Biotic soil crusts are communities of fungus, algae and mosses. Lichens (symbiotically associated fungus and algae) are also important components of biotic soil crusts (St. Clair et al. 1993). These crusts are usually dark in color and have an irregular topography. They can be very extensive in arid and semi-arid environments of the Southwest, often representing 70% of living ground cover (Belnap, 1999). Abiotic soil crusts are formed in deserts through physical processes such as mineral deposition. The picture at right is an example of a salt crust formed via evaporation in Death Valley California. However, the focus of this discussion is on biotic crusts. Therefore, when crusts are referred to it is implied that we only mean biotic crusts. We will look at the importance of soil crusts in the southwestern United States especially in arid and semi-arid deserts. By looking at the importance of soil crusts to the ecology of arid and semi-arid areas the impacts of human disturbance can be evaluated. These human impacts will be examined with a focus on trampling caused by recreation and grazing. Finally, the possibility of restoration and recovery in trampled areas will be covered. It should be noted that biotic soil crusts are important to many desert ecosystems and are not restricted to the Southwestern United States (Moore 1998).
Although water is most often the limiting resource in desert ecosystems, nutrients can also be in short supply (Smith 1996); in fact, nitrogen is often limiting in desert environments (Zak and Whitford in Belnap 1995). Biotic soil crusts are therefore, particularly important in a desert ecosystem because they can fix carbon and nitrogen from the atmosphere. The photosynthesizing activity of lichens and cyanobacteria within crusts are responsible for carbon fixation, whereas the nitrogenase activity of cyanobacteria and cyanobacterial components are responsible for the fixation of nitrogen. In one type of arid ecosystem (juniper woodland), crusts have been found to be the primary input of nitrogen (Evans and Ehleringer 1993). Additionally, Belnap and Gardner (1993) observed negatively charged clay particles bound to and incorporated into the polysaccharide sheath material of the cyanobacterium Microcoleus vaginatus. They suggest that these positively charged macro-nutrients could be held in the upper soil horizons, thus increasing soil fertility. Therefore, biotic soil crusts not only actively fix carbon and nitrogen, they might retard the nutrient depleting effects of leaching.
Crusts provide nutrients in a form that is available for vascular plants (Mayland and McIntosh 1966). Higher plants in areas with undisturbed crusts have higher concentrations of nitrogen and other macro-nutrients (Belnap 1993). Scientists have been reported saying that crusts increase the uptake of phosphorus, potassium, calcium, magnesium and iron in vascular plants (Gillis 1994). In addition, desert rodents and desert tortoises feeding on vascular plants may suffer from nutrient deficiencies in the absence of biotic crusts (Gillis 1994).
What is the effect of human disturbance in relation to the crucial role that soil crusts play in desert ecosystem, nutrient cycling? Disturbance of soil crusts result in significant spatial variability in the nitrogen content of the soil (Evans and Ehleringer 1993). Belnap et al. (1993) found a significant reduction in nitrogenase activity after disturbance caused by shallow raking, deep raking, one pass with a tank, ten passes with a tank and removing the top centimeter of soil (scalping). Nine months after these disturbances, a 77% to 97% reduction in the nitrogenase activity of the crusts was found, showing the negative effects of compaction (i.e. Tank passes), scalping, and raking disturbances on nitrogen input. Chlorophyll contents of crusts however, are quick to recover when the disturbance event leaves the disturbed crust in place (Belnap et al. 1993). Why is there such a difference between the recovery of photosynthetic activity and the recovery of nitrogenase activity? Belnap et al. (1993) suggests a mechanism for creation of an anaerobic environment that facilitates nitrogenase activity in Microcoleus vaginatus. Microcoleus vaginatus forms polysaccharide sheaths that surround the living filaments. The packing of multiple filaments into one extra cellular sheath or the packing of multiple sheaths together could exclude oxygen. Therefore, any activity that breaks up these tightly packed sheaths or filaments would introduce oxygen and prevent the fixation of nitrogen. It would require the full repair and recovery of these sheaths to again exclude oxygen. This could explain why nitrogenase activity is so slow to recover compared to photosynthetic activity.
It is clear that human disturbance of biotic soil crusts in arid and semi-arid environments will reduce the fertility of desert soils. Deserts have a low rate of primary production compared to other ecosystems (Ricklefs 1993) and decreased nutrient input could further reduce primary productivity. The entire desert ecosystem would undoubtedly suffer from the effects of reduced primary productivity.
Soil Stability and Water Retention
Biotic desert crusts are important in the stability of desert soils in that they are significantly more resistant to wind erosion than bare sand (Belnap and Gillette 1997). The irregular topography of soil crusts decrease the velocity of surface water flow and impede water erosion (Harper and Marble 1988 in Belnap 1995). The cyanobacteria Microcoleus vaginatus is an important constituent of desert soil crusts on the Colorado Plateau and may explain how crusts stabilize the soil. Belnap and Gardner (1993) found that Microcoleus vaginatus forms polysaccharide sheaths that surround the living filaments and are intertwined throughout the soil particles. This polysaccharide material was observed attaching to soil particles and binding them together. Therefore, it seems that cyanobacteria are extremely important to the soil stabilizing effect of crusts.
Belnap and Gardner (1993) found that sheath material can absorb eight times its weight in water. Suggesting it's important in increasing the water-holding capacity of desert soils. This is obviously an important attribute considering that water is the most limiting resource in almost all desert ecosystems. Belnap and Gardner (1993) also observed that, when wet, the sheath material swells and covers soil surfaces more extensively. This suggests that the soil binding capability of crusts are enhanced by moisture. The lichen Collema tenax is an important component of soil crusts in the intermountain region of the U.S. (St. Clair et al. 1993), and has also has a high water holding capacity (Lange et al. 1998). This water holding capacity improves the rate of photosynthesis of the lichen which is water dependent (Lange et al. 1998).
What are the effects of disturbance on the effects of biotic crusts mentioned above? Disturbance of soil crusts have been found to significantly increase the potential wind erodibility of the soil (Belnap and Gillette 1997, Belnap and Gillette 1998). Only soils that had undisturbed crustal development were able to withstand wind speeds that are regularly encountered in the desert (Belnap and Gillette 1997; Belnap and Gillette 1998). Belnap and Gardner (1993) found polysaccharide sheath material from Microcoleus vaginatus at depths in the soil much greater than light can penetrate. These sheaths are no longer associated with living cyanobacteria and exist due to long periods of sedimentation. Despite the fact that they are no longer associated with living cyanobacteria, they still contribute to soil stability, nutrient availability and water retention (Belnap and Gardner 1993). However, if these sheaths are crushed due to disturbance then they cannot be regenerated and their contributions to the soil are permanently lost.
Erosion hazard has been identified as an environmental indicator that can be used to assess the risk of desertification (Mouat et al. 1996). It is clear that disturbance of soil crusts leads to greater erodibility and greater erosion hazard indicating that, the risk of desertification is increased through the disturbance of soil crusts (Belnap 1993).
Other Ecological Effects
Belnap (1993) found a variety of differences between disturbed and undisturbed plots. The biomass of soil bacteria and nematodes are significantly decreased when soil crusts are trampled. Micro arthropod population sizes and diversity were found to be higher in untrampled areas. Shrubs are a more dominant component of vascular plant community structure in areas with undisturbed crusts. In areas where crusts have been disturbed, the plant communities are dominated by annual and perennial bulbs that only have above ground parts in the spring. In addition, the spaces between shrubs are larger in trampled areas compared to untrampled ones. There is also a higher percentage of invasive species in trampled areas (see picture at top of page). These results confirm that biotic crusts play an important role in the overall maintenance and development of vascular plant communities (Metting 1991). These differences in community structure could have many wide ranging effects on the ecosystem. Belnap (1993) suggests that decreased soil biota will lead to decreased nutrient cycling and decreased primary productivity. With less plant and crust cover, the soils will be more open to erosion and degradation eventually leading to desertification.
Albedo or reflectance is increased when soil crusts are disturbed (Belnap 1993). This could be a factor that contributes to decreased soil temperatures in trampled areas (Belnap 1993). Decreased soil temperatures could have a wide range of negative effects on vascular plants and soil biota (Belnap 1993).
Recovery and Restoration
In the long run, the impact that humans will have on the ecology of the Southwestern deserts will depend on how quickly biotic soil crusts regenerate after disturbance. Visual assessments of recovery have proven to be inadequate because they under-estimate the time of recovery (Belnap 1993). Visual assessments of recovery have shown 100% recovery within one year, while other assessments of recovery, such as chlorophyl a content, lichen cover, lichen species richness and moss cover, show almost no improvement during that time. Using recovery rates found after three to five years, Belnap (1993) estimated that full recovery could take 40 years for chlorophyl a content, 30-40 years for average depth ramified by sheath material, 40-65 years for maximum depth ramified by sheath material, and over 250 years for moss cover. Soil scientists investigating recovery of tank tracks have been reported estimating up to 1,000 years for full recovery (Brainard 1998). They have also reportedly found that crusts develop more slowly in open areas that in areas shaded by vegetation (Brainard 1998). Rates of recovery could be highly variable depending on the extent of the disturbance, the presence of surrounding undisturbed crust, rainfall and severity of disturbance (Belnap 1993, Belnap 1994).
Knutsen and Metting (1991) suggested that forced development of biological crusts could be feasible and valuable in some situations. They note two major obstacles to the forced development of crusts on a large scale: finding a source of suitable inocula and the timely provision of sufficient water. They also mention that water sources unusable to humans (due to salinity ect.) could be used for culturing crusts. The technology for micro algal mass culture exists and is currently being used to produce protein rich food and other products. Therefore, it is conceivable that this same technology could be used to produce mass quantities of inocula for use in desert restoration (Knutsen and Metting 1991). Belnap (1994) notes that a draw back to commercially cultured inoculum is its non-native composition. Locally-cultured inoculum would be preferable, but can require space, time and money that is not available. A third alternative proposed by Belnap (1994) is to collect preexisting crust and distribute it over the disturbed area. The impact of this method could be reduced by using crust from the edges of disturbed areas or from areas destined for development (Belnap 1994).
Belnap (1993) tested the effectiveness of inoculation on the recovery of crusts disturbed by scalping the top three centimeters of soil. Some of the plots were then inoculated with a crumbled mixture of the scalped material and the plots were allowed to recover for a period of either three or five years. Inoculated plots showed significantly greater chlorophyl a concentrations compared to uninoculated plots. This implies that green algae and cyanobacteria were recovering faster in the presence of inoculate. The use of inoculate on damaged plots significantly increased the rate of recovery for the percent lichen cover, lichen species richness, and moss cover as well (Belnap 1993). The effectiveness of inoculation can be enhanced through application of moisture to the disturbed site (Belnap 1994). It is clear that, to some extent, humans can increase the rate of crustal recovery; however, the feasibility of this strategy for large areas has not been shown.
What then is the impact of human caused soil surface disturbance to the ecology of desert areas? This question is particularly important now because the population and the popularity of the Southwest is increasing. The effects of grazing and of recreational activities, especially off-road vehicles, need to be understood so that their impact can be minimized. The discussion above has made it clear that human caused disturbance (including trampling by cattle) significantly impacts deserts in the following ways: nutrient availability in the soil is negatively impacted, moisture retention of the soil is decreased, community structure is altered, the erodibility of the soil is dramatically increased and the risk of desertification is intensified. Further study needs to be done on recovery rates of biotic soil crusts. However, it is clear that full regeneration will take more than just a few years. Brainard (1998) reported that dropping inoculum flakes from airplanes has been suggested as a strategy to restore disturbed desert soils. Despite such ambitious plans, significant obstacles and expenses need to be overcome before any kind of wide ranging project to increase recovery rates through inoculation will be possible. Therefore, the focus of management efforts in the arid and semi-arid areas of the Southwest need to be on preventing the existing crust from being disturbed and allowing disturbed areas to start the recovery process. This will certainly entail the restriction of recreational and pastoral activities in desert areas.
For some great Pictures
Authored by Christopher Kauffman
May, 2 1999