Now we've all heard about how organic material breaks down into humus. Humus is the not-so-secret component of "Black Gold"... that earthy well composted organic material that binds soil particulates into aggregates, makes gardeners proud and teary eyed to behold, and feeds our precious soil food web. Humus is broken down into two distinct kinds: active humus and stable humus. Many fertilizers contain a mined humate called Leonardite, the result of thousands of years of the decomposition of ancient freshwater marshes. For the longest time, humic acids were thought to be the main carbon constituent of soils. Now, after breakthrough research by Sara Wright in 1996 and further study by her and others, what might be the soil's most important component has been identified.
Most people have now also heard about Mycorrhizal fungi -fungi that extend and improve plant root's abilities to breakdown, make available, transport, and absorb many minerals and nutrients that would not otherwise be available to the plant were it not for this symbiotic relationship. Once the spores germinate, these fungi can only survive about three days before they must become associated with plant roots or they perish. Many organic fertilizers contain inoculants of these amazing organisms and their benefits can be dramatic. They are broken down into two groups: Endomycorrhizae and Ectomycorrhizae. Endomycorrhizae, as the name implies, grow and can produce spores inside the roots of plants and Ectomycorrhizae, which coat the outside of the roots of some plants.
It has also been known that the soil food web is to a large part orchestrated by plants. This is done through plant root exudates, the study of which is still in it's infancy and our understanding is, and may remain, very limited. When you consider the importance of soil bacteria, realize that there are an estimated 60,000 species, most of them having not yet been named, studied, or their role understood, and then think of the growing list of plant root exudates that help orchestrate their behavior, you start to appreciate the intricate web we are delving into. This is a good moment to feel awed at the complexity of an ecology we so often take for granted. Just as it was amazing for us to realize that the overwhelming majority of plant tissue is not derived from the soil, but from the air ( CO2) and water (H2O), it is likewise amazing to realize that a large portion of the hydrocarbons produced by plants through photosynthesis are not held in the plants, but exuded through their roots into the soil food web! The area around the roots where all this action takes place is called the rhizosphere. It's not so surprising then, to think that the endomycorrhizae fungi might likewise also have exudates.
What if I told you that there was a substance in the soil... a substance that was produced by certain fungi in symbiotic relationships with plants... that hold one of the main answers to our planet's health and is able to combat global warming? As Landscapers and Gardeners we're in a position to both appreciate and play a big part in increasing the amount of glomalin in our soils. Amazingly, this material makes up a whopping 27% of soil carbon! Depending on heat and moisture, glomalin can last from seven to twenty-four years. Many soil microbes only last for 20 minutes. fungal hyphae may last from days to weeks. The durability and longevity of glomalin makes it very important for soil health and as a carbon sink. As a glycoprotein, carbon is stored in both it's sugar component and it's protein component, making it 30-40% carbon, up to 24 times as heavy as humic acids, and soils can contain four times as much glomalin as humates! It provides more nitrogen and carbon to the soil than do hyphae or other soil microbes and can be found in quantities of over 100 mg/g in some soils. It's durability is also the reason it took so long to identify it. To find glomalin, you must put a soil sample in a bath of citrate and then heat it at 250 F for an hour!
Glomalin is named after the order of fungi that produce it: Glomales. This Arbuscular mycorrhizal fungi creates glomalin in order to give rigidity and strength to it's hyphae so as to better bridge gaps between soil particles and to help transport soil solutions. As the older hyphae outlive their short lifespan, and new hyphae are produced near the root tips, they release the glomalin in the soil. It's interesting to note that tilling soil greatly reduces the amount of glomalin in soils. Also interesting is that higher concentrations of CO2 in the air promotes rapid glomalin production. This may be one of the major (and overlooked!) feedback loops the Earth has to regulate CO2 content in our atmosphere -and yet another reason to be awed and amazed by the abilities of plants! Between a 3 year study on shrubs and a 6 year study on grasses in San Diego, Wright and scientists from Riverside and Stanford showed that when CO2 levels reached 670ppm (the level we will attain by 2050 given current trends), Glomales hyphae grew three times as long and glomalin production increased 500%. Wright says that the same increase can come from good soil management techniques. Longer hyphae can help plants reach and absorb more water, protect soil from erosion, and increase the fertility of soils. These are all important traits for plants to survive in a warming climate.
Why is this not headline news?! Here we have a major component of healthy top soil- the precious material that we all depend on for our survival, perhaps a key feedback loop in regulating our planet's CO2 levels, and an element of soil fertility that may prove more important to our future in the long run than the Haber process of fixing ammonium nitrate. This discovery is fifteen years old and yet when I ask my fellow gardeners and landscapers if they've heard of glomalin, I get raised eye brows and an inquisitive look... This has to change. This knowledge is precious. Hats off to Sarah and her team! I hope you all go out and include glomalin in your discussions regarding soil fertility and carbon sequestration.
(photo by Sara Wright)