Glomalin: Carbon stored in a protein you’ve probably never heard of

What’s soil made of? Take out the chunks of roots and twigs, take out the particles of minerals, and what are you left with? What makes it soil, brown and lumpy, rather than something like fine sand? It’s a mixture of organic matter: stuff produced by things living in or on the soil, that can’t readily be broken down, and it’s attracting attention now because it stores quite a lot of carbon across the world. One important part is ‘humic acid’, a mix of complex acidic chemicals from decaying plant matter. ‘Humin’ is a generic name for the stuff that won’t dissolve. But in the last decade, another important component has been found, a tough protein called glomalin.

Glomalin, highlighted in green, around mychorrhizae (thin threads and blobby spores) growing on a root (thick part). Image by Sara Wright, USDA.

Most plants team up with fungi to get nutrients, especially phosphorus, from the soil. Fungal threads, or hyphae, can be much thinner than plant roots, so they can explore soil more efficiently. Those fungi are called mycorrhizae, and the most important group of them, the arbuscular mycorrhizae, are responsible for producing glomalin, which is possibly important to their structure. And although the fungal threads die and are replaced constantly, glomalin seems to last for years in the soil.

Besides containing carbon itself, glomalin also helps to glue together organic matter in the soil, slowing its decomposition, and so keeping more carbon in the soil and out of the atmosphere.

Different soils have different amounts of glomalin. In farmland, for example, leaving soils unploughed, as in ‘no till’ cultivation, allows glomalin to build up. Glomalin molecules also include iron, and there are hints that soils rich in iron might hold more of it.

In Hawai’i, scientists found that older soils (up to 4 million years old) had more glomalin. It seems unlikely that it would just keep building up for such a long period, but the key could be phosphorus: soil gradually loses phosphorus over time, and one way for plants to keep getting the phosphorus they need is to put more into the mycorrhizae that absorb it. Those same mycorrhizae also produce glomalin.

References:

Glomalin: Hiding Place for a Third of the World’s Stored Soil Carbon, USDA Agricultural Research Service

Rillig, M., Wright, S., Nichols, K., Schmidt, W., & Torn, M. (2001). Large contribution of arbuscular mycorrhizal fungi to soil carbon pools in tropical forest soils Plant and Soil, 233 (2), 167-177 DOI: 10.1023/A:1010364221169

Wright, S.F., Starr, J.L., & Paltineanu, I.C. (1999) Changes in Aggregate Stability and Concentration of Glomalin during Tillage Management Transition Soil Science Society of America Journal 63, 1825-1829.