the cells to remain in a favorable niche and
form mutualistic relationships ( 141 ). The key
mechanism for changing soil hydraulic prop-
erties with biofilms is bioclogging, which is
one of two main applications of a new branch
of geotechnical engineering called microbial
geotechnology ( 142 , 143 ). It aims for a ma-
nipulated partial clogging of porous media
by microorganisms and their by-products, in
order to reduce soil porosity and hydraulic con-
ductivity. Another application, biocementation,
uses microorganisms to improve the engineer-
ing properties of soils and is beneficial for in-
creasing soil strength and stiffness.
Bacterial EPS formation can improve soil wa-
ter retention and reduce desiccation ( 144 Ð 146 ).
EPS improves soil water-holding capacity in
three different ways. First, certain polysac-
charides (e.g., xanthan, dextran, scleroglucan)
are a component of EPS and are known for
their hydrophilic properties. By formation of
clay- or sand-polysaccharide associations with
soil microorganisms, a general increase in water-
holding capacity may be observed ( 146 , 147 ).
In particular, a creation of hydrated micro-
environments via production of alginate as an
important component of EPS that decrease
desiccation stress was shown forPseudomonas
( 148 ). Second, EPS promotes the formation of
soil aggregates and thereby soil water reten-
tion as the small intra-aggregate spaces hold
water firmly ( 149 , 150 ). Third, EPS modifies soil
surface water repellency, causing an increase
in the number of hydrophobic micropores
that inhibit water evaporation ( 149 , 151 , 152 ).
A better understanding of the role of biofilms
in soil hydraulic properties requires more
studies, following a standardized methodol-
ogy (Box 1).
Microorganisms are also able to produce
substances other than EPSs to protect them
in dry conditions. In bacteria, such compounds
include disaccharide trehalose, which is re-
sponsible for osmotic stress response, and a
polysaccharidea-glycan that increases desic-
cation tolerance in bacteria ( 91 , 153 ). Microbial
activity affects soil micro- and macroporos-
ity via the exudation of binding agents and
particle enmeshment by fungi that promote
aggregate formation ( 154 ). Glomalin-related
soil proteins, soil particle gluing agents that
are produced by AMF, also improve soil mois-
ture retention properties by increasing aggre-
gate stability ( 155 , 156 ).
Soil water repellency or hydrophobicity
occurs when hydrophobic organic compounds
accumulate at the soil surface. Those com-
pounds can be derived from plant leaves and
roots as well as from microorganisms. Exam-
ples of hydrophobic microbe-derived com-
pounds are ergosterol and glomalin-related
soil proteins ( 157 ). Fungal hyphae and bacte-
rial excreta promote aggregate stability but
at the same time can increase soil water repel-
lency ( 158 ). On the other hand, some bacterial
species have hydrophilizing properties, or have
ability to degrade waxes, and thereby reduce
hydrophobicity ( 80 , 81 ).
BSCs in arid and semi-arid environments de-
velop when communities of extremely drought-resistant microorganisms dominated by cyano-
bacteria colonize the soil surface ( 159 ). They
influence soil hydrology because they alter
soil properties. Although there is abundant
literature on the effects of BSCs on soil water
repellency and infiltration, reports are con-
tradictory. Some authors observed lower in-
filtration rates and higher runoff for soils
with BSCs, relative to soils with low levels or
no microbial cover ( 159 , 160 ); others report
positive effects ( 161 ) or no effects at all ( 162 ).
These discrepancies can be attributed to meth-
odological approaches, rainfall characteristics,
soil factors, and/or natural variations of the
BSC composition on crust functioning ( 160 , 163 ).
Because infiltration rates are controlled by
soil surface structure, the rough surface micro-
topography of BSCs, characteristic for semi-
arid cool and cold drylands, increases water
infiltration and reduces runoff. On the other
hand, BSCs in hyper-arid warm regions tend
to be smooth and flat because of the absence
of frost heaving, thereby reducing water infil-
tration and increasing runoff. Mixed effects
are observed in arid regions ( 164 , 165 ).Conclusions and future prospects
Ecosystems vary greatly in the degree to which
they are able to recover naturally from human
disturbance. Conservation efforts have increas-
ingly focused on active or natural restoration
of degraded ecosystems in order to restore
ecosystem services and biodiversity ( 166 ). In
some cases, only minor interventions such
as removing human disturbances (e.g.,fire,Cobanet al.,Science 375 , eabe0725 (2022) 4 March 2022 6 of 10
Fig. 4. Comparison of healthy and degraded land.Left: Hydrological fluxes in healthy soil. Right: Possible changes in soil hydrological fluxes as a result of soil
degradation. Microbial processes are circled. Degraded land is experiencing a decline in microbial diversity as well as reduced amounts of extracellular polymeric
substances (EPSs) and soil organic matter (SOM) that lead to a decrease in water-holding capacity and other hydrological changes, as shown here.
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