Effects of seedbed structure and water content at sowing on the development of soil surface crusting under rainfall

This study was carried out to observe the dynamics of crust formation on the soil surface under field conditions and analyse the effects of seedbed structure and water content on soil surface crusting. Seedbed sensitivity to crusting was also estimated in the laboratory by stability tests on aggregates. We observed 57 plots during the sowings of spring and autumn crops in fields in Northern France (Estrees-Mons, 50°N latitude, 3°E longitude). The soil is an Orthic Luvisol according to the FAO classification (0.17–0.25 g g⁻¹ clay and 0.02 g g⁻¹ organic matter on average). Visual assessments in situ were performed and photographs taken of crust stages on delimited areas, each 5 mm of cumulated rainfall since sowing. In 2004–2005, the seedbeds were characterised by their distribution of aggregate sizes and tests of aggregate stabilities of surface samples kept with their water content at sowing. A penetrometer was used to measure crust resistance and estimate its thickness. These data were analysed to detect the cumulative rainfall values needed for the initiation and development of the successive stages of crusts. A fully developed structural crust (stage F1) required 13, 22, 27 mm cumulated rainfall respectively for seedbeds with proportions of clods over 2 cm ranging from 0 to 0.15 (fine seedbed), 0.15 to 0.30 (medium seedbed), >0.30 g g⁻¹ (coarse seedbed). Aggregate stability measured on samples kept at sowing water content was low for soil with low water content (<0.17 g g⁻¹) but increased sharply for water contents over 0.17 g g⁻¹. Stage F1 was reached more rapidly (only 11 mm versus 19 mm cumulated rainfall) only for fine seedbeds with less than 0.15 g g⁻¹ of clods over 2 cm and with a low water content at sowing, The stage of 50% of soil surface covered with sedimentary crusts was reached for 85 mm for fine seedbed versus 120 mm for medium seedbed. The mean penetrometer resistance of dry crusts was 0.55 ± 0.43 MPa for stage F1 and 3.54 ± 0.83 MPa for a sedimentary stage; mean penetrometer resistance increased continuously with cumulated rainfall and was much lower for wet crusts. These quantitative data gathered under field conditions constitute the first step towards the prediction of soil surface crusting. The cumulative rainfalls were used in order to estimate the risk of occurrence of structural and sedimentary crusts forming during crop emergence with several types of seedbeds.     

Effects of tillage and management practices on soil crust morphology under a Mediterranean environment

Soil crust formation can be affected by soil tillage. Alternative soil conservation practices consisting of reduced tillage were tested against traditional tillage, which involves mechanical weeding by frequent ploughing in rainfed vineyard soils in Catalonia, Spain. After 2 years of the experiment (1994–1996), thin sections of the surface crusts were studied to evaluate the effects of the soil management treatments on crust morphology and genesis, using micromorphological observations and pore characterisation with image analysis. Reduced tillage caused thicker and more complex crusts consisting of layers with different degrees of sorting and pore types, compared to traditional tillage. Total porosity of crusts did not differ from that of non-crusted areas, but pores in crusts were less interconnected, more horizontally distributed and more elongated than in the underlying non-crusted material. The soil type, especially structure and texture, affected crust morphology and played an important role in the process of crusting. The results show that reduced tillage may be limited as an alternative management practice when used to reduce crust formation in Mediterranean conditions, due to the difficulty to establish an effective groundcover.     

Physical characterization,spectral response and remotely sensed mapping of Mediterranean soil surface crusts

Soil surface crusting and sealing are frequent but unfavorable processes in Mediterranean areas. Soil crust and seals form on bare soil subject to high-intensity rainfall, resulting in a hard, impenetrable layer that impedes infiltration and hampers the germination and establishment of plants. The adverse consequences of overland flow and reduced fertility can lead to erosion and ongoing degradation. Therefore, information on the distribution of surface crusts and their physical properties is essential to combat the undesired effects of crust formation in e.g. soil erosion and soil compaction. We studied the occurrence of crusts in a study area in Mediterranean southern France. Our objectives were to compare the physical and hydrological properties of the crusts and underlying soil, to identify the spectral characteristics (400 to 2500 nm) of the crusted and non-crusted soil surfaces using high-resolution field spectra, and to investigate the potential of mapping crust occurrence using airborne, hyperspectral HyMap images. The differences in some physical properties between crusted and non-crusted surfaces are significant while others are only marginal. Crusting markedly reduces the infiltration capacity and crust strength varies between the different soil types. Spectral differences are small, mainly in albedo values (overall reflectance) and in absorption band depth and shape. Albedo differences range from 8 to 40%. Differences in absorption band features in the spectra of crusts and non-crusted surfaces are small. Sixty percent of the crusted soil surfaces showed stronger absorption features in the clay mineral absorption bands at 2200 nm than non-crusted soils. Increased absorption is due to a relative enrichment in fines. Spectral feature fitting and linear spectral unmixing algorithms were applied to airborne HyMap images to evaluate the possibilities of mapping surface crusts. Crusts could be mapped in fallow, agricultural fields, but the spectral response of natural badlands was too fragmented for crust mapping.     

中国三种典型土壤结皮的发育过程与机理

To compare the development of physical crusts in three typical cultivated soils of China, a black soil (Luvic Phaeozem), a loess soil (Haplic Luvisol), and a purple soil (Calcaric Regosol) were packed in splash plates with covered and uncovered treatments, and exposed to simulated rainfall. Meshes covered above the surfaces of half of soil samples to simulate the effects of crop residue on crusting. The results indicated a progressive breakdown of aggregates on the soil surface as rainfall continued. The bulk density and shear strength on the surface of the three soil types increased logarithmically as rainfall duration increased. During the first 30 min of simulated rainfall, the purple soil developed a 7--8 mm thick crust and the loess soil developed a 3--4 mm thick crust. The black soil developed a distinguishable, but still unstable, crust after 80 min of simulated rainfall. Soil organic matter (SOM) content, the mean weight diameter (MWD) of soil aggregates, and soil clay content were negatively correlated with the rate of crust formation, whereas the percentage of aggregate dispersion (PAD), the exchangeable sodium percentage (ESP), and the silt and sand contents were positively correlated with crusting. Mechanical breakdown caused by raindrop impact was the primary mechanism of crust formation in the black soil with more stable aggregates (MWD 25.0 mm, PAD 3.1%) and higher SOM content (42.6 g kg^-1). Slaking and mechanical eluviation were the primary mechanisms of crust formation in the purple soil with low clay content (103 g kg^-1), cation exchange capacity (CEC, 228 mmol kg^-1), ESP (0.60%), and SOM (17.2 g kg^-1). Mechanical breakdown and slaking were the most important in the loess soil with low CEC (80.6 mmol kg^-1), ESP (1.29%), SOM (9.82 g kg^-1), and high PAD (71.7%) and MWD (4.6 mm). Simulated residue cover reduced crust formation in black and loess soils, but increased crust formation in purple soil.     

Determination of soil surface strength with a needle-type penetrometer

Soil crusting was characterized by means of direct measurement of penetration resistance with a needle type penetrometer at intervals of 0.1 mm over a depth of 15 mm in intact soil samples, treated with soil stabilizers or not, as a function of soil water content and bulk density. Relationships were established between the penetration resistance of the needle and standard cones of 60° angle and 1 cm² or 26.4 mm² base area. The effect of water content was stronger in dense than in loose soil.The effect of soil surface strength on the emergence of salsify (Scorzonera hispanica) was monitored in a field experiment on a loamy sand of which the natural crust was stabilized with soil conditioners. The penetration resistance was affected by the nature of the soil stabilizers as some created more or less hydrophobic and spongy crusts. Seedling emergence was negatively correlated with the penetration resistance and positively with the water content of the crust.     

The implications of spatial variability in surface seal hydraulic resistance for infiltration in a mound and depression microtopography

Soil surface crusting has a major impact on water infiltration and erosion in many soils. Considerable progress has been made in describing crusting processes and in modelling the impact of crusting on infiltration. Most studies, however, have neglected the high spatial variability in crust characteristics observed in the field. The objective of this experiment was to determine the influence of runoff depth on infiltration rate in the presence of a surface seal varying in hydraulic characteristics with microtopography. The Blosseville silt loam has a low aggregate stability and forms crusts readily. The Villamblain silty clay loam has a greater aggregate stability due to its greater clay and organic matter contents, and it is more resistant to aggregate breakdown processes under rainfall. Samples of the soils were sieved to retain aggregates less than 2.0 cm and packed in 50×50×15 cm soil trays. The trays were surrounded by a 10 cm soil border to compensate for splash loss. After molding the surface into a mound and depression microtopography, the samples were subjected to simulated rainfall at an intensity of 22.8 mm h⁻¹. Hourly measurements of surface roughness showed that the original roughness was smoothed out due to the infilling of depressions by sediments detached from the mounds. For the final hour, runon was added to the top of the soil tray to increase the runoff rate and depth. For both soils, infiltration rate increased more than could be attributed to the increased ponding pressure head. The change in infiltration rate was particularly great for Villamblain. The measurements of hydraulic resistance showed that structural crusts had a lower hydraulic resistance than sedimentary crusts. They also showed that the crusts formed on Villamblain were of a lower hydraulic resistance than those of Blosseville. It appears that small changes in runoff depth can significantly increase infiltration rate when structural crusts of lower hydraulic resistance are inundated. The effect was less important in Blosseville which formed seals of relatively high hydraulic resistance everywhere. The results provide a suitable explanation for field observations of increasing infiltration rate with either increasing rainfall intensity or runoff rate. The results also have implications for the relationships between surface roughness, surface water storage, and infiltration.     

Soil crust and its impact on crop establishment: A review

Soil crusting is a worldwide problem occurring under a wide range of soil and climatic conditions. Soil crusts affect seedling emergence and reduce the infiltration rate causing loss of water and crop yield. Considerable research has been done in order to understand the process of crust formation and the factors affecting it. Soil crust strength and impedance to seedling emergence have been studied in detail and measures to avoid crusting and methods to ameliorate its adverse effects have been suggested. The findings of such studies are summarized in this paper.     

紫色土表土结皮发育特征的试验研究

通过模拟降雨试验,探讨了紫色土表土结皮的发育特征。结果表明：有、无雨滴打击下,紫色土在30min内均形成稳定结皮层,厚度约7～8mm;淋移作用是紫色土形成结皮的主导作用,降雨打击使表土在降雨前期迅速形成致密薄层,抑制了淋移作用并使盖网处理的结皮容重略高;孔隙的剖面分布对入渗、抗剪强度有较大影响,表现在发生层总孔隙度低但分布均匀的盖网处理具有更强的入渗能力,且由于降雨初期未发生致密上层使其抗剪强度略低。     

Mechanical and hydraulic resistance relations in crust-topped soils

The formation of soil surface crusts leads to increased mechanical and hydraulic resistances. In this study, changes and relationships of both resistances under simulated sprinkle irrigation (or rainfall), and sprinkle followed by flooding, were examined. Results indicated that a silt-loam soil developed a thicker surface crust than a clay soil for any given kinetic energy (KE). Crusts as thick as 3.9 and 2.6 mm formed on the silt-loam and clay soils, respectively. Mechanical resistance, R_m, increased with increasing KE, where the effect was greater in the silt-loam and was attributed to intrinsic resistance and crust thickness. Steady-state infiltration rate (i) was much lower in crusted clay than crusted silt-loam soil. Changes of both R_m, and i closely followed changes in crust thickness (z_c). Thicker crusts showed more resistance against external force than thinner crusts, due to more extended particle interlocking. Obtained functions indicated that the effect of thickness on strength was more significant in the lower range of crust thickness. The effect of z_c on i strongly followed a negative power function for both soils, with higher i in the silt-loam soil.     

An experimental study of crust development on chalk downland soils and their impact on runoff and erosion

The sealing of soil surfaces by rainfall, the development of soil crusts and their impact on runoff and erosion was investigated in the laboratory by means of simulated rainfall. The soils investigated were stone-free samples of chalk soils from southeast England, and soils with a 25% cover of stones. Vertical change to the surface and immediate subsurface of the soils was assessed through the examination of thin sections scanned into a computer and analysed with image processing software. Changes in roughness and microtopography of the soil surface were measured by use of a laser micromapper. Crusting occurred both in the presence and absence of stones and was inversely related to the organic matter content and aggregate stability of the soils. Crusting of stone-free soils was accompanied by a reduction in roughness of the soil surface, but roughness of the stone-covered surfaces increased as crusting developed. Increases in the particle density of the crust were related to silt content. Organic-rich soil from under permanent grass and from a soil recently brought into arable cultivation crusted less than soils used for arable cultivation for longer periods with lower organic content. The inwashing of silt into the pores of the soil during crusting reduces infiltration, and increases runoff and erosion. In the presence of a 25% cover of surface stones, the reduction in infiltration was 25% less than for stone-free soils and erosion ∼50% less.     

Variations in the disintegration rate of physical crusts induced by artificial rainfall in different alcohol concentrations

Disintegration is closely correlated with geological disasters and soil erosion. However, quantitative studies on the disintegration processes of physical crust controlling the soil surface erosion are limited. Therefore, we disintegration process in structural and sedimentary crusts induced by artificial rainfall on a typical cropland soil from the Loess Plateau, China. The physical crusts were immersed for 200 s at different alcohol concentrations applied for delaying disintegration process to obtain disintegration rate (DR). The content of organic matter and the sand percentage in the structural and sedimentary crusts decreased with increasing rainfall duration, while the bulk density, silt and clay percentages increased. The initial DR values ranged from ?0.01 to 1.82 in structural crusts and from ?0.01 to 1.47 in sedimentary crusts under different alcohol concentrations. DR decreased by [86.5%, 91.3%] in structural crusts and by [86.3%, 88.2%] in sedimentary crusts during the whole disintegration period. For both structural and sedimentary crust, the DR was the lowest when the rainfall lasted for 30 min, and finally stabilized at 0.19 and 0.18, respectively, at the disintegration time of 80 s. Notably, the 50% alcohol concentration slowed the disintegration process most efficiently. The structural crust had a lower erosion resistance than the sedimentary crust due to the lower DR. These results provide a theoretical method for evaluating disintegration process and timely information revealing the erosion resistance mechanism of physical crusts.     

藻菌混合结皮对土壤水肥保持及玉米幼苗生长的影响

[目的] 研究藻菌混合结皮对土壤水肥保持及玉米幼苗生长的影响,为土壤治理与改良策略提供理论依据。[方法] 使用来自内蒙古翁牛特沙漠的2种产胞外多糖的细菌（Sphingomonas sp.D3-1和Massilia armeniaca sp.ZMN-3）和2种藻类（Cladophora aegagrophila D3-a和Nostoc sphaeroides0 D3-25）制备成藻菌混合剂后,喷洒至土壤表面,形成土壤结皮。研究藻菌混合结皮与土壤水肥保持、微生物数量及酶活性的关系,及其对玉米幼苗的影响。[结果] 与对照相比,藻菌混合结皮明显延缓了土壤水分的流失速度,并使氮、磷和钾的平均淋失效率分别减少了76.9%,64.4%和47.8%,土壤中的细菌、真菌和放线菌数量分别增加了8.3,1.8和3.1倍,芽孢杆菌的数量下降了12.4%。另外,在玉米盆栽中,藻菌混合结皮使玉米幼苗的地上部鲜/干重、根部鲜/干重和地上/下部长度分别提升30.0%/55.6%,55.4%/38.5%和16.5%/18.0%,叶绿素含量增加15.0%。[结论] 藻类与产胞外多糖的细菌共同形成的藻菌混合结皮,明显改善了土壤的水肥保持效果,可作为治理土壤的新途径加以推广。     

瓦碱的碱度与板结

“瓦碱”是一种碱化的浅色草甸土,零星分布于我国黄淮海平原的盐碱土地区。瓦碱的土表板平,呈灰白色,没有盐霜或很少盐霜。早春干旱时,土表易于板结,抑制幼苗生长,常引起缺苗现象,土壤板结严重时,甚至全部死亡,形成光板地。瓦碱上难以出苗,卽使出苗,苗也弱,分蘖少,但是出苗以后植株的生长此在附近盐土上好得多,因此羣众说:“瓦碱发老苗,不发小苗”。黄淮海平原内瓦碱的情况已有报导^[1-6],但少有详细的理化研究。本文拟就瓦碱苗弱和不出苗的原因及其改良原则进行初步研究。     

Biological soil crust development affects physicochemical characteristics of soil surface in semiarid ecosystems

Water and nutrients are scarce resources in arid and semiarid ecosystems. In these regions, biological soil crusts (BSCs) occupy a large part of the soil surface in the open spaces surrounding patches of vegetation. BSCs affect physicochemical soil properties, such as aggregate stability, water retention, organic carbon (OC) and nitrogen (N) content, associated with primary ecosystem processes like water availability and soil fertility. However, the way BSCs modify soil surface and subsurface properties greatly depends on the type of BSC. We hypothesised that physicochemical properties of soil crusts and of their underlying soils would improve with crust development stage. Physicochemical properties of various types of soil crusts (physical crusts and several BSC development stages) and of the underlying soil (soil layers 0–1 cm and 1–5 cm underneath the crusts) in two semiarid areas in SE Spain were analysed. The properties that differed significantly depending on crust development stage were aggregate stability, water content (WC) (at −33 kPa and −1500 kPa), OC and N content. Aggregate stability was higher under well-developed BSCs (cyanobacterial, lichen and moss crusts) than under physical crusts or incipient BSCs. WC, OC and N content significantly increased in the crust and its underlying soil with crust development, especially in the first centimetre of soil underneath the crust. Our results highlight the significant role of BSCs in water availability, soil stability and soil fertility in semiarid areas.     

模拟降雨条件下土壤结皮的发展和聚丙烯酰胺对黄土上结皮形成的影响

This study evaluated the morphological characteristics and dynamic variation in characteristics of soil crust and iden-tified the relationships between soil crust and splash erosion under simulated rainfall.The effect of polyacrylamide (PAM) on soil aggregate stabilization and crust formation was also investigated.A laboratory rainfall simulation experiment was carried out using soil sample slices.The slices were examined under a polarized light microscopy and a scanning electron microscope (SEM).The results revealed that the soil crusts were thin and were characterized by a greater density,higher shear strength,finer porosity,and lower saturated hydraulic conductivity than the underlying soil.Two types of crusts,i.e.,structural and depositional crusts,were observed.Soil texture was determined to be the most important soil variable influ-encing surface crust formation;depositional crust formation was primarily related to the skeleton characteristics of the soil and happened when the soil contained a high level of medium and large aggregates.The crust formation processes observed were as follows:1) The fine particles on the soil surface became spattered,leached,and then rough in response to raindrop impact and 2) the fine particles were washed into the subsoil pores while a compact dense layer concurrently formed at soil surface due to the continual compaction by the raindrops.Therefore,the factors that influenced structural crust formation were a large amount of fine particles in the soil surface,continual impact of raindrops,dispersion of aggregates into fine particles,and the formation of a compact dense layer concurrently at the soil surface.It was concluded that the most important factor in the formation of soil crusts was raindrop impact.When polyacrylamide (PAM) was applied,it restored the soil structure and greatly increased soil aggregate stabilization.This effectively prevented crust formation.However,this function of PAM was not continuously effective and the crust reformed with long-term rainfall.In conclusion,this study showed that soil micromorphological studies were a useful method for evaluating soil crust formation.     

Effect of surface crusting on emergence of soybean (Glycine max L. Merr) seedlings II. Influence of tillage treatment,planting method and time of crust formation

The influence of tillage treatment and the time of crust formation after planting on the degree of surface crusting and seedling emergence of soybean (Glycine max L. Merr.), planted on the flat and on ridges, are described. Crusts were developed artificially at specific times after planting. Under non-crusted conditions (control), more rapid drying of the soil in the seed zone occurred in conventionally-tilled, ridge-planted (Tcr) and minimum-tilled, flat-planted (Tmf) seedbeds compared with excessively and conventionally-tilled, flat-planted (Tef and Tcf) seedbeds. The formation of surface crusts induced a reduction in moisture loss from the seed zone. On drying, large crust blocks with a smooth surface developed in Tef plots whereas, in Tmf plots, small crust blocks with a rough surface appeared. Both the rate of emergence and the ultimate emergence of soybeans were in the order Tef > Tcf > Tcr > Tmf under non-crusted conditions and, on average, in the order Tcr > Tcf > Tef > Tmf under crusted conditions. Crusts which formed within two days of sowing were more injurious because these were hard and impenetrable, and were present at the normal time of seedling emergence. Crusts induced 72 h after planting had less effect on seedling emergence because, by the time they could form, emergence was already almost completed.     

Laboratory methods for the estimation of infiltration rate of soil crusts in the Tabernas Desert badlands

Soil crusting is a crucial factor for runoff generation in the Tabernas Desert badlands; however, very few methods have been developed for the measurement of infiltration in crusts, which are often distributed on steep slopes where experimental devices are very difficult to install, making measurement difficult. We have used the trickle irrigation (TI) method and mini-disk infiltrometers (MDI) in the laboratory to measure steady infiltration rate under nearly saturated condition and tensions, respectively, in soil crust samples removed from the field. Steady infiltration rate under tensions were performed at three water pressure heads (h=−0.5, −2.0, and −6.0 cm).Steady infiltration rate of soil crusts in the Tabernas Desert badlands ranged from 21.3 to 30.7 mm h⁻¹ measured by TI method. Steady infiltration rate under tensions decreased with increasing tensions; it was 9.9–18.4 mm h⁻¹, 0.4–5.7 mm h⁻¹, and 0.2–3.3 mm h⁻¹ at −0.5-, −2.0-, and −6.0-cm pressure heads, respectively. Steady infiltration rate measured by TI method and that at a tension of 0.5 cm by the MDI is consistent with the results from the previous simulated rainfall studies in the same soil crust surfaces, suggesting that the TI and MDI methods may be potentially a useful lab measurement for approximating field infiltration rate.     

Controlling effects of surface crusts on water infiltration in an arid desert area of Northwest China

Purpose

Surface crusts are important features in arid desert areas and are critical to hydrological processes and ecosystem development. This paper aims to understand the effects of crusts on water movement in the soil and the factors that affect this and to provide the soil parameters for estimation of saturated hydraulic conductivity (K _s) in ecohydrological models.

Materials and methods

The study area was located in the middle and lower reaches of the Heihe River Basin, an arid desert area in Northwest China. There were three crust types in this region: physical soil crusts (PSCs, formed by water drop and erosion), biological soil crusts (BSCs, formed by microorganisms, moss, algae, lichen, and soil materials), and salt soil crusts (SSCs, formed by soluble salts). The infiltration rates of different soil and crust types and scalped soils were determined in situ in the field conditions using a disc infiltrometer with three repetitions. Crusts and soils were collected, and their properties were determined in the laboratory.

Results and discussion

The K _s of crust were significantly lower than that of scalped soils with a decrease of 13–70 %. The K _s of crusts were related to the type of crust and the properties of soil beneath the crusts. In this region, the soil textures are similar throughout, due to ubiquitous loess sedimentation, so textural differences had no significant effect on K _s. Soil organic matter (SOM) played a weak negative role on K _s because most crusts had higher SOM than the underlying soil. However, both crust thickness and electrical conductivity (EC, an index of salt concentration) showed significantly negative exponential relationship with K _s. Therefore, the SSC with high EC and thick crust have the lowest K _s among all crust types. Because soil development is related to salt accumulation, structure, and crust formation, the K _s follows the order of Solonchaks < Cambisols < Regosols, from lowest to highest.

Conclusions

Crusts have different characteristics compared with original soils and are the limiting layer of water infiltration in these arid soils. Therefore, the characteristics of crust must be considered in ecohydrological models. The main apparent controlling parameters of water infiltration rate in this area are crust thickness and EC.

     

Soil crusting and infiltration on steep slopes in northern Thailand

Predicting the rate at which rain infiltrates on steep slopes is very uncertain. There is no consistent information in the literature. We have therefore related infiltrability to slope gradient under field conditions by experimenting on a gravelly loamy soil occupying the upper half of a cultivated convex hill in northern Thailand. Fifteen 1 m × 1 m plots with slope gradients ranging from 16 to 63% were established, and simulated rain was allowed to fall on them at controlled rates and for fixed times. We obtained the following results. The surface fell 0.4–7.2 mm due to compaction and soil loss. The proportions of crust (0–40%) and embedded gravel (10–60%), the runoff coefficient (0.05–0.78 mm mm^?1), the mean sediment concentrations (0–5.6 g l^?1), and soil detachment (10–313 g m^?2) were more pronounced on the gentle slopes than on the steep ones. The steady final infiltration rate (1–107 mm hour^?1) increased sharply with increasing slope gradient. Microaggregates tended to behave like sand and become tightly packed on gentle slopes (packing crust). These results suggest that the vertical component of kinetic energy, which is greater on gentle slopes, has a dominant role. Nevertheless, the differences in compaction and in sediment concentration could not be ascribed to the vertical component of kinetic energy alone. On steep slopes the horizontal component of the kinetic energy is transformed into shear stress, hampering the development of crusts so that water can still infiltrate. On steeper slopes, the water film was thinner, thereby limiting the role of splash. We conclude that the relationship between slope gradient and infiltrability depends on the nature of the soil and must be examined in the light of surface crusting processes.     

Geologic composition influences distribution of microbiotic crusts in the Mojave and Colorado Deserts at the regional scale

Abiotic and biotic factors influencing distribution of microbiotic crusts within hot deserts, such as the Mojave and Colorado Deserts, are poorly known. Our objective was to examine microbiotic crust distribution with reference to soil and parent material characteristics as well as plant functional groups in wilderness areas of Joshua Tree National Park (JTNP). A total of 75 sites were visually assessed for crust abundance and plant community composition; soil physical and chemical factors also were measured. Microbiotic crusts of JTNP, in particular lichen and moss crusts, were not as well-developed or as widely distributed as in other arid regions of North America. Algal crusts were most prevalent, lichen crusts were sparse, and crusts containing mosses were rare, with average percent land surface absolute (and relative) cover for these three cover categories being 11.4% (17.4%), 1.7% (2.7%), and 0.02% (0.02%), respectively. Previously reported individual drivers of crust development, such as pH, electrical conductivity (EC), and soil texture, did not appear to strongly influence crust development in this study of the Mojave and Colorado Deserts. Proximity to granitic bedrock and grusy granitic soils associated with it were the key determinants of microbiotic crust distribution in the wilderness areas of JTNP. In particular, crusts were best developed in grusy granitic soils. Overall, our study emphasized the importance of geology in driving crust distribution and its potential value as a predictor of where crusts may occur in the hot deserts of North America.