Microphytic Soil Crusts and Desert Ecosystems
Communities of micro-organisms create crusts on soils throughout semi-arid and arid regions of the world. These microphytic (also called cryptogamic) crusts are formed when all or some of a diverse array of photosynthetic cyanobacteria (blue-green algae), fungi, bacteria, lichens and mosses, bind together with inorganic particles in the first few millimeters of a soil
Microphytic crusts are dominant feature in desert
soils; they are estimated to represent approximately 70% of desert soil biomass world wide (Belnap 1993). Un-restricted human activity (farming, livestock grazing, recreation) results in the denigration or destruction these prominent crusts. Many claim that soils and soil mechanisms are at the base of other ecosystem functions (Vitousek, Walker, Syers in Gillis 1994). In order to better understand and manage desert ecosystems
, it is important to begin to understand how cryptogamic crusts form, what role crusts play in shaping desert soil properties, and further, how crust removal might effect soil quality and ecosystem stability.
It is generally thought that the formation of microphytic crusts begins with the establishment of cyanobacteria or agal communities on the soil surface (Campbell et. al. 1989). There are many different types of algae and cyanobacteria which exist in the new crusts, however it is difficult to ascertain which types of organisms are responsible for which processes of early crust formation. Johansen postulates that crusts begin to form when filamentous cyanobacteria (as opposed to diatomic and nonfilamentous cyanobacteria or other algae) colonize the surface of soils in a period of moist weather (1993). As cyanobacteria begin to metabolize in the moist and light conditions, they excrete polysaccharides which cause them to adhere to other soil particles. During periods of limited moisture, the filaments dehydrate and become brittle, and the organisms stop photosynthesis. When water is available to the organisms again, they expand out into new areas, leaving behind their old filaments. This process creates a complex of soil particles bound together by old filaments. Eventually the soil surface is covered by a mat, and the soil below the mat has been consolidated by a net of polysaccharides.
After the initial establishment of agal mats, other organisms may take advantage of biomass created by cyanobacteria. Generally, the diversity of organisms present in crusts increases over time. Lichens form when fungi associate with cyanobacteria and/or algae. This association requires long periods of time (on the order of 50+ undisturbed years to establish an well developed crust), and takes place in soils which are relatively silt and clay rich (Skujins ). Lichen hyphae increase the structural integrity of soil crusts. This leads in one way or another to the formation of pinnacles (microtopography on the surface of the soil). It is not entirely understood whether pinnacles result from erosion of areas surrounding lichen cover or if lichen cover enables wind blown particles to accrete to the soil surface (Johansen 1993). Certainly a case can be made for either theory. Crusts are found in predominantly erosional landscapes, and spots with lichen caps are more resistant than surrounding areas. However, secretion of polysaccharides during cyanobacteria metabolism in conjunction with the increased structural integrity afforded by the lichens could lead to increased rates of wind-blown sediment capture.
Because of their relatively simple structure, low nutrient requirements and their ability to become dormant in periods of drought (resistance to desiccation), algae and cyanobacteria can exist in almost any soil (Isichei 1990). However there are limits to the establishment and development of extensive microphytic mats. Mats do not form in extremely arid regions where there is rarely water for the cyanobacteria to utilize. Cyanobacteria cannot proliferate in environments with little available light. Microphytic crust productivity is limited in areas which receive a high percentage of their moisture during the hot summer months (Eldridge et al. 1994); cyanobacteria and lichens are know to be intolerant of the combination of high temperature and high humidity. In such environments, the structure of the cyanobacterial filaments and lichen thalli collapse due to internally high water pressure (Metting 1991). Extreme soil textures also limits crust formation; crusts are unable to develop in unconsolidated sand (high wind-erosion) or in soils which are excessively rocky (Eldridge et. al 1994, Skujins )
It is interesting to note that cryptogamic crusts do not form in the moister soils of temperate forest zones. Soil pH is the probable culprit - cyanobacteria favor high pH's and forest soils are on the average more acidic than desert soils. This is due in part to the accumulation of woody litter in forest soils (Johansen, 1993). Here, water is still an important factor that determines whether crusts develop or not. In wetter regions, vascular plants can establish themselves just as readily as cyanobacteria. However, vascular plants decrease soil pH and then discourage cyanobacterial growth. This cycle and others like it make it difficult to isolate initial variables which lead to the proliferation of certain crusts. As is the case in most ecosystem processes, cause and effect are not linear. Certain conditions favor crust formation, but the formation of crusts effects those very same conditions. It is important, therefore, to look at cryptogamic crusts as communities that have evolved with the soil, and not developed only in response to initial soil conditions.
Influence of Crusts on Physical Soil Properties
Cryptogamic crusts change the surface of soils. Accordingly, they affect many physical properties of the soil. Most obviously, microphytic crusts affect the erosion potential of soils (crusts are often the primary soil cover in a generally eroding environment). Booth (1941 in Isichei 1990) found that soil loss rate is 22 times higher in un-crusted soils than in crusted soils. Kleiner and Harper (1972, 1977 in Johansen 1993) observed outwash gullies and partially buried grass in grazed areas with disturbed cryptogamic crusts. Undisturbed locations showed little evidence of these erosional processes.
Crusts probably stabilize the soil surface and reduce erosion of smaller fractions of soil material through the continual production of sticky polysaccharides. In addition to holding soil material in place, crusts act as traps to wind-blown sediment: sediment sticks to filamentous ooze in the crusts during periods of high crustal productivity. Crusts create a micro-topography in an otherwise flat soil-scape. The micro-topography slows the speed of wind across the surface of the soil, and provides still areas for sediment to drop out from the air. This leads to the accumulation of silt and clay in the upper parts of soils with crusts. Accretion of new soil material could lead to increased soil fertility through soil renewal.
In addition to slowing soil erosion, crusts change hydrologic properties of soils. There is some dissent as to how microphytic crusts affect soil permeability and infiltration rates. Many theories are extrapolations and not based on experimental results (Eldridge et. al. 1994, Johansen 1993). Most agree that crusts decrease soil permeability. This is a little misleading. Microphytic crusts do lead to the formation of more stable soil aggregates (Loope and Gifford 1972), and one might think that this would lead to greater soil porosity. However, t is not the soil aggregates which control permeability in soils with crusts. In the presence of water, cyanobacteria filaments swell and take on water, leaving smaller pore spaces for water to trickle downward. Very dense lichen cover has been shown to effectively eliminate percolation (Loope and Gifford 1972).
The same process which limits water percolation generally increases water infiltration and moisture retention. Wang found that crusts in Inner Mongolia took on seven times their weight in water over a short period (1981, in Campbell et. al. 1989). The capacity of crust organisms to swell in the presence of water allows desert soils to capture large volumes of water at a time, unless porosity is drastically reduced by lichens (Loope and Gifford 1972). In this instance, very low permeability leads to only a small amount of water reaching the filaments in lower parts of the crust. This leads some to believe that soil crusts cause increased watershed erosion by increasing runoff rates (Stanley 1983 and West 1990 in Johansen 1993). However, most have observed that water retaining properties of crust organisms prevent erosion and increase soil moisture.
Microphytic Crust's Role in Creating and Cycling Soil Nutrients
Perhaps the important function of microphytic crusts in desert environments stems from their ability to fix nitrogen and carbon. Microphytic crusts are the basis for the production of organic carbon in desert ecosystems where plant biomass is low. Cyanobacteria are photoautotrophs, meaning that they need only water, air and sunlight (and some other inorganic nutrients) to produce organic carbon and grow. This primary production of organic carbon enables heterotrophs in the ecosystem to exist (Beymer and Kloptatek 1991).
Fixed nitrogen quantities are generally very low in desert soils. Microphytic organisms could be the primary source of N imput in these soils (Metting 1991). Cyanobacteria are known to fix N2 in the laboratory, and thus most believe that the cyanobacteria component in crusts is responsible for the majority of N2 fixation. Unfortunately, N-cycling is not very well understood; it is difficult to determine whether cyanobacteria activity is directly responsible for N availability, or whether other factors mediate the process. Jefferies et al studied N fixation in desert brush communities with pinnacled crust formations (1992). They found that N2 fixation curves were not directly related to the moisture dependent wax and wane of cyanobacteria metabolism. They hypothesize instead that heterotrophic bacteria associated with the dominant cyanobacteria in crusts are responsible for the majority of N fixation.
Jefferies and others (Skujins , Metting 1991) have shown that crustal organisms fix nitrogen in soils. It is unclear, however, whether this fixed nitrogen remains available in the soil for vascular plant and animal use. When the crust absorbs water, conditions are reducing and thus favorable to N2 fixation. Much of the N fixed during these favorable conditions is lost to either volatilization (of NO3 gas) or to the reduction of NH4+ to N2. Other crust organisms metabolizing organic carbon produced by flourishing cyanobacteria are responsible for the second reaction. Rate of fixed nitrogen loss is therefore related to the amount of organic carbon in the soil (which is related to the productivity of crustal cyanobacteria) and oxidation-reduction cycles. N-removal rates might also be related to the number of cation exchange sites available in a soil. Cation exchange sites can retain NH4+. Crusts trap silt and clay particles and increase soil organic material which both have relatively high cation exchange rates. Thus increased silt and clay content could increase the retention of any nitrogen that crustal organisms produce (Metting 1991).
Though it is almost impossible at this point to sort out specific mechanisms, we can notice that water plays an important role in determining how nitrogen is cycled through cryptogamic soils. Different moisture regimes influence factors which control rates of nitrogen fixation and also rates of fixed nitrogen loss. It seems reasonable to expect that under certain moisture regimes, cryptogamic crusts might constitute the most important imput in the nitrogen budget. Under different circumstances, crustal N-fixation might not ultimately control levels of available N.
Based on the complex nature of many relationships established by microphytic crusts in soil, it seems impossible to say how, exactly, cryptogams control the stability of desert ecosystems. We have seen that crusts do indeed stabilize soil surfaces, do influence soil moisture, and do affect, to varying degrees, the nutrient cycling in desert soils. It is still not clear how these factors combine to define a clear relationship between microphytic crusts and, for example, vascular plant communities. Some preliminary research has been done in this area, but only general trends were studied. Harper (in Gillis and Miller 1994) noticed that vascular plants and vertebrates are nutrient deficient in areas where microphytic crust has been eliminated by over grazing. He also found that absorption of nutrients by native plants is greater in areas with crusts.
Until we have a better understanding of nutrient cycling, it will be impossible to define exact relationships between soil and ecosystem. As we have seen, this does not by any means imply that there is no relationship or that the relationship is negligible. There is a relationship between cryptogamic crusts and endemic desert ecology. Desert soils should be managed to reflect this clearly complex relationship.
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