What Role Do Deep Soil Minerals Play in Carbon Storage?

An estimated 600 megatons of carbon are currently held by reactive minerals deep within terrestrial soils around the world  —more than twice the amount of carbon that humans have added to the atmosphere since the Industrial Revolution began. Understanding the pathways and variables that influence carbon sequestration in soil could lead to new ideas to combat climate change and protect vulnerable ecosystems.

A new study by Marc G. Kramer and Oliver A. Chadwick examines the role of minerals in facilitating carbon sequestration in soil. "Climate-Driven Thresholds in Reactive Mineral Retention of Soil Carbon at the Global Scale," published in Nature Climate Change in November 2018, quantified carbon stabilized by reactive minerals in soil from different ecosystems around the world. Their analysis used megapit soil samples from 47 NEON field sites augmented by a set of additional soils ranging from arid to humid climates. NEON megapit soil samples are collected and archived from each terrestrial field site across the network during initial site characterization. The soil is collected from multiple soil horizons at a single soil pit (a "megapit") that is up to 2m deep. The results of the Kramer, Chadwick study demonstrate that carbon bound to reactive minerals play a much bigger role in global carbon sequestration than has previously been realized. 

A Carbon Sink Six Feet Under

Soil is one of nature's most important carbon sinks, holding more carbon globally than all terrestrial vegetation and the atmosphere combined. Much of this carbon is stabilized against microbial decay by chemical bonding with reactive minerals.  Carbon compounds near the soil surface are often rapidly decomposed,  but about ¼ of the total soil carbon stock is bound to minerals deeper within soil profiles. This deep carbon is more stable than that stored closer to the surface and is an important long-term carbon sink.

One of the keys to carbon storage stability is the reactive mineral content in the soil. When dissolved organic carbon in the soil comes into contact with reactive minerals (here defined as minerals containing iron (Fe) or Aluminum (Al) ions), they react with the minerals and form strong chemical bonds. This process keeps carbon bound in the soil rather than allowing it to be biologically decomposed and returned to the atmosphere as carbon dioxide (CO₂). The greater the reactive mineral content, the greater the carbon storage potential of the soil.

Climate is a critical control on soil carbon storage because it controls both the amount of carbon added to soil annually and the rate of reactive mineral formation. Rainwater dissolves organic matter from decaying plants, animals and microbes and moves dissolved carbon compounds into deeper soil horizons where they can encounter fresh reactive mineral surfaces. In general, this means that the mineral storage process is more efficient in wetter climates.

The paper by Kramer and Chadwick quantified the carbon bound with reactive soil minerals across a broad suite of climate zones or biomes. Marc Kramer says, "This is one of the most important pathways for carbon retention, but it hasn't been extensively studied in relation to climate. We wanted to  to study links between climate and reactive-mineral – carbon storage, and how future shifts in rainfall may change this storage pathway." 

Kramer and Chadwick used soil samples from the NEON project because they represented climate zones across all of North America, including deserts, grasslands, dry forests, wet forest and tundra ecosystems. NEON collects physical soil samples from terrestrial field sites and sediments from aquatic sites for biogeochemical, physical and organismal analysis. Archival samples are available to outside researchers in need of geological samples to conduct their own analysis. This study used samples collected from "megapits" dug during the construction of each of the terrestrial NEON field sites, providing a comprehensive set of samples of known provenance collected from each of the 20 North American ecoclimatic domains. The team also used a global soils data set from North America, New Caledonia, Indonesia, Europe, Costa Rica and Brazil.

The Link Between Climate and Carbon Storage

The study found a strong relationship between effective moisture and the amount of carbon retained by reactive minerals. Effective soil moisture is the amount of moisture that ends up in the soil—in other words, total precipitation minus moisture lost to direct evaporation and transpiration from plants. The more moisture that makes its way into the soil, the stronger the pathway for carbon retention with reactive minerals. The percentage of carbon stored in reactive minerals varied widely, ranging from 72% in wet forested biomes to 3% in arid deserts.

It is clear that soil carbon storage is sensitive to variation in climate and therefore it is likely that future changes in climate and soil moisture will lead to changes in carbon storage capacity. As climate patterns change, some ecosystems are getting drier. Rising temperatures also cause more water to evaporate. Both of these effects reduce the effective moisture therefore capping total mineral storage potential.

Some biomes are more sensitive to shifts in climate than others. Kramer and Chadwick identified biomes that are likely to see significant changes in the carbon storage potential of soil due to climate changes. Understanding these impacts will help climate scientists build better models of the interrelationship between climate and carbon sequestration. If some biomes are less able to sequester carbon in deep soil than previously predicted, this could result in higher-than-expected increases in atmospheric carbon over the next few decades.

Protecting a Valuable Carbon Sink

Developing a greater understanding of the pathways and mechanisms for mineral-based carbon storage in soil could help researchers develop ways to protect these mechanisms. Kramer believes that changes in land use policies and agricultural methods could have a big impact on the reactive mineral storage pathway.

For example, agriculture releases large amounts of carbon into the atmosphere as carbon-containing soils are disturbed through plowing. Farmers already add organic matter back to the soil to increase agricultural productivity. Adding key minerals back into the mix could support chemical reactions that would result in more stable forms of carbon that would stay in the soil instead of migrating to the air.

The study also identified vulnerable biomes, such as temperate moist forests and wet tundra, where shifts in climate could have an outsize impact on carbon storage potential in soils. Land use policies could be put in place to protect deep carbon sinks in these areas and to enhance growth of deep-rooted plants that would add carbon to deep soil horizons.

Another key takeaway from the study is that there are many areas with mineral-rich soil that have the potential to sequester much more carbon than they are holding currently. Marc says that this finding opens up exciting new possibilities for addressing climate change. "We're just starting to understand what is possible with soil in terms of how we can sequester carbon," he says. "Until we start doing the tests, we don't know the answers. If we think outside the box and use our understanding of soil chemistry, there may be lots of opportunities we could explore there."

More work will be needed to refine our understanding of the chemical pathways involved in mineral-based carbon sequestration and the total storage potential for different soil types and different biomes. The NEON project will continue to be a resource for researchers wishing to further explore relationships between climate, soil types and carbon storage.

Using NEON to solve key questions in research studies of carbon sequestration and climate

In addition to soil samples, a wide variety of meteorological measurements are collected at NEON field sites, including temperature, humidity, precipitation, and fluxes of carbon, water, and energy between terrestrial ecosystems and the atmosphere. NEON also collects measurements of soil moisture, soil CO₂ and temperature as well as organismal field sampling data to better understand how our ecosystems are changing over time.

Learn more about how to request Megapit soil samples.