Chapter 6. Lichens and Microfungi in Biological Soil Crusts: Community Structure, Physiology, and Ecological Functions

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Bryum bicolor agg./Brachymonium exile

Peltigera rufescens

Fulgensia fulgens



Nostoc sp. Calothrix parietina

Endocarpon/

Catapyrenium sp.



Nostoc sp.

Macrochloris

multinucleata

Microfungi



Microcoleus

spp.



Figure 6.1 Schematic block diagram of a biological soil crust with typical colonizers. The

thickness of the layer is about 3 mm, but organisms are not drawn to scale. Peltigera rufescens

(cyanobacterial foliose lichen), Fulgensia fulgens (crustose–squamulose green algal lichen), and

Endocarpon and Catapyrenium (placoid chlorolichens) are illustrated. (Illustration by Renate KleinRödder. Adapted from Belnap et al., in Biological Soil Crusts: Structure, Function, and Management,

J. Belnap and O.L. Lange, Eds., Springer-Verlag, Berlin, 2003a, pp. 3–30.)



2003), approximately 10% with Cyanophyta (creating cyanolichens), and the remainder

are associated simultaneously with both groups (Honegger, 1996). About 40 genera of

photobionts have been identified in lichens: 25 are green algae and 15 are cyanobacteria.

The autotrophic lifestyle of lichenized fungi requires a long-time exposure of the

green thallus to light. Most lichens are long-lived organisms with high habitat specificity.

They are especially ecologically successful in polar, alpine, and warm arid and semiarid

areas where competition with phanerogamous vegetation is reduced. It is estimated that

approximately 8% of the Earth’s terrestrial surface has lichens as its most dominant lifeform (Ahmadjian, 1995). One of their most important habitats is biological soil crusts,

which lichens often dominate.

In the present chapter we concentrate on those widely distributed crusts in which

lichens play a dominating role. We describe their community structure, analyze the special

properties of lichens as key members of biological soil crusts, discuss lichen function

within the crusts, and then discuss the function of lichen-rich soil crusts as components

of larger ecosystems and landscapes (for details and specific literature, see Belnap and

Lange, 2003).



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6.2



119



COMMUNITY STRUCTURE OF SOIL CRUSTS AND THEIR

DISTRIBUTION



The low moisture requirement of biological soil crust organisms enables them to exist

where moisture deficits limit vascular plant cover and productivity. Therefore, biological

soil crusts occur in almost all ecoregions where light can reach the soil surface either

temporarily (e.g., tree fall gaps) or permanently (e.g., deserts). This is true on a global,

regional, landscape, and microsite scale. Thus, crust communities occur in a large variety

of vegetation zones worldwide, including winter-cold steppes, grasslands, and most conspicuously, hot and cold semiarid and arid areas where plants are widely spaced. Vegetational communities in these more arid regions range from evergreen and deciduous woodlands, saltbush communities, grassland, and shrub and succulent formations to areas with

fixed dunes or where vascular plants are restricted to water-collecting depressions. Soil

crust communities also colonize the spaces between vascular plants in polar and alpine

areas. On a small scale, soil crust communities are even found in temperate climatic

regions, such as xerothermic local steppe formations in Central Europe and in the pine

barrens of the U.S.

Biological soil crusts can be grouped into four types, based on habitat conditions,

taxonomic composition, physical appearance, and function. Smooth crusts are found in

hyperarid regions (e.g., hyperarid Australia, Atacama and Negev Deserts), where soils do

not freeze. High potential evapotranspiration (PET) prevents growth of lichens and mosses,

except in a few moist microhabitats. Thus, these crusts are almost exclusively endedaphic

cyanobacteria, algae, and microfungi, and they actually smooth the soil surface. The other

three crust categories generally have epedaphic colonizers such as lichens and mosses in

addition to the endedaphic biota. Rugose crusts occur in areas with lower PET than smooth

crusts. Although dominated by endedaphic cyanobacteria, algae, and microfungi, they

also support scattered clumps of lichens and mosses that give the soils a slightly roughened

surface (<2 cm of roughness). This crust type is found in hot deserts that lack soil freezing

(e.g., Sonoran and Mojave Deserts, southern Australia, Central Negev, coastal fog zone

of the Namib, and Mediterranean-type climates). Pinnacled and rolling crusts occur in

regions with lower PET than rugose or smooth crusts and where soils freeze annually.

Pinnacled crusts are dominated by cyanobacteria but locally can have up to 40% lichenmoss cover. Soils are frost-heaved upward and then differentially eroded downward,

creating pinnacles up to 15 cm high. This crust type occurs in regions such as the Colorado

Plateau and central Great Basin, U.S., and the mid-latitudes in China. Rolling crusts occur

in regions where relatively low PET results in fairly continuous lichen-moss cover that

is frost-heaved upward in winter. Unlike pinnacled crusts, the cohesive lichen-moss mat

resists downward erosion, creating gently rolling surfaces up to 5 cm high. This type of

crust is widely distributed in the northern Great Basin, U.S., and in the steppes of the

Eurasian subcontinent.

Desert habitats with fog and dew (as in the Namib and Central Negev Deserts) favor

chlorolichens, whereas lack of dew, less rain, and higher temperatures (as in the Arava

Valley, Dead Sea area) favor cyanolichens (Galun et al., 1982). Lichens grow on almost

all soil types across the pH gradient, although species composition may change. Extensive

lichen cover is found on highly stable soils, such as gypsum and calcite, which also have

high water-holding capacity and high levels of phosphorus and sulfur.

The floristic inventory of soil crusts of the world is still poorly known. Nevertheless,

the number of crust-building lichen genera now known is already surprisingly high. Büdel

(2003) reports 69 green algal lichen genera and 35 cyanobacterial lichen genera from soil

crusts around the world. It is striking how similar biological soil crusts are throughout the



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42

NA



34



24



44



EU



16

19

?

26



Figure 6.2 Floristic similarity on a genus basis between lichens in soil crust communities in the

different areas of the world. The magnitude of the Sörensen coefficient (see text) is indicated by

lines: thick lines, 60 to 90% similarity; thin lines, 40 to 60% similarity. Number of lichen species

in South America is unknown. Numbers indicate the absolute number of genera. NA = North

America; EU = Europe. (From Büdel, in Biological Soil Crusts: Structure, Function, and Management, J. Belnap and O.L. Lange, Eds., Springer-Verlag, Berlin, 2003, pp. 141–152. With permission.)



world. This is true not only with respect to the structural appearance of the communities,

but also in terms of taxonomic composition. Büdel (2003) calculated the floristic similarity

of soil crust lichens on a generic basis using a Sörensen coefficient (the ratio of identical

genera to the sum of all lichen genera present; Figure 6.2). Soil crust lichens show several

strong floristic relationships among continents, with a generic similarity of 60 to 90%

between Asia, the Middle East, and Africa. A similar relationship exists between Africa

and Australia. The relationship among the other continents, although weaker, is still high.

There are even some lichen species (e.g., Psora decipiens, Collema tenax) with a worldwide distribution and that occur in soil crust communities on almost all continents.

Representatives of almost all types of lichen growth forms can be found in soil crust

communities. Crustose lichens cover the soil with an appressed, more or less flat layer of

thalli. More or less isolated, crustose thallus scales occur in placoid genera (such as Psora,

Buellia, and Trapelia), and shield-like scales can form peltate thalli that are attached by a

central holdfast (e.g., Endocarpon, Peltula). When thalli are more continuous the thallus

surface is usually divided into small areoles (e.g., Diploschistes, Lecidella, Acarospora).

Squamulose genera such as Squamarina represent a transition to the foliose lichens. Here,

the margins of the individual thallus lobes are raised above the substrate (e.g., Peltigera,

Xanthoparmelia). The transition to the fruticose form is represented by genera such as

Toninia with inflated thallus lobes, whereas examples of soil crust fruticose species include

Cladonia and Cladia species. Most of these lichens have a heteromerous (stratified) structure. Several cyanobacterial lichens have homoiomerous (unstratified) thalli and a gelatinous

consistency, with the most important species of this group belonging to the genus Collema.



6.3



ECOPHYSIOLOGICAL FUNCTIONING OF SOIL

CRUST LICHENS



The microenvironment in which the soil crust biota are found, the soil surface, is one of

the most extreme habitats for autotrophic organisms on Earth. Here, the danger exists that

high levels of solar radiation might damage tissue and DNA and might cause photoinhi-



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Net photosynthesis, % of maximum



Lichens and Microfungi in Biological Soil Crusts



121



100



Diploschistes diacapsis

75



50



Collema tenax

25



Fulgensia fulgens

0

0



0.5

1

1.5

2

2.5

Water content, mm precipitation equivalent



Figure 6.3 Dependence of net photosynthesis (percent of maximum) on water content for different types of soil crust lichens: Fulgensia fulgens (from local steppe formation, Würzburg, Germany)

and Diploschistes diacapsis and Collema tenax (both from Colorado Plateau, UT). (From Lange,

in Biological Soil Crusts: Structure, Function, and Management, J. Belnap and O.L. Lange, Eds.,

Springer-Verlag, Berlin, 2003, pp. 217–240. With permission.)



bition. This zone is also where the highest and lowest temperatures occur within the soil

atmosphere profile. Even in temperate regions, the temperatures of lichen thalli at the same

site can be up to 70°C in summer and down to –20°C in winter. Temperatures are likely

to be even more extreme for soil crust lichens in hot deserts or in polar habitats. Thus,

the ability to tolerate extreme temperatures (at least in the desiccated state) is a requirement

for soil crust organisms. All soil crust components are poikilohydric and are often exposed

to long periods of strong dehydration between infrequent moistening events. Cladonia

convoluta from a soil crust site in southwest Germany was not impaired after 56 weeks

of experimental drying (Lange, 1953). Dry-weight-related water content of lichen thalli

can reach 5% or less, terminating all metabolic processes. In deserts, precipitation events

are infrequent and generally less than 3 mm (Sala and Laurenroth, 1982). Therefore,

lichens must be able to use these small events, as well as snowmelt, fog, dew condensation,

or even high air humidity, for reactivation and photosynthesis.

6.3.1

Carbon Exchange

Water content (WC) is the most important parameter that determines photosynthetic

productivity of a soil crust lichen. The moisture compensation point (MCP) denotes the

minimal WC that is required to reach positive net photosynthesis (NP), while optimal

WC results in maximal rates of NP. The water-holding capacity of a lichen is the maximal

amount of water that can be absorbed by the lichen thallus. Various species have different

thallus structures and specific physiological features that result in large differences in

WC, MCP, and NP. Thus, individual species have very different carbon exchange

response patterns (Figure 6.3). The chlorolichen Fulgensia fulgens, with a very low MCP,

is capable of using very slight hydration by dew or fog (Lange et al., 1997). This species

is even able to reactivate photosynthesis by using water vapor from very humid air, i.e.,

without moistening by liquid water. However, the water-holding capacity of F. fulgens

is low and NP is heavily depressed at high thallus water content (suprasaturation), as

the presence of water increases the diffusion resistance for CO2 uptake. Lichen types

such as Fulgensia are best adapted to regions where small, nonrain moisture events are

frequent. In contrast, both the moisture requirement and water-holding capacity of the



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Belnap and Lange



Net photosynthesis, µmol m−2 s−1



3



2



1



0



0

200

W 400

at 600

er

co 800

nt 1000

en

t, 1200

%

1400

1600 0



1800

1600

1400

1200

1000

−1

800

−2 s

600

lm

400

mo

200

,µ

FD

PP



Figure 6.4 Response of net photosynthesis (at 17°C) to PPFD at several thallus water contents

(percent of dry weight) for Collema cristatum (local steppe formation, Würzburg, Germany). (From

Lange et al., Journal of Experimental Botany, 52, 2033–2042, 2001. With permission.)



gelatinous cyanobacterial lichen Collema tenax are much higher (Lange, 2000). This

species begins photosynthetic carbon gain at a WC that is higher than the optimal

hydration for Fulgensia. Obtaining such a high WC usually requires a rain or snow event.

However, with its high water storage, Collema can make better use of these larger

moisture events, giving it long-lasting periods of activity. Such cyanobacterial lichens

are frequently found in deserts and semideserts where rain is the predominant source of

moisture, even if it is sparse. Diploschistes species are highly favored in all habitats due

to lack of suprasaturation depression and through substantial water-holding capacity.

Soil crust species of this genus are very widely distributed, ranging from the temperate

and Mediterranean regions, across different types of deserts, and to the cold steppe

formations in Asia and the U.S.

Typical light response curves of soil crust lichen’s NP reveal sun plant characteristics

with relatively high light compensation points and light saturation points that exceed 1000

µmol m–2 s–1 photosynthetic photon flux density (PPFD). Figure 6.4 shows a suite of light

curves at different degrees of hydration for a Collema species. There is no observable

photoinhibition even at the highest light levels. Maximal NP rates are attained at optimal

WC of 600% of thallus dry weight. The character of the light curves remains identical at

suboptimal WC when photosynthesis becomes increasingly limited by desiccation, as well

as at supraoptimal WC when thallus diffusion resistance increases.

The different types of terrestrial lichens can tolerate a large range of temperatures

for effective net photosynthetic productivity. Upper temperature compensation points for

CO2 exchange are very high (>40°C) for cyanobacterial lichens and slightly lower for

chlorolichens (~30 to 35°C; see Figure 35.2 in Chapter 35). For some of the green algal



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Lichens and Microfungi in Biological Soil Crusts



123



soil lichens, maintenance of low, but still measurable, rates of net CO2 fixation could be

detected under controlled conditions (Lange, 1965) down to thallus temperatures of –12°C

(Cladonia rangiformis from central Germany) and –22°C (Cladonia convoluta from the

Mediterranean area of southern France).

Photosynthetic carbon gain and respiratory carbon loss under field conditions are

the result of a complicated interplay between environmental conditions and the functional

response patterns of individual soil crust species. Certain characteristic weather conditions occur repeatedly and have resulted in four main types of diel courses of CO2

exchange in soil crust lichens. (There are also dry days without any metabolic activity.)

These four response types are illustrated with typical days for Cladonia convoluta from

the local steppe formation of Würzburg, Germany (Figure 6.5). Panel a shows that

moistening by dew, frost, or high air humidity results in a very short peak of NP in the

early morning hours. In panel b, one can see that thorough wetting with rain during the

night enables the lichen to be active longer (the next morning once light conditions

become favorable) than when wetted with dew, until the thallus desiccates again. Panel

c portrays the frequent changing of moist and dry periods due to the quick responses of

the lichen’s metabolism, and the most productive situation for C. convoluta. Panel d

depicts days when the thallus is continuously moist under favorable light conditions.

These four types of weather conditions potentially occur for soil crust lichens in many

habitats and regions. However, the frequency at which each weather condition type

occurs, the duration of crust activity, and the magnitude of activity rates will vary by

region. Dew and fog can be the main sources of hydration for many soil crusts, especially

in hot coastal deserts, and winter frost can be a significant hydration source in interior

cool and cold deserts.

The photosynthetic capacity of soil crust lichens is remarkably high. Maximal rates

of NP under optimal hydration, light, and temperature conditions are in the range of 7.0

(Collema tenax), 5.9 (Diploschistes muscorum, Lecidella crystallina), 5.5 (Squamarina

lentigera), and 5.1 (Fulgensia fulgens) µmol CO2 m–2 s–1. This is close to the 10 to 20

µmol CO2 m–2 s–1 typical maximal rates for light-saturated leaves of sun plants (Lange,

2003). However, in contrast to phanerogamous leaves, optimal water content is a rare and

short event for poikilohydrous crust lichens, and they can only transiently make use of

their high photosynthetic capacity. In addition, their photosynthetic productivity is limited

due to the short and infrequent hydration times. Under temperate habitat conditions (central

Germany), metabolic activity time ranges from 35 to 65% of the year, with Fulgensia

fulgens having the lowest activity time. Photosynthesis occurs only 13 to 27% of the year,

with Fulgensia again having the lowest activity time (Figure 6.6). On the Colorado Plateau

of Utah, photosynthetically active times are estimated at 9 to 11% of the year for a Collema

soil crust (Belnap, 2002). In the coastal fog zone of the Namib Desert, total metabolic

activity time for soil crust lichens is estimated at 10 to 12% of the year, while this

proportion is likely still smaller for arid regions with even less atmospheric moisture

(Lange et al., 1991).

Projections from CO2 exchange measurements in the field and from modeling efforts

based on laboratory studies allow estimates of the order of magnitude of annual productivity of soil crust lichens (Table 6.1). The area-related C (carbon) balance (net primary

productivity) is highest for the foliose–fruticose species Cladonia convoluta. Crustose

Namib lichens profit from low respiratory carbon losses such that their production is

similar to the nonfruticose temperate species. For mixed-lichen or moss-dominated soil

crust communities, annual C balances are estimated at 120 to 370 kg of C ha–2 year–1

(Evans and Lange, 2003). This is a substantial contribution to the C budget for the semiarid

and arid ecosystems where the vascular plant productivity is low.



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Belnap and Lange

Cladonia convoluta



8

7

6

5

4

3

2



June 19, 1997

(a)

Dew moistening



1

0

−1

8

March 26, 1997

(b)



7

6

5

4



Rain in the night,

drying during day



CO2 exchange, nmol g−1 s−1



3

2

1

0

−1

8

7



June 23, 1997

(c)



8

7

6

5

4



Frequent rain (arrows)

and drying during day



Sept. 25, 1996

(d)



6

5

4

3

2

1

0

−1



Lichen continuously

moist



3

2

1

0

−1

0



2



4



6



8



10 12 14 16 18 20 22 24



Figure 6.5 Natural diel time courses of CO2 exchange of Cladonia convoluta (local steppe

formation, Würzburg, Germany) that are typical for the characteristic weather types (Panels a–d,

see text). (From Lange and Green, Bibliotheca Lichenologica, 86, 257–280, 2003. With permission.)



DK3133_book.fm Page 125 Tuesday, April 12, 2005 4:01 PM



Metabolic activity, % of total time



Lichens and Microfungi in Biological Soil Crusts



125



100

80

Inactive

60

40

CO2 loss



20



CO2 gain

0



h.



fu



co



s



m



s



a



ru



en



lg



us



er



en



sc



um



tig



fe



en



m



.l



ru



sia



sc



a



at



a



ut



ol



nv



ist



co



cr



a



ar

m



er



a

m



ni



en



lo



ua



ltig



le



do



lg



Fu



p

Di



Sq



Pe



l

Co



a

Cl



Figure 6.6 Duration of metabolic activity (metabolically active with photosynthetic CO2 gain or

respiratory CO2 loss, respectively, or inactive) as a percentage of the total time of measurement

period for different soil crust lichens (species from local steppe formation; measurements under

quasi-natural conditions in the Botanical Garden at Würzburg, Germany). Data are representative

for the course of 1 year. (From Lange and Green, Bibliotheca Lichenologica, 86, 257–280, 2003.

With permission.)



Table 6.1 Estimates of Annual Carbon Budget (ΣC) for Single Lichen Thalli of Squamarina

lentigera, Cladonia convoluta, Collema cristatum and a Community of Crustose Lichens of the

Coastal Fog Zone of the Namib Desert

ΣCa

–2



–1



g C m year

Squamarina lentigerab

Cladonia convolutab

Collema cristatumb

Crustose lichens of the Namib

Desertc

Crust communities, lower ranged

Crust communities, higher ranged



28.2

142.3

25.8

32



mg C gDW–1 year–1



mg C (gC)–1 year–1



41.15

98.3

84.3



157.1

225.8

199.7



0.4–2.3

12–37



Note: The lower range of annual production estimates for cyanobacteria-dominated soil crust communities

and the upper range for lichen- and moss-dominated communities are obtained from the literature (Evans and

Lange, 2003).

a



Related to projected thallus area, dry weight, and carbon content.

Local steppe formation, Botanical Garden, Würzburg, Germany (Lange, 2000; Lange and Green, 2003,

2004).

c Lange et al., 1994.

d Cyanobacterial-dominated soil crust communities, lower and higher ranges of estimated annual production

(Evans and Lange, 2003).

b



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Belnap and Lange



6.3.2

Nitrogen Fixation and Loss

Nitrogen (N) levels are low in desert ecosystems relative to other ecosystems. Atmospheric

input is low (Peterjohn and Schlesinger, 1990), the distribution and cover of N-fixing

plants is limited (Farnsworth et al., 1976), and heterotrophic bacterial fixation is also low

(Wullstein, 1989). Consequently, cyanolichens and free-living soil cyanobacteria can be

an important, or the dominant, source of fixed N for plants and soils in many desert

ecosystems (Evans and Ehleringer, 1993). Because N can limit plant productivity in deserts

(Ettershank et al., 1978; James and Jurinak, 1978; Romney et al., 1978; Nobel et al., 1988),

maintaining normal N cycles is critical to maintaining the fertility of desert soils.

Most soil crusts in the western U.S. are dominated by N-fixing soil cyanobacteria

(Microcoleus, Scytonema, Nostoc) and cyanolichens (Collema, Heppia, Peltula). A large

range of N input has been previously reported for these organisms (reviewed in Belnap,

2003b). The most recent estimates for soil crust communities range from an average of

up to 1 kg of N ha–1 year–1 for Microcoleus-dominated cyanobacterial soil crusts to an

average high of 9 kg of N ha–1 year–1 for Collema-dominated soil crusts (Belnap, 2003b).

Nitrogen fixation is highly dependent on many factors (Figure 6.7; Belnap, 2003b).

Nitrogen fixation requires the products of photosynthesis, and thus factors that influence

C gain also influence N fixation. As a result, N fixation generally begins only after soil

crusts have been wet, in the light, and able to fix C. Nitrogen fixation occurs mostly in

the light but can also occur for a limited time (about 4 to 6 h) in the dark. Maximal N

fixation rates occur at lichen water contents of approximately 20 to 80% and soil surface

temperatures of about 25 to 27°C (Rychert and Skujins, 1974; Pearson et al., 1981; Paerl,

1990; Belnap et al., 1994). The timing, extent, and type of past disturbance are also a

critical factor in amounts of N inputs because disturbance often reduces the biomass and

flora of crusts (Belnap, 1995, 1996). Reduction of crust biomass after disturbance means

fewer N inputs. Lichens (e.g., Peltula, Collema) have much higher fixation rates than a

similar surface area of free-living cyanobacteria (Belnap, 2003b), but lichens are much

less tolerant of disturbance than cyanobacteria. Therefore, postdisturbance crusts are

generally dominated by cyanobacteria with a greatly reduced N fixation potential. Over

time, as lichens recolonize and crust biomass increases, N inputs increase as well.

Nitrogen contributed by soil crusts can be lost via gas losses, overland flow, and

leaching downward through the soil profile (Barger et al., in press). Recent estimates for

soil crusts on the Colorado Plateau show gaseous losses to be less than 1 kg of N ha–1

year–1, with losses higher under lichen crusts relative to cyanobacterial crusts. Overland

flow events, which occur every few years, can remove up to 6 kg of N ha–1 per event for

cyanobacterial crusts, mostly via large sediment losses. Losses via overland flow for lichen

crusts are very low (~1 kg of N ha–1), as sediment losses are limited. Nitrogen losses via

leaching have not been investigated.

Nitrogen fixed by crust organisms is made available to surrounding soils and cooccuring organisms in two ways. First, N is released when the crust organisms die. Second,

5 to 88% of newly fixed N is released with wetting events into surrounding soils (Figure

6.8; reviewed in Belnap, 2003b). This N is utilized by nearby vascular plants (see next

section), microbes, and invertebrates. Therefore, biological soil crusts can be an essential

source of N for otherwise infertile desert soils.

6.3.3

Other Aspects of Soil Fertility

Roughened soil surfaces, protuding lichen and moss tissue, and the mucilaginous sheath

material of cyanobacteria capture dust, increasing the amount of fine particles in soils

(Reynolds et al., 2001). Soil crusts increase soil pH from 8 to 10.5 (Garcia-Pichel and

Belnap, 1996), affecting the availability of many plant-essential nutrients. As noted above,



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Figure 6.7 Biweekly measurement of acetylene reduction assay (ARA) for 2 consecutive years

in field-collected Collema from southeastern Utah, measured in the lab under standard conditions

(fully moistened, light, 25˚C). Top panel: Average air temperatures for the sampling date. Samples

were collected in early morning. Sample dates with rainfall within 3 days of sample collection are

indicated by circle color: dark, rain; gray, snow; clear, none; * snowmelt that added water to soils.

Middle panel: Amount of precipitation. Precipitation was measured 10 miles from the sample site;

thus amounts recorded estimate only those at the study site. Bottom panel: Nitrogenase activity

(NA). Samples were collected, moistened, and incubated under light at 25˚C for 4 h. In general, if

soils had been moist within 3 days of collection, NA levels were highly correlated with daily average

temperature (r2 = 0.93) unless temperatures were below 1˚C or above 26˚C. The following letters

refer to the vertical lines labeled at the bottom of the figure. (a–d), when temperatures are above

1˚C and soils are moist, NA is observed. Even if soils are moist, low air temperatures preclude ARA

activity; however, once temperatures increase, ARA activity resumes. Levels are positively correlated

with temperature; (e) temperatures are at the maximum for NA. Soils are moist, but this moisture

follows a long dry period, and thus NA levels are moderate; (f) air temperatures exceed the maximum,

and although soils are moist, no NA is detected; (g) this sample point was anomalous, as no soil

moisture was recorded, but moderate NA were still observed; (h) low temperatures and soil moisture

result in low NA levels; (i–k) snow melts, and NA levels soar until air temperatures get too low; (l)

in spite of optimal air temperature, lack of moisture precludes NA; (m–o) rain after a long dry period

initiates NA. As soils continue to receive moisture, NA increases, although air temperatures are

similar. As temperatures decrease, so does NA.



their presence can increase soil N by up to 200% (Shields and Durrell, 1964) and C by

up to 300% (Rao and Burns, 1990; Rogers and Burns, 1994). Crusts also increase soil

organic matter, known to ameliorate compaction, reduce inorganic soil crusting, reduce

nutrient leaching losses, and increase soil moisture retention (Tongway and Ludwig, 1990).

Exopolymers secreted by soil crusts also modulate metal-ion concentrations at the

microbial cell surface by creating a mosaic of polyfunctional metal binding sites (Greene



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Belnap and Lange

Nitrate concentration, µM



−1



0



200



400



600



800



Depth, mm



0



1



2



3



4



Figure 6.8 Profiles of nitrate concentrations under the lichen Collema within 30 min of wetting

in the light. (Adapted from Garcia-Pichel and Belnap, in Biological Soil Crusts: Structure, Function,

and Management, J. Belnap and O.L. Lange, Eds., Springer-Verlag, Berlin, 2003, pp. 193–201.)



and Darnall, 1990). These polymers act to prevent heavy metals from approaching the cell

surface while concentrating growth-promoting nutrients (Lange, 1976; Geesey and Jang,

1990). Soil fines, with attached nutrients, also bind to crustal organisms. Most binding is

extracellular; thus, bound nutrients remain plant-available (Geesey and Jang, 1990). Soil

crust organisms secrete powerful metal chelators such as siderochromes (Schelske et al.,

1962; Lange, 1974; McLean and Beveridge, 1990) that form complexes with polyvalent

metals, keeping them in a plant-available form. Chelators are also effective in sequestering

essential trace metals that otherwise occur at very low concentrations in the soil (Paerl,

1988). Secretion of peptide nitrogen and riboflavin combine with siderochromes to keep

phosphorus, copper, zinc, and iron plant-available (Bose et al., 1971; Lange, 1974; Gadd,

1990; Geesey and Jang, 1990). Crusts also secrete glycollate (which stimulates uptake of

phosphorus), various vitamins (e.g., B12), auxin-like substances, and other substances that

promote growth and cell division in plant and animal tissue (Fogg, 1966; Venkataraman

and Neelakantan, 1967). Thus, there are many ways in which biological soil crusts increase

the fertility of desert soils.



6.4



BIOLOGICAL SOIL CRUSTS AS AN ECOSYSTEM

COMPONENT



6.4.1

Water Relations

Biological soil crusts influence both the infilitration and soil retention of precipitation.

Infiltration is determined by a balance among the permeability of the soil surface, the

water storage capacity of the soil crust organisms, and the effect of soil surface roughness