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Biological Soil Crusts: Out of Small Things Come Those Which Are Great
Volume 14, Number 46: 16 November 2011

In a recent study of biological soil crusts (BSCs), Liu et al. (2011) begin by describing their characteristics and listing some of the important ecosystem services they provide.

The six scientists first note that (1) BSCs are also known as microbiotic crusts, microphytic crusts, cryptogamic crusts and cryptobiotic crusts, that (2) they are comprised of varying assemblages of cyanobacteria, algae, fungi, lichens, mosses, and other bacteria at or below the soil surface, that (3) they are found throughout the world in arid and semi-arid landscapes, and that (4) "the progression of BSCs in desert ecosystems after disturbance and dune stabilization is usually considered a course in which the pioneer cyanobacteria are gradually replaced by desert algae, lichens and mosses (West, 1990; Langhans et al., 2009)." They also state that numerous important ecosystem services have been ascribed to BSCs, such as preventing soil erosion (Warren, 2003; Barger et al., 2006), enhancing soil fertility and stability (Belnap, 1996; Li et al., 2005; Guo et al., 2008), increasing soil moisture (George et al., 2003), increasing soil aeration and porosity (Harper and Marble, 1988) and adjusting local hydrology (Evans and Johansen, 1999, Belnap and Lange, 2003).

In their own study of BSCs, Liu et al. explored their effects on soil nematode communities at the southeastern edge of the Tengger Desert of Northern China, working in a transitional substrate zone between desertified steppe and sand dunes, where they collected soil samples in July 2010 from different vegetated areas that were stabilized in 1956, 1964, 1981 and 1991, using mobile sand dunes as a contrasting control equivalent to a temporal starting point. This they did by determining (1) the Shannon-Weaver index (H'), which is a measure of the richness of nematode genera present in the BSCs (Shannon and Weaver, 1949), (2) the nematode maturity index (MI), as per Bongers (1990), (3) the nematode enrichment index (EI), which is based on "opportunistic bacterial- and fungal-feeding nematodes representing enrichment," and (4) the nematode structure index (SI), which is based on "the relative weighted abundance of disruption-sensitive guilds representing structure."

So what did the Chinese researchers find? Quoting from their paper, they say that "nematode abundances, generic richness, H', MI, EI and SI were positively correlated with crust ages," and that "nematode abundances and generic richness under moss crusts were higher than those under cyanobacteria-lichen crusts." Thus, they concluded that BSCs could have "positive effects on soil nematode communities in the re-vegetated areas," due to the presence and succession of BSCs that increase the thickness of topsoil after dunes have stabilized, "creating suitable habitats and providing an essential food source for nematodes," which can then proceed to work their own wonders on the developing soil matrix.

So what does all of this have to do with CO2? It has plenty to do with it, because the biological activity of BSCs tends to rise dramatically when the air's CO2 content rises.

Brostoff et al. (2002), for example, collected pieces of algal-dominated soil crusts from dune tops and playa bottoms in the western Mojave Desert of California (USA). Bringing them back to the laboratory, they measured their photosynthetic responses to variations in atmospheric CO2 when all other environmental parameters were maintained at values that promoted optimal rates of net photosynthesis. Under these conditions, Brostoff et al. observed the net photosynthetic rates of the soil crusts from both the dunes and the playas to rise in linear fashion as the air's CO2 concentration rose from 150 to 1000 ppm (which was the highest concentration they were able to test with their instrumentation). And in the case of the playa crusts, the net photosynthetic rate of the algae rose by a factor of two in going from the ambient CO2 concentration characteristic of their normal environment (385 ppm) to the maximum value they investigated (1000 ppm), while in the case of the dune crusts, the net photosynthetic rate tripled.

But what happens when environmental conditions are not optimal? Lange et al. (1999) studied this situation with lichens, finding that when water contents were supra-optimal -- and net photosynthesis rates were depressed below their normal maximum values because of too much water -- atmospheric CO2 enrichment almost always alleviated the photosynthetic depression. In many instances, in fact, the CO2-enriched air actually boosted the lichens' rate of CO2 uptake to 20 to 30% above the maximum values observed under optimal moisture conditions. At the other end of the spectrum, Tuba et al. (1998) studied the effects of sub-optimal water contents, when lack of water depressed the photosynthetic rates of lichens. In this case, a doubling of the air's CO2 content allowed photosynthetic carbon gains during experimental dry-downs to be maintained 14% longer than what was typically observed in normal air. In addition, the total assimilation of carbon during the dry-downs was determined to be 50% greater in the CO2-enriched air than in the ambient air.

And thus it can be seen that "lowly" biological soil crusts, helped along by "dangerous" anthropogenic CO2 emissions, can be a great boon to arid and semi-arid ecosystems and the important services they provide, as well as to the nematodes that appear later in the ecosystem succession process and the many benefits that the majority of them provide, such as enhancing the decomposition of organic materials, which promotes the recycling of nutrients and their return to the soil where they are accessible to plant roots, plus the tendency of many of them to seek out and eliminate various plant pests that have a subterranean soil stage. It all has to start somewhere, and for many forlorn landscapes, it begins with BSCs.

Sherwood, Keith and Craig Idso

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Bongers, T. 1990. The maturity index: an ecological measure of environmental disturbance based on nematode species composition. Oecologia 83: 14-19.

Brostoff, W.N., Sharifi, M.R. and Rundel, P.W. 2002. Photosynthesis of cryptobiotic crusts in a seasonally inundated system of pans and dunes at Edwards Air Force Base, western Mojave Desert, California: laboratory studies. Flora 197: 143-151.

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Guo, Y.R., Zhao, H.L., Zuo, X.A., Drake, S. and Zhao, X.Y. 2008. Biological soil crust development and its topsoil properties in the process of dune stabilization, Inner Mongolia, China. Environmental Geology 54: 653-662.

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