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A Crust of Dust: Degradation of Desert Topsoil by Human Activities May Wreak Havoc with the Environment




Raymond Okonski/Geograph Project/Wikimedia Commons

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Desert soil has a living crust that is essential for fixing nitrogen, a critical plant nutrient, and for avoiding erosion that produces a swirl of itinerant dust. When the crust is damaged, dust storms well up, residents of nearby communities develop hacking coughs, snow melts early and a whole array of untoward consequences ensue. Jayne Belnap of the U.S. Geological Survey is the world's foremost expert on biological crusts and has issued a clarion call that we should stop treating this ecological treasure like dirt. Measures such as restricting off-the-road vehicles are needed to protect desert crusts. A continuation of an interview that appeared in the January issue of Scientific American follows. 

Can the crust still function even if it's broken?
No, actually. Nitrogen fixation is something that needs to happen in an anaerobic environment, and so when you break the crust up, you aerate it, and when you aerate it then the nitrogen fixation basically stops. Microorganisms in the crust are also photosynthetic, so they have to be on the on the surface, and if the soil gets churned and they get buried, then they die. We talk about nitrogen and carbon fixation, but really the most important role these organisms play is stabilization of the soil, because deserts are places where there's low plant cover and a lot of wind, and without these organisms to make the soil crusts, you wouldn't have soil in place.

Are there crusts in other deserts around the world?
They're in every desert, however, there are areas within a given desert where there aren't very many. With a shifting, moving sand dune, there's hardly gonna be anything, if anything, on the dune itself but, in the inner-dune areas, you're probably going to have good stuff. Still, if you look at the globe, as a whole, there's not that many deserts that are shifting sand dunes. Most deserts are semi-fixed dunes or fixed dunes or more clay soil, and they have great soil crusts on them. All of them, with the exception of the hyper, hyper-arid like the Atacama, have them. You also have areas like central Asia that has stony deserts like the Gobi. They have so much stone cover and they're dark black and they get really hot but the amount of life contained on the soil surface is very minimal. It's not nonexistent but it's pretty minimal and so I'm not sure that I would count those really as being soil, as biological crusts; they have really good physical crust but they don't really have biological crusts.

Are other places doing a better job at protecting their crusts?
Nobody is proactive because, frankly, deserts are still, basically, wastelands to almost everybody, and they are definitely more disturbed in other countries. It's just like the U.S. in that people don't know the damage they cause. People just don't know that there's a reason to protect them and, even if they did know, frankly, when it comes down between eating and worrying about it, you're gonna eat.

The issues in the other countries are so much more life-and-death than in the U.S. that I'm not sure that they'll ever be in a position to worry about crusts, which is a negative feedback loop, of course. Because the less they worry about them then the more infertile the soils get, so it's a spiral downward. The good and bad news is it takes a really long time to spiral downwards, so that's the good news, I guess. After 100 years, they won’t know what they lost.

You've done a lot of your research in national parks. How are they preparing for climate change?
Well, the parks are really in a quandary because they're supposed to be conserving a landscape, which is kind of hard with climate change. They are going through a soul-searching exercise of what does it mean to be a park? I mean we kind of know what it means when, you know, you can say, "Well, we don't graze cattle" but what do you do about regional nitrogen solutions, what do you do about regional air pollution, what do you do about regional dust production? The first thing that I think we all need recognize is that there are no natural systems left. Everything is managed because everything is impacted by humans. We don't have any nonimpacted systems, and so the question now is how do you keep stuff as natural as possible, given the fact you do have regional air pollution, whether it be nitrogen deposition or dust or whatever—and that's really gonna be hard for the parks. So I think the one thing is to make sure that they keep up their monitoring, which they're doing, so they understand the status and trends of their resources.

I've always encouraged them to compare used areas with unused areas, for example. Most parks have areas that are much more used—by the public—than other areas and so they can compare those areas. We are lucky at Canyonlands and Arches because we have areas, actually, that have never been grazed by livestock and are almost never visited by humans, so we have a pretty good idea of what is just regional change versus what is direct-visitation change. We have been measuring vegetation for 20 years, so we can also use that to say, "Wow, we are losing grasses," for instance—which we are—and say, "Okay, parks, you know, you are losing grasses, and you're losing grasses in areas that have never been grazed and are not visited by people, and you're losing that lichen, Collema, in an area that has never been grazed and has never been visited by people. So if you see it happening in areas where there's a lot of people it's not a result of management.” It also makes the parks incredibly valuable as a comparison for more utilized landscapes, like the Bureau of Land Management lands, because we can now distinguish between what's climate change and what's land use. A Let me give one example: I always assumed that the reason that they were losing their lichens and grasses was because of grazing and, you know, frankly, I was wrong. I think they've hastened it but they haven't created it, per se.

Finally, a vitally important question: Why do desert flowers bloom after rains?
When you do have a wet year then you have large flushes of annual plants, so, yes, moisture is important, but it's not just moisture. It's also crust. When you have a long time between wet years, you have a lot of nutrient buildup in the soils, and that nutrient buildup is due these organisms. They are activated by tiny, tiny rain events, like one millimeter will turn 'em on and they'll fix nitrogen and they'll fix carbon. So when it rains there's a huge flush of nutrients.

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