or whitish appearance, whereas organic carbon gives soil its darker colour.

Soil carbonates arise either through the weathering of rocks or through reactions between soil minerals and carbon dioxide in the atmosphere.

Inorganic carbon generally accumulates more readily in arid and semi-arid soils, including those in Australia. This is because water moving through soil tends to remove and carry away some carbonates.

Our calculations indicate that Australia’s top two metres of soil contain around 160 billion tonnes (7%) of the world’s inorganic carbon. As a result, Australia has the world’s fifth-largest store of soil inorganic carbon.

In more humid locations, soil carbonates can also occur beside rivers, near lakes and in coastal zones, as calcium-rich alluvial sediments or calcareous rocks. Soils in karst landscapes – places with abundant rocks such as limestone, commonly featuring caves and sinkholes – often contain carbonate within their rocks. In regions including central Asia, large deposits of wind-blown sediment, known as loess, also help carbonate minerals to build up.

Why should we care about soil inorganic carbon?

Environmental change can affect this enormous carbon store, particularly through soil acidification. Acids dissolve calcium carbonate, causing the carbon either to dissolve in water or to be emitted as carbon dioxide gas.

Soils across many parts of the world, including China and India, are becoming more acidic because of acid rain and other pollution associated with industrial activity and intensive agriculture.

Researchers have traditionally regarded soil carbonates as a comparatively stable carbon pool that alters only gradually. Human activity, however, has increased the mobility of soil inorganic carbon.

Irrigating and fertilising agricultural land accelerates the dissolution of soil inorganic carbon and its leaching from the soil.

Soil has built up inorganic carbon across immense spans of Earth’s history. Disrupting this carbon can have far-reaching consequences for soil health.

Interference with this carbon reduces soil’s capacity to neutralise acidity, manage nutrient concentrations, support plant growth and stabilise organic carbon. Soil inorganic carbon is not simply a carbon reservoir; it also underpins many of the vital functions soils perform within ecosystems.

Our study found that soils lose 1.13 billion tonnes of inorganic carbon to inland waters every year. This movement has major, but frequently neglected, implications for carbon transfer among land, freshwater systems, the atmosphere and the oceans.

What should be done?

Soil carbon is increasingly acknowledged as a central element of nature-based solutions for tackling climate change. To date, though, attention has largely centred on organic carbon. Our findings demonstrate that inorganic carbon deserves the same consideration.

Better land-management practices could limit disruption to the global soil inorganic carbon pool, and may even increase it. In farming, tailoring irrigation and fertilisation more closely to plants’ growth requirements can lessen their effects on inorganic carbon. In certain soils, organic additions such as compost and manure may guard against acidification, raise calcium concentrations and boost soil inorganic carbon.

Our research indicates that climate-change mitigation efforts seeking to sequester carbon in soil need to include inorganic carbon alongside organic carbon.

Inorganic carbon in soils responds to global pressures, including climate change, industrial pollution and soil overuse, differently from organic carbon. Nevertheless, certain approaches intended to retain more carbon in soil – including enhanced rock weathering, afforestation and trapping organic carbon in soil minerals – may also raise inorganic carbon levels.

International soil-carbon schemes already exist, such as the 4 per mille initiative, which seeks to raise global soil carbon storage by 0.4% each year. These programmes could become still more ambitious by accounting for the essential contribution of inorganic carbon to sustainable soil management and the achievement of climate targets.

Yuanyuan Huang, Senior Research Scientist, Chinese Academy of Sciences; Pep Canadell, Chief Research Scientist, CSIRO Environment; Executive Director, Global Carbon Project, CSIRO, and Yingping Wang, Chief research scientist, CSIRO

This article is republished from The Conversation under a Creative Commons licence. Read the original article.