Organic Soil Crusts of Western Rangelands

George Nibler

An often-overlooked component of rangeland ecosystems is the minuscule assemblage of plants and animals that covers the ground between the more obvious vascular plants such as grasses and shrubs. This biotic community, variously known as the organic or biological soil crust, microbiotic soil, or cryptogamic soil, is present in many environments but is especially important in the arid rangelands of the western US. People fortunate enough to have walked through an undisturbed, healthy sagebrush steppe landscape may have noticed the sparse and patchy distribution of vascular plants, and that the soil surface between the plants is not bare ground but is covered with a mat of tiny vegetation. This biological soil crust can cover up to 3/4 of the total ground surface in some areas, and it plays a vital role in the overall health of the rangeland, controlling soil erosion, preventing the invasion of noxious weeds, and promoting soil fertility.

Biological soil crusts are composed of a community of organisms that live on, or within a few centimeters, of the ground surface. These include mosses, lichens, algae, fungi, and bacteria. Some, like mosses and lichens, may be up to a few millimeters in size and are visible to the naked eye, while most of the algae and bacteria are only visible with a hand lens or microscope. Within this organic matrix live numerous animals, ranging from microscopic protozoans and nematodes, to macroscopic arthropods like mites, springtails, centipedes, and many others.

This organic community forms a textured, porous layer a few centimeters thick above the ground surface, with a "root zone" (actually fungal hyphae, bacterial filaments, etc.) that may extend many centimeters below ground. In the sagebrush steppe environment of the Great Basin the biological crust commonly develops to cover essentially the entire ground surface between the vascular plants, except in rocky areas, steep or unstable slopes, or disturbed areas.

For all practical purposes, biological crusts are the soil surface where they are present, and so control erosion and affect the infiltration of precipitation. The mosses and lichens, fungal and bacterial filaments, and associated organic materials form a protective cover that limits soil erosion from both wind and water. In many areas the crust acts like a sponge, soaking up precipitation and snow melt and slowly releasing it to the underlying soil, reducing runoff and increasing infiltration and soil moisture.

Biological crusts are very effective at controlling the spread of noxious weeds. A healthy crust presents a near-continuous surface cover that seeds must penetrate in order to germinate. While native plants have evolved mechanisms to penetrate the crust, exotic weeds are generally not able to. Cheatgrass, skeleton weed, and many others will rarely germinate and become established in areas with a healthy crust. This corresponds directly to the susceptibility and response of the land to fire. Healthy weed-free rangeland does not ignite as readily as weed-infested areas, and fires that do start generally burn slower, less hot, and do not spread as rapidly as in degraded weedy land.

Biological crusts are integral parts of the soil food cycle, and are important contributors to soil fertility. Crusts may cover a significant fraction, if not the largest part of the ground surface, and cycle the majority of organic matter, especially organic carbon, to the soil where it acts as a food source for soil microorganisms. Many crust organisms are photosynthetic, converting atmospheric carbon to organic carbon and adding it to the soil. Other organisms are decomposers that break down complex organic matter and convert it to forms usable by plants. Some bacteria and lichens are nitrogen-fixing, and transfer nitrogen from the atmosphere to the soil in a usable form.

Although soil crusts are a ubiquitous part of western rangelands, they are quite sensitive and susceptible to disturbance. The sagebrush steppe ecosystem has evolved in the presence of relatively low populations of large animals such as elk and antelope, compared with the Midwest prairie and its until-recently vast herds of bison. The sagebrush ecosystem, soil crust organisms as well as vascular plants, did not develop adaptations to intensive trampling as did the sod-forming grasslands of the Midwest. Consequently soil crusts are poorly-equipped to survive where they are heavily trampled, crushed, mashed, pulverized, and buried due to intensive cattle grazing.

Disturbance (from grazing, off-road vehicles, mountain bikes and even foot traffic) results in a crust that is reduced in species diversity and nutrient production and cycling, as well as a degradation of each of the benefits just mentioned. Infiltration of precipitation is reduced while runoff and erosion increase. Bare ground exposed where the crust is destroyed is often quickly colonized by noxious weeds, and the potential for hot and fast-burning range fires is greatly increased. Organic carbon and soil nutrient levels are diminished and the overall productivity of the land decreases.

Of course Nature is resilient and the organic crust may eventually recover. The time required for reestablishment of a healthy organic crust following disturbance is quite variable due to the complexity of the crust community and the range of differences in site-specific conditions, but is clearly on the order of at least few decades. In areas that become infested with noxious weeds and are subject to recurring range fires, the organic crust may recover, though on a time frame best appreciated by a geologist. Land managers have been slow to recognize and incorporate organic crust health in land management "planning", but a healthy rangeland cannot exist without a healthy organic soil crust.

George Nibler is an environmental geochemist who lives in the desert outside Boise, Idaho.