Biogeochemical cycling[ edit ] Carbon cycling Biological soil crusts contribute to the carbon cycle through respiration and photosynthesis of crust microorganisms which are active only when wet. Respiration can begin in as little as 3 minutes after wetting whereas photosynthesis reaches full activity after 30 minutes.
Desert Soils Joseph R. Desert soils are downright unusual! They vary tremendously in texture; many are sandy and gravelly, while others contain layers of sticky clay, or even rock-hard, white limy layers.
Desert soils may be gray-colored, brown, or even brick red. In the more arid parts of the Sonoran Desert, surfaces of some soils are covered by a layer of small stones that can be as tightly interlocked as pieces of an ancient Roman mosaic, and are coated with dark, shiny rock varnish.
Many of these diverse features of desert soils have taken thousands of years or more to form. Characteristics of these soils also greatly affect, and are greatly affected by, desert organisms.
Soil Formation The varied geological terrains of the Sonoran Desert provide many different kinds of parent materials in which soils form. The mountains themselves possess various rock types, slopes, and exposures that offer a complex array of different soil-forming environments.
The monstrous heaps of wind-blown sand in the dune fields of the Gran Desierto in northwestern Sonora and the Cactus Plain east of Parker, Arizona, provide yet another kind of Microphytic soil crusts and desert ecosystems essay parent material.
Soils initially form within the physical matrix of the parent material. Water is responsible for most of this work of alteration and transportation of materials in soils, even in deserts.
Physical and chemical weathering reduces the sizes of coarse particles. Chemical reactions convert some minerals contained in the parent materials into clay minerals. All of these processes take time; desert soils that have formed in parent materials that were deposited long, long ago differ considerably from those that have formed in younger deposits.
Photo by Kyle House You can readily observe the effect of time on the degree of soil development in alluvial fans deposited at different times throughout the region.
The alluvial deposits on the southwest side of the Tortolita Mountains near Tucson provide an example. Water in Wild Burro Wash transported large volumes of gravelly to rocky alluvium out of Wild Burro Canyon and deposited these materials in alluvial fans on the broad, gently sloping piedmont see the photograph and matching landscape diagram above.
Some of these materials were deposited perhaps more thanyears ago, and the surfaces of some of these ancient alluvial fans, although somewhat eroded, still persist. Beneath these old land surfaces are soils that have developed thick, red- colored clayey layers underlain by accumulations of white caliche defined and discussed below.
In contrast, soils in nearby, considerably younger Holocene deposits lack the bright red coloration and accumulations of clay and lime. Soils in both the Pleistocene and Holocene deposits started out with the same kind of parent materials: The great contrast in the amount of time over which the soils have developed is responsible for the differences in soil characteristics.
Clay, Colors, and Caliche Many soils of the piedmonts and basin floors of the Sonoran Desert start out as deposits of gravelly or stony alluvium which have fairly uniform characteristics throughout. With the passage of time, however, pronounced horizontal layers called soil horizons develop.
You can often easily see these horizons in older, well-developed soils because they differ from each other in color. You may also detect them by changes in texture and other characteristics.
Soil horizons form in arid and semi-arid regions through the downward transport of materials by water and the accumulation of various materials at characteristic depths. The depth at which materials accumulate depends mostly on how much precip-itation is received greater amounts of precipitation soaking into the soil transport materials to greater depths and on the kind of material that is being transported.
Calcic Horizons Many desert soils contain prominent, whitish layers called calcic horizons. These are accumulations of calcium carbonate, the same material found in chalk, concrete, and agricultural lime.
In the Sonoran Desert, the tops of these horizons are typically less than twenty to forty inches 50 to cm below the soil surface. Calcic horizons may be very thin six inches; 15 cm in some soils and contain only small amounts of calcium carbonate. In other soils, these horizons may be very thick greater than three feet; 1 m and strongly cemented.
These nearly impenetrable, cemented layers, or petrocalcic horizons, are commonly called caliche. Calcic horizons are most typically unique features of soils of arid and semi-arid regions and are usually absent from parts of the world receiving much precipitation.
The amount of precipitation that infiltrates into the soil is the most important factor that determines the depth to which calcium carbonate is transported and accumulates. In relatively moist parts of the Sonoran Desert, such as areas near Tucson, Arizona, where annual precipitation averages ten inches 25 cm or more, calcium carbonate tends to accumulate at depths exceeding ten inches 25 cm.
In extremely arid regions, though, such as the area around Yuma which annually receives four inches 10 cm or less precipitation, calcium carbonate may accumulate within a few centimeters of the surface, or even at the surface. In contrast to desert regions, areas such as the eastern United States receive so much rainfall that calcium carbonate never accumulates in pronounced soil horizons, because it is readily leached from the soil and flushed into the groundwater.
Thick, strongly-cemented calcic horizons take a long time to form.
They start as thin, patchy coats of whitish calcium carbonate on the lower surfaces of pebbles and small stones.Biological Soil Crusts: Ecology & Management This publication was jointly funded by the USDI, BLM, and USGS Forest and Rangeland Ecosystem Science Center.
Soil C and N pools in patchy shrublands of the Negev and Chihuahuan Deserts Thomas L. Thompsona,, Eli Zaadyb,c, and biological soil crusts. Within such ecosystems ‘‘fertile C4 grasses and soil crusts dominating microphytic patches. Very small organisms, including cyanobacteria, algae, lichens, mosses, and liverworts, form living crusts on many desert soils.
Scientists call these coverings microphytic crusts. Depending on the environment, they may be a mix of many of the above kinds of photosynthetic organisms. This crust plays a major role in the redistribu- more about the interaction between the microphytic crusts tion of water (Eldridge et al., ), in¯uencing ecosystem and the soil surface dynamics in S.
tenacissima steppes, we functioning and dynamics in these areas (West, ). Microphytic Soil Crusts and Desert Ecosystems Essay - Microphytic Soil Crusts and Desert Ecosystems Communities of micro-organisms create crusts on soils throughout semi-arid and arid regions of the world.
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.