How Wide-scale Desert Soil Disturbance Releases Stored Carbon
by Robin Kobaly
Most people are not aware of the vast amount of carbon that is captured and stored underground in desert soils. When comparing carbon storage in desert landscapes to that in forest landscapes, for example, it is easy to underestimate carbon stocks in the desert, because desert plants in most arid habitats are small and short in stature and are often well-spaced, with very little leaf litter or mulch on the ground. This might lead to a conclusion that, with such sparse biomass in foliage and wood and little mulch to hold carbon on the ground, the desert must not be able to sequester or store much carbon. We need to look deeper to find out how wrong this perception is.
Desert plants store much of their captured carbon deep underground in a massive network of connected roots and fungal root-partners, unlike forests which store most of their carbon aboveground or near the soil surface. Historically, much of the desert’s “soil organic carbon” has been missed by soil scientists, because many soil studies conclude at “plow-line depth,” or between 6 and 12 inches. These studies aren’t of much relevance in the desert because most of the carbon that desert plants capture is stored extremely deep in the soil. Roots of most desert plants grow incredibly deep, up to ten times longer than the plant is tall in their critical quest to find soil moisture, and the subterranean biomass of this network of deep roots is filled with carbon.
California Evening Primrose (Oenothera californica).
As with other desert plants, the long, water-seeking roots of the California Evening Primrose partner with miles of mycorrhizal filaments, and together they store large amounts of carbon underground.
Some of this carbon is stored in the tiny but numerous filaments of root-partnering fungi, called mycorrhizal fungi, that live in partnership with plant roots. The filaments, or mycelium, of one large group of these mycorrhizal fungi are coated with a “sealant” called glomalin made from carbon that was captured aboveground by the plant host. Because there can be so many miles of fungal hyphae (covered with glomalin) in each cubic foot of desert soil, glomalin is attributed with storing one-third of the world’s soil carbon.
Much of the carbon these plants capture aboveground from the air and convert into sugar is eventually turned into inorganic carbon underground. When the long roots breathe out carbon dioxide deep into dark moist soil, this carbon dioxide combines with the abundant calcium in our arid soils to create mineralized deposits called caliche (calcium carbonate). These deposits start as tiny crystals but eventually grow to large crystals, then chunks, and into layers of caliche that can start at the surface or form at various depths underground. These caliche deposits can store captured carbon in this inorganic form for hundreds, to thousands, to even hundreds of thousands of years . . . if not disturbed. Most of the caliche in our desert soils was actually formed during the Pleistocene when the climate supported more dense and productive vegetation. In fact, Dr. Michael Allen at the UCR Center for Conservation Biology commented on the desert’s capacity to store large amounts of carbon dioxide as caliche, noting that “The amount of carbon in caliche, when accounted globally, may be equal to the entire amount of carbon as carbon dioxide in the atmosphere.” Despite its long-term storage capacity, caliche releases its sequestered carbon when vegetation is removed and soils are disturbed and exposed to erosion. As caliche degrades in disturbed soils, its calcium and carbon molecules are uncoupled, releasing the carbon to again reenter the atmosphere.
Unfortunately, agencies planning for California’s 30X30 Initiative have been slow to include the desert’s carbon stocks (both organic and inorganic) in their modeling processes. By extension, they undervalue the California Desert as a landscape capable of contributing to sequestered carbon stocks in their planning. If the value of the desert’s role in carbon sequestration is not recognized, no one should be surprised when vast swathes of the desert are scraped bare of vegetation to build industrial-scale solar to meet the state’s ambitious goals for reaching net-zero carbon. There will be a temptation to put almost the entire burden of the state’s climate goals squarely on the desert – and no one will much care because “the desert isn’t helping us in the carbon capturing/storage column anyway.”
I feel that a new story, a new tagline, a new slogan is needed to help convince land managers to consider the value that our intact deserts offer before it’s too late. One argument used to justify excluding the desert in carbon balance equations is that no one knows exactly how much carbon is stored underground in the desert. No one has yet figured out how to measure deeply buried carbon across such a vast landscape that is so diverse in topography, soils, climate, and vegetation. Carbon-storing caliche deposits are distributed in patches in some places and in vast layers in other places. Also, it is distributed at varying soil levels depending upon rainfall and the depth of desert plant roots that can deposit carbon all the way down to groundwater. Arriving at a total value for stored underground carbon in the diverse desert is much more challenging than for other more homogeneous landscape types. But should we feel free to destroy the desert because we don’t yet understand it and can’t yet precisely quantify its stored carbon? Could we convince non-desert-trained scientists to protect the desert if a reasonable estimate were made?
One approach might be to calculate the amount of carbon that a single, long-lived desert plant could capture in its lifetime, because that is a value we can estimate with reasonable accuracy. We might first look at Mojave yucca (Yucca schidigera), an ancient, extremely slow-growing plant that is very common across both the Mojave and Colorado Deserts, and that has been found to reach ages of 2000+ years. We could calculate how much carbon one plant captures each year, then extrapolate how much carbon an individual yucca plant would sequester in say, a 1000-year lifespan. Then figure how many Mojave yuccas are expected to be ripped from the ground in a typical industrial solar field such as the newly-approved 5,000-acre Yellow Pine Solar Project in the Mojave Desert (Pahrump, Nevada) – in this case, over 80,000 Mojave yuccas will meet their demise during the construction of an array expected to operate for perhaps twenty years before becoming obsolete. Will the reduction in carbon that would have been sequestered (and stored underground) by those 80,000 Mojave yuccas actually be offset by possibly twenty years of the solar project that replaced them?
If we could do the same for the creosote bush that also can live for thousands of years, we might gain still more perspective on the critical question of net carbon gain or loss through various management practices in our ancient desert landscapes. More precise estimates of the carbon-capturing contributions by our long-lived desert plants could raise awareness of their value in helping us fight climate change . . . before we lose them needlessly to large-scale solar developments.
If few people realize the intrinsic value of the desert’s carbon contributions, it becomes more difficult to protest when thousands and thousands of acres of desert habitat are scraped bare for solar fields. It appears that the California Deserts may be sacrificed to meet California’s climate goals without even considering the full consequences of doing so. Where will our carbon footprints lead us . . . down a path that leads to a slashed-apart, industrialized desert where throngs of people once flocked for solitude and for vast uninterrupted vistas of an ancient landscape? Let’s not lose this treasure when there is a smarter path forward, including solar panels on rooftops, parking lots, fallowed agricultural lands, and even exposed aqueducts.
With a Master’s Degree in biology, Robin Kobaly had a twenty-year career as a botanist with the BLM, also serving as a wildlife biologist, and natural history interpreter. She is currently executive director of the SummerTree Institute, a 501(c)3 nonprofit corporation dedicated to providing responsible viewpoints toward our environment, our place in it, and our responsibility to it. <firstname.lastname@example.org>