Microbiotic Crusts Research Articles

TWO CAUSES FOR RUNOFF INITIATION ON MICROBIOTIC CRUSTS: HYDROPHOBICITY AND PORE CLOGGING

Hydrophobicity and pore clogging are suggested as two mechanisms responsible for generating runoff over microbiotic crusts overlying dune sand. Although natural microbiotic crusts in the Hallamish dune field (Negev Desert, Israel) did not show any hydrophobicity, that was not the case with natural crusts subjected to long periods of continuous wetness in the lab. Monoalgal crusts, grown in the lab, also showed high hydrophobicity when dry. The hydrophobicity vanished, however, once the surface was wetted. Runoff on monoalgal lab-grown crusts was obtained when (i) the dry crust exhibited hydrophobic properties and (ii) the wetted crust no longer showed water repellence. Although runoff generation caused by hydrophobicity is expected, it is suggested that runoff initiation when the crusts do not exhibit any hydrophobicity stems from the high water absorption and swelling of the exopolysaccharide cyanobacterial sheaths causing pore clogging. The multi-layered structure of the crust and filament migration to the surface may enhance pore clogging. The experiments and their interpretation are supported by reinterpretation of published data.

Microbiotic crusts in the high equatorial Andes, and their influence on paramo soils

Although microbiotic crusts are known from many lowland deserts, they have apparently never been reported from alpine areas. This paper describes microbiotic crusts from an equatorial high Andean location (Páramo de Piedras Blancas) in Venezuela. The main objectives are to describe the basic characteristics of the crusts, to examine the properties of the soils on which they occur, and to discuss their possible geomorphic significance for paramo soils. Properties of crusts and of the soils beneath them were compared with those of adjacent bare soils at three sites, between 4360 and 4550 m altitude. Measurements on unconfined soil compressive strength and infiltration rates were taken in the field. Crust specimens were also gathered, and later examined and sectioned under a microscope to ascertain micromorphological details. Laboratory data included soil texture, organic matter content, color, pH, water-storage at field capacity, and stability of soil aggregates. Crusts formed discontinuous, garland-like patches parallel to contours, or covered small round soil buds, produced by frost sorting. Microbiotic crusts contained many plants, including an unclassified hepatic (genus Marsupella Dumort.), a moss ( Grimmia longirostris Hook.), and several lichen species. Crusts consisted of a thin organic layer (5–35 mm) overlying mineral soil, and showed three parallel bands with diffuse boundaries. The outer layer had green-reddish brown, live plant shoots and stems with few fine mineral grains. The middle layer had anatomically recognizable dark-brown plant material and a greater content of mineral grains. The light-colored basal layer had little organic matter mixed with coarse mineral particles; it also contained many small orthovughs (vesicles) and some larger arched chambers, caused mainly by frost activity. Soils with a microbiotic crust were darker than bare soils. Organic matter content in crust areas was two to three times higher than in adjacent bare soil; pH dropped ∼ 0.3 units below crusts. Soils with crust had finer texture than bare ground: the content of fines in crusts was 1.5 times higher than in bare soils, but gravel in these was four times higher than below cryptogams. Indices of particle concentration showed that grains < 0.5-1 mm were more abundant in crusts. Finer soils in crusts may result from a combination of (a) prior random texture variation; (b) cryptogam colonization of fine-soil patches sorted by frost; (c) interception and capture of aeolian dust by cryptogams, especially by mosses; and (d) reduction of fine-particle erosion by crusts; as coarse grains are lost, mean particle size within crusts drops. Geomorphic effects of crusts were also significant: (a) Infiltration rates were 70% greater in crust than in noncrust soils (67.0 vs. 40.4 mm/min); (b) Crust soils stored more water at field capacity (48.7–88.4%) than bare areas (20.5–42%); (c) Crust soils had a higher resistance to raindrop erosion than bare areas (47.5 drops/0.1 g vs. 10.2 drops/0.1 g). Undisturbed crust specimens were even more effective in resisting drop impact, surviving up to 1100 drops/0.1 g; (d) Cryptogams increased soil compressibility more than 200%, from 54 g/cm 2 in bare soils, to 118.1 g/cm 2 in crust areas. All these geomorphic changes should result in lower rates of soil erosion in paramo areas with dense crusts.

Trapping of airborne dust by Eig's meadowgrass (Poa eigii) in the Judean Desert, Israel

North-facing slopes above sea level in the Judean Desert are covered with a nearly continuous carpet of the meadow grassPoa eigii. The grass functions as a trap for aeolian dust and protects the soil from erosion. The microbiotic crust, composed of mosses, liverworts, lichens, cyanobacteria and fungi, which occur among the tufts ofPoa eigii, also contributes to trapping and protection. The presence of allochtonous quartz and clays throughout the 60-cm deep soil profile suggests an aeolian origin of the soil. Stones > 2 mm in diameter contribute to only 13–33% of the soil weight. South-facing slopes have a thinner soil mantle, with 67–92% stones, quartz and clays occurring only at the surface layers and are devoid of the meadow grass. Microbiotic crust composed mainly of cyanobacteria protects the crust from erosion and assists trapping of aeolian material in this layer.

Microbiotic soil crusts - a review of their roles in soil and ecological processes in the rangelands of Australia

Microbiotic crusts are assemblages of non-vascular plants (mosses, liverworts, algae, lichens, fungi, bacteria and cyanobacteria) which form intimate associations with surface soils. They play a major role in infiltration processes through changes to soil physico-chemical properties, and through their influence on soil surface roughness. Whilst some research suggests that they may restrict infiltration, Australian experience is that they are generally associated with enhanced infiltration. Unlike physical soil crusts, microbiotic crusts stabilize the soil against water and wind erosion, increasing landscape stability, particularly in areas of low vascular plant cover. Microbiotic crusts are thus useful indicators of soil surface condition, and cyanobacteria in the crusts fix nitrogen which may be utilized by developing vascular plant seedlings. Little is known, however, about how they interact with vascular plants and soil invertebrates. Their role in rangeland ecosystems has received renewed attention over the past few years with an increasing interest in ecologically sustainable development of arid and semi-arid grazing systems. In this review we discuss the characteristics and distribution of microbiotic crusts in the rangelands of Australia, their roles in soil and ecological processes and the impacts of fire and grazing. Finally we propose a new system for classifying crusts into functional groups and identify areas requiring further investigation.