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Soil hydrology is independent ofmicrophytic crust cover: Furtherevidence from a wooded semiaridaustralian rangelandD. J. Eldridge a , M. E. Tozer b & S. Slangen ba Department of Land and Water Conservation The GraduateSchool of the Environment, Macquarie University, New SouthWales, 2109, Australiab Department of Land and Water Conservation The GraduateSchool of the Environment, Macquarie University, New SouthWales, Australia

Available online: 09 Jan 2009

To cite this article: D. J. Eldridge, M. E. Tozer & S. Slangen (1997): Soil hydrology isindependent of microphytic crust cover: Further evidence from a wooded semiarid australianrangeland, Arid Soil Research and Rehabilitation, 11:2, 113-126

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Soil Hydrology Is Independent of MicrophyticCrust Cover: Further Evidence from a Wooded

Semiarid Australian Rangeland

D. J. ELDRIDGEM. E. TOZERS. SLANGEN

Department of Land and Water ConservationThe Graduate School of the EnvironmentMacquarie UniversityNew South Wales, Australia

Rainfall simulation experiments were performed on 25 plots of varying microphyticcrust cover in a wooded semiarid rangeland in eastern Australia. Under a rainfallintensity of 45 mm h-1, steady-state infiltration ranged from 5 mm h-1 to 41 mm h-1,but there was no effect of cover on this or any of the other soil hydrological variablesmeasured. When disturbed plots with low cover (<15% cover) were excluded from theanalyses, significant increases in time to ponding were associated with increases incrust cover. Despite some significant relationships, however, crust cover was aninsignificant predictor of soil hydrological status at this site. We attribute this to thewell-structured nature of the soils at the site, which have not been subjected to grazingby domestic animals for almost 20 years. The results support earlier work suggestingthat in the short term, crust cover is only an important moderator of soil hydrologywhen soils are degraded.

Keywords cyanobacteria, infiltration, lichen, microphytic crusts, moss, semiaridrangelands

Microphytic or cryptogamic soil crusts are important and common components of theground flora in arid and semiarid southern Australia (Eldridge, 1993; \996b; Eldridge &Greene, 1994; Rogers & Lange, 1971). Microphytic crusts form through intimate rela-tionships between cyanobacteria, lichens, fungi, liverworts, mosses, and bacteria, and thetop few millimeters of the soil (Chartres, 1992).

Microphytic crust cover varies at a range of scales. On a broad regional or landscapescale, crust cover varies according to such environmental factors as rainfall amount anddistribution (Rogers, 1971, 1972) and such soil factors as calcium carbonate content(Downing, 1992; Downing & Selkirk, 1993; Rogers, 1972). In semiarid and arid NewSouth Wales, crust cover and diversity of crust organisms is greatest on sandplains

Received 23 September 1996; accepted 14 November 1996.Bruce Reardon, Ceciel Overgoor, and Ingrid Loggers assisted with field work and some data

analysis. We thank the New South Wales National Parks and Wildlife Service for permission toperform these experiments and for providing logistical support. The work was supported by theLand and Water Resources Research and Development Corporation and the New South WalesDepartment of Land and Water Conservation.

Address correspondence to Dr. D. J. Eldridge, Graduate School of the Environment, MacquarieUniversity, N.S.W., 2109, Australia.

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Arid Soil Research and Rehabilitation, 11:113-126, 1997Copyright © 1997 Taylor & Francis

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114 D. J. Eldridge et al.

dominated by calcareous earths and red earths (Eldridge & Tozer, 1996). At a finer scale,the distribution of crust cover is correlated with vascular plant cover, geomorphic location,soil physical and chemical properties, and local microtopographic location. Extensivelevels of crust cover are often associated with areas of sparse vascular plant cover or withsurfaces regarded as being in good condition (Greene & Tongway, 1989). An extensivecover of microphytic crust on landscapes dominated by calcareous earths and red earthsis generally regarded as an indicator of good condition (Tongway, 1994).

Although there is a large body of evidence demonstrating the importance of soil crustsin ecological functioning in arid and semiarid regions (e.g., Eldridge & Greene, 1994;Harper & Marble, 1988; Scarlett, 1994; West, 1990; Williams et al , 1995; Yair, 1990),their role in hydrological processes is poorly understood and often contradictory (Eldridge& Greene, 1994). For example, in degraded soils where macropores are scarce and themajority of flow is through matrix pores, cyanobacteria and the hyphae of free-living fungicould reduce infiltration by occupying matrix pores (Eldridge, 1993). Alternatively,mosses in soil crusts might absorb large amounts of rainfall, channeling this into the soilvia their rhizines and protonema, thereby increasing infiltration.

Knowledge of the intimate relationships among soil crusts, soil surfaces, and vascularplants leads to various hypotheses about how crusts influence soil and ecological pro-cesses. Eldridge (1993) proposed that the importance of crusts in moderating water flowthrough soils increased as the soil surface became more degraded. This concept wasextended to explain how vascular plant cover influences soil hydrology (Eldridge, 1996a).

In August 1994 we undertook a series of infiltration measurements at Mungo NationalPark in eastern Australia. The site chosen has been ungrazed for almost 20 years and isregarded as being in good rangeland condition. The aim of the study was to investigate theeffect of varying cover of microphytic crusts on sorptivity, infiltration, times to ponding,time to runoff, and depth to penetration of infiltrating water.

Vegetation and Soils

The study was carried out in Mungo National Park, 80 km northeast of Buronga, NewSouth Wales, Australia (33°45' S, 142°59' E). The site was chosen because it had beenungrazed by domestic livestock since 1977, and therefore supported a relatively undis-turbed and extensive soil crust community (Downing & Selkirk, 1993; Eldridge & Kin-nell, 1997). In addition, the site provided an ideal opportunity to examine changes in soilhydrology with changes in crust cover at a site regarded as being in good rangelandcondition. Condition was assessed using a combination of soil surface features outlined byTongway (1994), as well as such vegetation characteristics as cover, composition, and agestructure of perennial plants, and the degree of recruitment. Good range-condition status,therefore, signified that the site had an extensive cover and diversity of perennial plants,shrubs, and trees, with obvious evidence of recruitment and little or no erosion.

Median annual rainfall for the site is 264 mm (Bureau of Meteorology, 1961) and ispredominantly winter-dominant, with approximately 25% more rain falling during Marchto August compared with the period September to February. The general area in which thestudy was carried out consists of Pleistocene sediments overlain by calcareous Quaternarysands and clays, occurring as areas of aligned west-east trending dunes with low relief (to4 m). The soils are classified as Hypercalcic Calcarosols (Isbell, 1996) or Calcic Aridisolsin the U.S. Soil Taxonomy System (Soil Survey Staff, 1975). The soils were characterizedby massive, yellowish-red earthy profiles, ranging from loams with fine sand in thesurface horizons to sandy clay-loams in the B horizon. The soils were typified by profiles

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Soil Hydrology and Microphytic Crusts 115

with more than 20% of soft, finely divided carbonate in the earthy fraction, and less than20% of hard calcrete nodules. The calcareous loams of this general area support a highdiversity of cryptogamic organisms (Downing & Selkirk, 1993; Rogers & Lange, 1972),which dominate the soil surface even during droughts.

Vegetation at the site is classified as the belah-rosewood (Casuarina cristata Miq.-Alectryon oleifolius Desf.) association (Cunningham et al., 1981), and is dominated by thetrees belah, rosewood, sugarwood (Myoporum platycarpum R. Br.), and wilga (Geijeraparviflora Lindl.). The sparse understorey and ground cover was dominated by perennialchenopod shrubs Maireana pyramidata (Benth.) P.G. Wilson, Rhagodia spinescens R.Br., Enchylaena tomentosa R. Br., and perennial grasses (Stipa spp. and Aristida spp.).

Methods

Plot Measurements

During August 1994, 25 plots measuring 0.8 m x 0.8 m were selected from a level (<1%slope) sandplain over an area of approximately 40 m x 40 m. Plots were selected toencompass a range of microphytic crust cover (Table 1) ranging from low (<10% cover)to extensive (>80% cover). Before rainfall simulations, soil microtopography and surfacecover were measured on each plot with a profilemeter (Semple & Leys, 1987). This deviceprovides a series of points using a row of 16 vertical steel rods spaced at 50-mm intervals.Ten rows were used, spaced 90 mm apart, giving a total of 160 points. The profilemetermeasured microtopography across the slope, after the ends of each of the pins came intodirect contact with the soil surface or a particular cover component such as microphyticcrust or vascular plant. The vertical position of each pin was also measured so that anassessment of soil surface roughness (microtopography) could be made to the nearest1 mm. Microtopography was expressed as the standard deviation of the heights of the 160pins. Foliage cover was measured at the same time and location as microtopography, andrecorded in the following categories: bare soil, litter, microphytic crusts, and vascularplants. Soil crust lichens and mosses found at the site are listed in Table 2.

Physical and Chemical Soil Analyses

Laboratory analyses were carried out on two replicate samples each from two depths(0-25 mm and 25-50 mm) collected adjacent to each of the 25 plots. The followingmethods were employed:

1. Gravimetric soil moisture content.2. Bulk density using intact cores 50 mm diameter by 25 mm depth.3. Aggregate stability: 2- to 6-mm size fraction using the modified Kemper and

Rosenau (1986) method (after Greene, 1992). This modified wet-sieving techniqueinvolved the rapid immersion of the 2- to 6-mm fraction of the soil in waterfollowed by wet-sieving for 10 min (at 30 oscillations min"1). Four sieve sizes (2.0mm, 1.0 mm, 0.5 mm, and 0.25 mm) were used, and the results are expressed asa mean weight diameter (MWD) value. The mean weight diameter is the sum ofthe percentage of the soil remaining on each sieve multiplied by the mean diameterof adjacent sieves.

4. Particle size analysis of the <2.0-mm fraction according to Loveday (1974) usingdispersed samples.

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Table 1Soil surface characteristics and soil hydrological properties for the simulations carried out at Mungo National Park

ON

Plotno.

5191

12142

2325221873

20164

13698

171015241121

Surface

Crust

1.34.48.8

10.011.913.023.126.331.331.333.033.135.637.541.045.046.348.852.554.963.866.366.376.383.8

cover (%)

Plant

1.81.20.70.01.21.30.00.00.50.05.07.01.90.01.00.03.11.10.60.01.80.01.10.00.6

Surfaceroughness

0.4060.4990.5170.7100.5170.5260.3700.4310.4780.4390.4110.6950.3990.4550.3510.5810.4110.9580.3840.5680.4420.9580.3080.2660.698

Time toponding(min)

4.305.603.782.333.223.582.152.532.572.871.832.75 .2.603.333.003.924.002.952.423.633.982.603.983.404.00

Time torunoff(min)

5.636.27

10.705.437.004.355.033.174.704.234.426.134.974.678.725.005.755.786.007.175.305.104.335.025.00

Depth ofwetting

front(mm/25 mm

rainfall)

21.3113.4410.7815.5216.3322.974.095.385.29

12.1414.3318.407.59

16.9019.3418.0613.0613.8511.5513.8911.3012.509.31

13.8413.94

Soil hydrological properties

Sorptivity(mm sec"05)

1.305.56

25.4520.8624.33

1.9435.4928.2332.7320.499.280.43

24.326.918.802.188.65

23.7128.535.63

13.1112.8710.012.376.76

Steadystate

infiltration(mm h"1)

40.95432.9436.929

14.07026.70336.6034.811

13.5098.858

25.91032.83434.50711.01528.41431.07836.47526.90918.05515.72131.56730.25526.11616.84038.31126.089

Cumulative infiltration dnm1

4 min

2.81022.76452.76182.79402.73042.87102.78052.73882.59742.77522.59742.73562.69682.60472.60472.67142.64642.80202.76192.86822.70962.81562.75662.76722.6539

6 min

4.19314.14684.14294.17984.09554.17334.09313.97503.82954.10723.84064.10343.97863.90703.90703.91833.96964.15874.14284.30223.97574.20124.04614.09533.9586

8 min

5.57605.49575.52375.55465.46075.57545.01714.97814.93955.31725.06165.44895.24935.15385.20935.20975.14855.37105.44605.69195.19735.57585.36895.45675.2412

at:

10 min

6.95896.85586.90476.90716.28206.96655.65265.80365.63886.33856.17166.78346.40906.41186.40076.51216.29416.36136.53847.05946.24136.89486.64736.81806.5015

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Soil Hydrology and Microphytic Crusts 117

Table 2Lichens and bryophytes found on each of the monolith surfaces

used in the rainfall simulation experiments

Lichens Catapyrenium lachinulatum (Ach.) BreussCatapyrenium pilosellum BreussCollema coccophorum Tuck.Endocarpon pusillum Hedwig.Endocarpon simplicatum (Nyl.) Nyl. var. bisporum

McCarthyEremastrella crystallifera (Taylor) G. SchneiderHeppia despreauxii (Mont.) Tuck.Lecidea ochroleuca Pers.Peltula patellata (Bagl.) Swinscow & Krog subsp.

australiensis (Muell. Arg.) BuedelPsora decipiens (Hedwig) Hoffm.Toninia sedifolia (Scop.) TimdalXanthoparmelia spp.

Bryophytes Aloina bifrons (De Not.) DelgadilloBryum pachytheca C. Muell.Crossidium davidai Catches.Didymodon torquatus (Tayl.) Catches.Goniometrium énerve Hook. & Wils.Riccia lamellosa RaddiRiccia limbata Bisch.

5. Organic carbon content using the Walkley-Black wet combustion technique (Col-well, 1969).

6. pH and electrical conductivity (EC): 1:5 soil-water suspension shaken for 1 h.

Rainfall Simulations and Runoff Collection

Artificial rainfall was applied to all plots, measuring 0.8 m x 0.8 m, at an intensity of 45mm h"1 using a Morin-type revolving disk rainfall simulator (Grierson & Oades, 1977).The simulator delivers rainfall from a height of 2.05 m, producing drop sizes of 2.5 mmdiameter using 52 kPa pressure. Rainfall was applied at a constant rate until steady-stateinfiltration was achieved, which in most cases was less than 45 min. Time to ponding wasrecorded for each plot and is defined as the time taken for free water to cover about 60%of the soil surface. This has been shown to be more reliable than the use of tensiometers(I. Packer, personal communication). Once time to runoff occurred, a vacuum pump wasactivated, and runoff samples were collected at 1-min intervals in a flume at the lower endof the plot. Infiltration was calculated as the difference between applied rainfall andrunoff, and assumes that surface detention was minimal. The mean depth to the wettingfront was measured at the cessation of simulation at 10 locations on each plot, and isexpressed as the depth to the wetting front per 25 mm of applied rainfall, as all plotsreceived different amounts of rainfall depending on the time taken to achieve steady-stateinfiltration. The relationship between crust cover and soil erodibility is reported elsewhere(Eldridge & Kinnell, 1997).

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118 D. J. Eldridge et al

Statistical Analyses

Simple linear regression (Minitab, 1994) was used to determine the contribution of thevarious independent variables to the variance in the dependent variables. The variables aredefined in Table 1. Variables were logi0 transformed where necessary to normalize thevariances.

Results

Crust Cover and Soil Hydrology

Across the full range of cover values, there was no significant effect of microphytic crustcover on time to ponding, time to runoff, depth of water infiltration, or final steady-stateinfiltration rate (p > 0.05). The non-significance of most relationships between cover andsoil hydrological properties can be attributed to the high variability in soil hydrologicalproperties, particularly at low levels of microphytic crust cover. For example, time takento pond water on essentially bare (i.e., <15% cover) surfaces ranged from 2.3 to 5.6 min(Table 1).

At this site there were few areas where the soil surface was completely devoid ofmicrophytic crust. Small patches of almost bare ground can be attributed to disturbance byrabbits, kangaroos, or uprooted trees, or represented the above-ground cappings of sub-terranean termite mounds (Noble et al., 1989). These soil surface condition states areknown to influence soil hydrology (Eldridge, 1993), which varies markedly over smallspatial scales. Disturbed sites could therefore be expected to exhibit a high spatial vari-ability in infiltration. Consequently, we re-analyzed the data to seek relationships betweencrust cover and soil hydrological properties on plots with more than 15% crust cover.Excluding these "bare" plots revealed significant increases in time to ponding only (Fl 17

= 10.85, p = 0.004, R2 = 0.354).

Soil and Vegetation Properties and Hydrology

Organic carbon levels were low, though significantly higher at the surface compared withat depth (Table 3). Organic carbon levels of less than 1% are not uncommon for Australiansoils with high sand contents (Stafford Smith & Morton, 1990). Bulk density and aggre-gate stability levels were significantly greater on the surface compared with at depth (p <0.05).

Significant relationships between soil hydrological properties and vegetation vari-ables are given in Table 4. Time to ponding increased significantly with increases in theorganic carbon content of the soil in both the 0- to 25-mm layer {R2 = 0.269, p = 0.005)and the 25- to 50-mm layer (R2 = 0.134, p = 0.004). Increases in time to runoff wereassociated with significant increases in aggregate stability at the 25- to 50-mm depth (R2

= 0.135, p = 0.04). Increases in steady-state infiltration and the depth to wetting front weresignificantly correlated with increases in the litter and the sand (coarse sand and fine sand)fraction of the soil, and decreases in the fine (clay and silt) fraction of the soil (Table 4).

Sorptivity varied markedly across the 25 plots, ranging from 0.43 to 35.49 mm sec"0 5

(mean ± s.e.m. = 14.37 ± 2.19 mm sec"0'5; Table 1). Although there were no significantassociations with crust cover (p > 0.05), increases in sorptivity were associated withincreases in the cover of bare soil (R2 = 0.159, p = 0.028) and decreases in the cover oflitter (R2 = 0.277, p = 0.004; Table 4).

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Soil Hydrology and Microphytic Crusts 119

Table 3Mean soil physical properties from the two depths (0-25 mm and 25-50 mm)

measured adjacent to each simulation plot

Soil depth

Soil property 0-25 mm 25-50 mm

Organic carbon (%) 0.527a (0.042)" 0.378b (0.028)Bulk density (Mg m~3) 1.50a (0.02) 1.45b (0.020)Aggregate stability (MWD) 150.87a (6.30) 94.44b (6.400)Electrical conductivity (dS m"1) 0.074a (0.011) 0.070a (0.010)pH 7.89a (0.152) 8.06a (0.173)Particle size analysis*

Clay (%) 12.5 (0.679) 12.3 (0.795)Silt (%) 6.5 (0.599) 6.0 (0.719)Fine sand (%) 42.6 (1.28) 42.6 (0.571)Coarse sand (%) 38.4(1.08) 39.1(0.991)

"The standard error of the mean is given in parentheses. The appearance of different letters withineach row indicates significant differences at p < 0.05.

6No significant differences in particle size analysis between depths.

Discussion

At Mungo National Park, variations in soil hydrological properties were independent ofcrust cover. This occurred despite the wide variation in crust cover, which ranged from1.3% to 83.6% (Table 1). Although statistically significant relationships were detectedbetween time to ponding and some soil properties and crust cover when sites of low crustcover (<15%) were excluded from the analyses, we are cautious about ascribing ecologicalsignificance to these results.

The weak relationships between crust cover and soil hydrological properties are notunexpected, given the widely conflicting evidence reported in the literature (see Eldridge& Greene, 1994; West, 1990). Although some work has indicated increased infiltration onmicrophytic crust-dominated soils compared with crust-free soil (e.g., Blackburn, 1975;Eldridge, 1993; Fletcher & Martin, 1948; Gifford, 1972), other work (Brotherson &Rushforth, 1983; Danin, 1978; Loope & Gifford, 1972) has demonstrated reductions ininfiltration on soils as cover increases. More recently, Williams et al. (1995) showed thatinfiltration capacity of soils was unaffected by crust cover.

Inconsistencies in the reported relationships between crust cover and soil hydrologyare at first difficult to explain, but they could result from a number of factors related tothe biological characteristics of the crusts—e.g., crust floristics and morphology—and/orto the physical characteristics of the soil surface. For example, differences in soil hydrol-ogy may result from changes in the proportion of cyanobacteria to mosses or lichens.Alternatively, differences may be strongly related to such soil physical properties asporosity and aggregate stability upon which the crusts are formed. Below we discuss howphysical and biological "factors might influence the relationship between soil crust coverand soil hydrology, with reference to published and ongoing research.

Influence of Soil Physical Properties on Soil Hydrology

A large body of predominantly Australian research suggests that soils with a well-devel-oped microphytic crust cover have more favorable soil physical properties and enhanced

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120 D. J. Eldridge et al.

Table 4Significant relationships between soil hydrological properties (response

variables) and vegetation and soil (predictor) variables

Response"variable

TPTPTRWFWFWFWFWFWFWFWFWFSORPTSORPTSSISSISSISSI

Predictor*variable

+ organic carbon (0-25 mm)+ organic carbon (25-50 mm)+ stability (25-50 mm)+ litter- clay (0-25 mm)- clay (25-50 mm)- silt (0-25 mm)- silt (25-50 mm)+ fine sand (25-50 mm)+ coarse sand (0-25 mm)+ coarse sand (25-50 mm)+ stability (0-25 mm)+ bare- litter- clay (0-25 mm)- clay (25-50 mm)+ fine sand (25-50 mm)+ litter

Fi.23value

9.834.714.76

24.597.05

12.044.349.418.934.915.504.285.52

10.194.727.556.43

18.35

R2

(%)

26.913.413.549.620.131.512.226.024.814.015.812.015.927.713.421.418.442.0

P

0.0050.0410.0400.0000.0140.0020.0480.0050.0070.0370.0280.0500.0280.0040.0400.0110.0180.000

"TP, time to ponding; TR, time to runoff; WF, depth to wetting front; SORPT, sorp-tivity; SSI, steady state infiltration.

*The sign on the predictor variable indicates the effects of increases in the responsevariable.

stability—and, consequently, higher rates of soil hydrological properties that are inde-pendent of crust cover per se. The classification of the surface morphology of massive redearths in the semiarid woodlands near Cobar in eastern Australia revealed a patchworkpattern of non-eroded (Class 1) to seriously eroded (Class 4) soil surfaces (Greene &Tongway, 1989). The cover of microphytic soil crusts is an essential component of thisclassification system, with cover ranging from extensive on Class 1 surfaces to no coveron Class 4 surfaces. Final ponded infiltration rates measured on these classes showedsignificant decreases in infiltration on the eroded Class 4 surfaces (8 mm h"1) comparedwith the Class 1 surfaces (49 mm h"1). Class 4 surfaces, devoid of soil crusts, probablyhave intrinsically lower infiltration rates because of the eroded nature of their surfaces(Miicher et al., 1988). Grazing-induced erosion is often associated with a loss of macro-pores, leaving only the matrix pores (micropores) to conduct water into the soil. Thisevidence suggests that infiltration rates are determined by the erosional history of the soilsurface rather than the absence of soil crusts per se.

In our study at Mungo National Park, we measured median steady-state infiltra-tion rates of 26.7 mm h"1 under an application rate of 45 mm h"1. This represents aninfiltration efficiency of almost 60%, or a runoff coefficient of 40%. These relatively highinfiltration rates could have resulted from either increased levels of favorable soil physicalproperties, stem flow, or flow through soil macropores (Dunne et al., 1991). Macroporesat the base of grass and shrub tussocks are generated by rootholes, microfaunal burrows,

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Soil Hydrology and Microphytic Crusts 121

and interpedal spaces between large soil aggregates stabilized by organic matter (Greeneet al., 1994). Macropore flow, provided by the roots of such grasses as Stipa spp. and theperennial chenopod shrubs Maireana pyramidata and Maireana sedifolia (F. Muell.) P.G.Wilson, was probably responsible for the large increases in infiltration at this site. Plotswere selected on the basis of microphytic cover only, and although grass cover wastypically <5% cover on the plots (Table 1), shrubs or grasses were often present imme-diately adjacent to the plots. Large macropores, commonly up to 5-10 mm across, rep-resenting the feeding galleries of subterranean termites, were often observed during plotexcavation when wetting front depths were being measured. These macropores wereresponsible for the relatively deep levels to which infiltrating water penetrated into thesoil. At the cessation of simulations, wetting fronts had often reached more than 130 mminto the profile.

At the Mungo site, soil hydrology was independent of crust cover. Similar resultswere obtained from a soil surface in good condition near Cobar, approximately 300 kmnortheast of Mungo National Park (Eldridge, 1993). At Cobar, the overriding influence ofsoil macropores was thought to be responsible for the lack of any influence of crust cover,with water effectively bypassing the soil crust layer. This was confirmed by analysis of theratio of saturated (+10 mm) to unsaturated (-40 mm) infiltration. This ratio provides auseful index of the macropore status of the soil (White, 1988). High values (1:6 or greater)suggest that the majority of flow is through macropores that are generally >0.75 mm indiameter (Greene, 1992). Conversely, ratios of 1:5 or less indicate that macropores arescarce and matrix pores are important conductors of water. Although unsaturated infil-tration was not measured in the Mungo experiments, the abundance of macropores wasapparent when the plots were being set up. The presence of macropores is probably dueto the absence of sheep and cattle grazing since 1977, and to the buildup in pasture levelseven in the presence of large numbers of kangaroos.

In degraded systems, unlike the Mungo site, matrix pores are essential for conductingwater into the soil, and given the likely paucity of plant cover and organic matter, and thecompacted nature of the soil surface, any biological components on the surface are likelyto enhance soil hydrological properties. In these systems, large responses in soil hydrologyhave been associated with small changes in microphytic crust cover (Eldridge, 1993).

Effect of Crust Composition and Morphology on Soil Hydrology

In Australia rangelands, crusts comprise an assortment of bryophytes (mosses and liver-worts; Eldridge & Tozer, 1996), lichens (Eldridge, 19966), cyanobacteria (Belnap, 1995;Rogers, 1989), and other assorted bacteria and fungi. Depending on the component or-ganisms, crusts are morphologically and structurally diverse, and range from black algalscums with little surface microrelief to strongly patterned, thick, often pedicelled crustscomprising lichens and bryophytes. As different crust morphologies interact in a special-ized way with the surface layers of the soil, intuitively it would be expected that theywould have a differential influence on soil hydrology. Unfortunately, we are unaware ofany empirical data to support this assertion.

Crust biota such as mosses and liverworts act as roughness elements, reducing rain-drop impact and influencing overland flow processes (Eldridge & Kinnell, 1997). Al-though the physical structure of mosses and some liverworts, for example, may resemblevascular plants—in that they have stems, leaves, and roots—these plants are most efficientat reducing raindrop action when in the wetted state. In the dry state, mosses and liver-worts shield only a small area of the soil surface. This is because when dry, the leaves of

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many dry area species such as Didymodon torquatus (Tayl.) Catches, and Barbula crinitaSchultz are twisted around their stems, thereby reducing their surface area. This twistingmechanism is thought to protect the photosynthetic tissues, enabling them to survive longperiods of no rainfall. Small falls of rain are sufficient, however, to réhydrate mosses fromthis dry state (Scott, 1982), and within a few minutes of rehydration, which occursnaturally in the field after rainfall or periodic flooding (Rogers, 1994), they may increasetheir surface area up to fourfold.

During rainfall events, moss- and liverwort-dominated crusts would probably in-crease the time taken for runoff to occur by increasing absorption of water on the surface.They would also maintain the integrity of macropores and matrix pores adjacent to mosstufts by increasing aggregate stability in the top few centimeters of the soil as well asincreasing the water-holding capacity of the soil (Danin & Gaynor, 1991). The architec-ture of moss leaves may also influence soil hydrological properties. Many moss leaveshave long tips or hairpoints that channel water into the center of the plant. Apart fromenhancing infiltration, moss leaves increase the surface area of the plant, protecting thesoil against erosion or helping to trap airborne dust and incorporating it into the upper soilhorizons (Danin & Gaynor, 1991). However, where soil crusts are dominated by thalloseliverworts, such as in the summer rainfall areas of northern Australia (Eldridge & Al-brecht, unpublished data; Rogers, 1989), they are likely to shed water once they have fullyimbibed and have reached their interception capacity (Rogers, 1989).

Crustose and squamulose lichens dominate the soil lichen flora in arid regions (Rog-ers, 1974). Unlike the mosses, arid zone lichens generally do not alter their shape inresponse to rainfall, but merely imbibe water as the algal cells turn green and begin tophotosynthesize. Research from eastern Australia (Eldridge, 1996i>) suggests that de-graded sites are likely to be dominated by cyanobacteria and primitive cyanolichens suchas Collema coccophorum Tuck., Peltula patellata (Bagl.) Swinscow & Krog, and Heppiadespreauxii (Mont.) Tuck. These cyanolichens are characterized by a lichen thallus wherethe cyanobacterial cells are uniformly dispersed among the fungal hyphae (homiomerous),rather than a structure that is stratified. These homiomerous species can absorb up to 13times their own body weight in water (Galun et al., 1982), whereas other stratified lichensthat are more typical of healthier sites can only absorb up to 3 times (Blum, 1973). Thedegree to which this interception of water on the surface influences subsequent watermovement through the soil is open to question. West (1990) contends that these sponge-like lichens could hold water on the surface for longer periods, preventing it from beingused by vascular plants and increasing the possibility of evaporation.

Compared with mosses and lichens, cyanobacteria probably provide the bulk of theirsoil protection below the soil surface, within the top few millimeters. The gliding motionof these cyanobacteria as they migrate in search of light and moisture (Campbell et al.,1989) results in the trapping of silt and clay particles in a fine polysaccharide web.Cyanobacterial sheath material accumulates near the soil surface as the motile cyanobac-teria are expelled from their sheaths upon rehydration. Well-developed cyanobacterialsheaths can absorb up to 8 times their weight in water (Wang et al., 1981), rapidlyabsorbing rainwater, and in some situations prolonging the onset of ponding. Althoughcyanobacteria have been shown to absorb water, they may also inhibit absorption in thetopsoil by occupying soil pores near the surface.

On coarse-textured soils, crusts are poorly developed and dominated by physicalcrusts interspersed with free-living fungi and cyanobacteria such as Microcoleus spp.Cyanobacterial sheaths and fungal hyphae associated with poorly developed soil crustsmay reduce infiltration if they exclude water by occupying matrix pores (Chartres, 1992;

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Soil Hydrology and Microphytic Crusts 123

Greene & Tongway, 1989), or may increase infiltration where they help to maintaincontinuous pores by their growth habit through the soil. Evidence of the hydrophobicnature of the crust comes from studies on a dunefield near Cobar in eastern Australia,where removal of the crust from the dune flank enhanced infiltration and delayed the onsetof runoff by 1 hour. When runoff finally occurred, it was significantly less than that on theundisturbed crust (Greene, unpublished data). Similarly, removal of a thin (<2 mm thick)biogenic crust from the surface of a sandy dune soil in the western Negev Desert, Israel,completely halted runoff generation (Yair, 1990). These studies suggest that cyanobac-terial crusts block infiltration even on sandy soils. Other studies (Eldridge, 1993) haveindicated that cyanobacterial sheaths and fungal hyphae enhance infiltration under tension,i.e., through matrix pores, by maintaining entry points on the surface through which smallamounts of rainfall can infiltrate. Clearly more work is required to determine how andunder what conditions these crust components influence water movement through aridsoils.

Conclusions

These studies at Mungo National Park indicate that crusts are a minor determinant ofinfiltration in these soils. Other factors such as soil macropore status, perennial grass andshrub cover, and soil physical properties were probably more influential in determininginfiltration capacity of this soil.

Because rainfall is the most limiting resource in arid and semiarid systems, and istherefore a primary determinant of net primary productivity, strategies that enhance in-filtration and therefore reduce runoff will maximize pasture production and ultimatelyyields and profit. In areas where crusts have been removed by overtrampling or inappro-priate land management, crust recovery will be associated with enhanced infiltration in theshort term (Eldridge, 1993), and therefore with increased productivity and reduced ero-sion. On surfaces naturally well endowed with soil crusts, infiltration is governed byintrinsic soil properties that are "independent" of crust cover. In these areas, a loss ofcrust cover may indicate land degradation and may be associated with a loss in macroporestatus, vegetation cover, and favorable soil physical properties.

For land managers the answer is clear. Where soil crusts are a component of thelandscape, management in the long term should aim at maintaining a healthy crust on thesurface. Alternatively, where soil crusts have been disturbed, management should aim toincrease surface stability to increase regeneration of the crust, and to enhance macroporesthrough manipulation of grazing.

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