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How soil cover types and rainfall patterns influence carbon (C) release in temperate desert ecosystems has largely been unexplored. We removed intact crusts down to 10 cm from the Shapotou region, China, and measured them in PVC mesocosms, immediately after rainfall. C release rates were measured in soils with four cover types (moss-crusted soil, algae-crusted soil, mixed (composed of moss, algae, and lichen)-crusted soil, and mobile dune sand). We investigated seven different rainfall magnitudes (0-1, 1-2, 2-5, 5-10, 10-15, 15-20, and >20 mm) under natural conditions. C release from all four BSCs increased with increasing rainfall amount. With a rainfall increase from 0 to 45 mm, carbon release amounts increased from 0.13 ± 0.09 to 15.2 ± 1.35 gC m in moss-crusted soil, 0.08 ± 0.06 to 6.43 ± 1.23 gC m in algae-crusted soil, 0.11 ± 0.08 to 8.01 ± 0.51 gC m in mixed-crusted soil, and 0.06 ± 0.04 to 8.47 ± 0.51 gC m in mobile dune sand, respectively. Immediately following heavy rainfall events (44.9 mm), moss-crusted soils showed significantly higher carbon release rates than algae- and mixed-crusted soils and mobile dune sands, which were 0.95 ± 0.02, 0.30 ± 0.03, 0.13 ± 0.04, and 0.51 ± 0.02 μmol CO m s, respectively. Changes in rainfall patterns, especially large rain pulses (>10 mm) affect the contributions of different soil cover types to carbon release amounts; moss-crusted soils sustain higher respiration rates than other biological crusts after short-term extreme rainfall events. [ABSTRACT FROM AUTHOR]

Abstract: Many soils in arid and semi-arid regions of the world are affected by crusting, a process by which a compact layer or thin mantle of consolidated material is formed at the soil surface. Crusts can increase the resiliency of the soil to wind erosion, but characterization of soil crust formation to light precipitation events common to the Columbia Plateau region of the Inland Pacific Northwest is unknown. Our objective was to evaluate the effect of light rainfall events on crust formation of five soil types prominent in the Columbia Plateau. The five soils were Athena silt loam, Palouse silt loam, Ritzville silt loam, Walla Walla silt loam, and Warden sandy loam. Soil crusts were formed in the laboratory using a rainfall simulator that applied water at a rate of 2.5mm h−1 for various durations to achieve a total rainfall application of 0, 0.15, 0.30, 0.60 and 1.0mm. Crust strength was measured by a penetrometer while crust thickness was measured by a ruler. Silt and clay content of the five soils ranged from 23 to 66% and from 9 to 17%, respectively. An increase in crust thickness and strength was observed with an increase in rainfall for all soils. Crust thickness was similar among the soils whereas crust strength varied among soils at any given level of rainfall. For example, crust strength of soils when subject to 0.15–0.60mm of rainfall decreased in order of Walla Walla>Athena and Palouse>Ritzville and Warden. These results are consistent with a decreasing order of clay and silt contents. The strongest crust was formed on Palouse silt loam under the highest rainfall amount. A logarithm relationship adequately described the relationship between crust thickness and rainfall for the five soils (r 2 =1.00). This relationship over-predicted crust thickness based upon observations in the field, but performed better than the algorithm used by the Wind Erosion Prediction System. [Copyright &y& Elsevier]

Summary: Runoff in arid and semiarid areas is characterized by high spatial and temporal variability. The spatial component is largely associated with the high spatial heterogeneity of soil surface attributes, such as vegetation and rock fragment covers, topography, and soil crust typology. Biological soil crusts (BSCs) are a common soil cover in arid and semiarid areas, and they play an essential role in local hydrological processes, since they affect many soil surface attributes associated with hydrologic properties. Although several publications have reported on the influence of BSCs on runoff at microplot spatial scales, only a few have examined their influence on larger spatial scales. Moreover, very few studies have analyzed the effect of BSCs on runoff under natural rain conditions. This is difficult, since a complex pattern of interactions among rainfall properties, BSC characteristics and some local characteristics, such as topography or type of soil is expected. In addition, in order to achieve a realistic model of how BSCs and rainfall affect runoff, it would be necessary to consider the level of human-driven degradation of BSCs. In this study, runoff was analyzed in plots with varying cover of cyanobacterial and lichen BSCs at microplot and small hillslope scales for two hydrological years in badlands in SE Spain. Structural equation modelling (SEM) was applied to test both direct and indirect relationships of BSC cover, slope gradient, rainfall characteristics and runoff. Our model showed that rainfall characteristics were the main factors controlling runoff yield. The slope positively affected runoff at small hillslope scales, but did not influence runoff at microplot scales. Runoff decreased at both scales with increased lichen-BSC cover. However, this effect was only significant during low-intensity events. Under high rainfall intensities, neither the BSC cover nor the slope had a causal effect on runoff. Our results suggest that incorporation of BSC-crusted surfaces in models should improve their capabilities for low-intensity rainfall events in semiarid areas similar to ours, but would be less important for high-intensity events. In addition, our approach, which takes direct and indirect relationships of factors affecting runoff into consideration, provides a very accurate picture of the process. [Copyright &y& Elsevier]

Abstract: Biological soil crusts are receiving increasing attention due to their potentially positive effects on desert ecosystems. Artificial propagation of crusts is one way to restore degraded desert soils. However, little is known about their effectiveness on overland flow or water infiltration. Consequently, a study was done on soils of the Loess Plateau of China that had crusts propagated on different slopes. The results show that: (1) it is feasible to artificially grow moss-dominated biological soil crusts (moss crusts) in laboratory; when inoculated by sprinkling crushed fragments of stems and leaves of natural moss crusts they will cover the soil surface almost completely after about 10 months; (2) the artificially propagated moss crusts significantly increase infiltration consequently decreasing overland flow and this increase in infiltration is positively correlated to the soil coverage extent; (3) overland flow results increased by slope in moss-protected and bare soils, consequently the action of moss crusts is particularly important for protecting steep slopes; (4) the start time of runoff process is delayed by moss crusts, and the soil water redistribution process is also greatly affected. These results may be useful for helping to control desertification on the Loess Plateau of China or similar regions. [Copyright &y& Elsevier]

Abstract: In Northern Mexico, long-term grazing has substantially degraded semiarid landscapes. In semiarid systems, ecological and hydrological processes are strongly coupled by patchy plant distribution and biological soil crust (BSC) cover in plant-free interspaces. In this study, we asked: 1) how responsive are BSC cover/composition to a drying/wetting cycle and two-year grazing removal, and 2) what are the implications for soil erosion? We characterized BSC morphotypes and their influence on soil stability under grazed/non-grazed conditions during a dry and wet season. Light- and dark-colored cyanobacteria were dominant at the plant tussock and community level. Cover changes in these two groups differed after a rainy season and in response to grazing removal. Lichens with continuous thalli were more vulnerable to grazing than those with semi-continuous/discontinuous thalli after the dry season. Microsites around tussocks facilitated BSC colonization compared to interspaces. Lichen and cyanobacteria morphotypes differentially enhanced resistance to soil erosion; consequently, surface soil stability depends on the spatial distribution of BSC morphotypes, suggesting soil stability may be as dynamic as changes in the type of BSC cover. Longer-term spatially detailed studies are necessary to elicit spatiotemporal dynamics of BSC communities and their functional role in biotically and abiotically variable environments. [Copyright &y& Elsevier]

Abstract: In arid zones, runoff is frequently generated as a result of the crust development on the soil surface. This crust is a thin layer of greater density, high shear strength, finer pores and has a lower saturated hydraulic conductivity than the underlying soil. The objective of the research reported herein was to study the factors that influence the generation of runoff in small plots under natural rainfall conditions. Factors studied were crust permeability, roughness, soil salt content and time gaps between the rain showers. The field trial was carried out in the Mashash experimental runoff farm in Israel''s Negev desert. Runoff was measured on eight plots using a tipping-bucket system (resolution 0.01 mm s−1). Rainfall intensity was recorded on-site with a rainfall gauge (resolution 0.25 mm s−1). Two treatments were studied: long-term rainfall-induced crusts (LTC) that had developed over a period of years (three plots), and complete destruction of the crust (ICU) by cultivation to a depth of 0.2 m with a rotary tiller before the beginning of the trial (five plots). Surface roughness was characterized by the surface RMS height obtained from laser micro-relief measurements before and during the season. Prior to the onset of rain, roughness was similar for all the ICU plots. One month thereafter, roughness had decreased sharply, but exhibited no further change until the end of the season. Roughness of LTC plots did not change during the season and was lower than that of the ICU plots. After ~21 mm of cumulative rain, the average runoff yield was similar for both ICU and LTC plots, even though roughness in the former did not reach the low values of the LTC plots. Although the variability in roughness among individual LTC plots was very small, large differences were observed in the collected runoff. The same phenomenon was observed for the ICU plots. Moreover, the runoff yields in two ICU plots were consistently higher than those in two LTC plots while three other ICU plots produced much less runoff. Apparent saturated hydraulic conductivity (AHC) was measured on mounds and depressions. In the ICU plots the average AHC of the mounds was markedly higher than that of the depressions while the AHC values of mounds were very similar. No mounds or depressions were observed in the LTC plots, and their AHC was similar to that of depressions in ICU plots. The results indicate that the apparent saturated hydraulic conductivity of the upper soil layer was not directly linked to the runoff generation. Treatment effect was significant only for the first two rainfall events, but the presence of salts in the upper soil layer significantly affected runoff generation during the last four rain events of the season. Analysis of runoff and time gaps between the runoff-producing rain showers showed a clear relationship between runoff yields and average rainfall intensity, the degree of correlation between them improving with a decrease in the length of the gap. [Copyright &y& Elsevier]

Biological soil crusts, a community of cyanobacteria, lichens, and mosses that live on the soil surface, occur in deserts throughout the world. They are a critical component of desert ecosystems, as they are important contributors to soil fertility and stability. Future climate scenarios predict alteration of the timing and amount of precipitation in desert environments. Because biological soil crust organisms are only metabolically active when wet, and as soil surfaces dry quickly in deserts during late spring, summer, and early fall, the amount and timing of precipitation is likely to have significant impacts on the physiological functioning of these communities. Using the three dominant soil crust types found in the western United States, we applied three levels of precipitation frequency (50% below-average, average, and 50% above-average) while maintaining average precipitation amount (therefore changing both timing and size of applied events). We measured the impact of these treatments on photosynthetic performance (as indicated by dark-adapted quantum yield and chlorophyllaconcentrations), nitrogenase activity, and the ability of these organisms to maintain concentrations of radiation-protective pigments (scytonemin, beta-carotene, echinenone, xanthophylls, and canthaxanthin). Increased precipitation frequency produced little response after 2.5 months exposure during spring (1 April-15 June) or summer (15 June-31 August). In contrast, most of the above variables had a large, negative response after exposure to increased precipitation frequency for 6 months spring-fall (1 April-31 October) treatment. The crusts dominated by the soil lichenCollema, being dark and protruding above the surface, dried the most rapidly, followed by the dark surface cyanobacterial crusts (Nostoc-Scytonema-Microcoleus), and then by the light cyanobacterial crusts (Microcoleus). This order reflected the magnitude of the observed response: crusts dominated by the lichenCollemashowed the largest decline in quantum yield, chlorophylla, and protective pigments; crusts dominated byNostoc-Scytonema-Microcoleusshowed an intermediate decline in these variables; and the crusts dominated byMicrocoleusshowed the least negative response. Most previous studies of crust response to radiation stress have been short-term laboratory studies, where organisms were watered and kept under moderate temperatures. Such conditions would give crust organisms access to ample carbon to respond to imposed stresses (e.g., production of UV-protective pigments, replacement of degraded chlorophyll). In contrast, our longer-term study showed that under field conditions of high air temperatures and frequent, small precipitation events, crust organisms appear unable to produce protective pigments in response to radiation stress, as they likely dried more quickly than when they received larger, less frequent events. Reduced activity time likely resulted in less carbon available to produce or repair chlorophyllaand/or protective pigments. Our findings may partially explain the global observation that soil lichen cover and richness declines as the frequency of summer rainfall increases. [ABSTRACT FROM AUTHOR]