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

Abstract

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.