This document discusses soil microbial biomass as a key driver of ecosystem functioning. It makes three main points: 1. Soil microbial biomass plays a key role in ecosystem sustainability by maintaining essential soil functions and nutrient cycling. Larger microbial biomass results in higher ecosystem stability and ability to withstand disturbances. 2. Environmental drivers like climate, soil properties, vegetation composition, and land use influence patterns of microbial biomass across different ecosystem types. Nutrient-poor ecosystems rely more on microbial biomass as a nutrient reservoir. 3. Restoring degraded lands requires considering microbial activities and biomass development in response to environmental drivers. Mixed plantations can help recover lost microbial diversity and biomass faster than monocult

1. Short Communication
Soil microbial biomass: A key soil driver in management of
ecosystem functioning
Jay Shankar Singh a,
⁎, Vijay Kumar Gupta b
a
Department of Environmental Microbiology, Babasaheb Bhimrao Ambedkar (Central) University, Raebarelly Road, Lucknow 226025, Uttar Pradesh, India
b
Department of Chemistry and Biotechnology, ERA Chair of Green Chemistry, School of Science, Tallinn University of Technology, Akadeemia tee 15, 12618 Tallinn, Estonia
H I G H L I G H T S
• Soil microbial biomass as a key ecologi-
cal driver has been described.
• Soil microbial diversity and biomass
play key roles in the ecosystem services.
• Soil micro-flora N uptake is an adapta-
tion for short-term storage of N in ter-
restrial ecosystems.
• Soil microbial biomass is key ecological
driver in rehabilitation of degraded
lands.
G R A P H I C A L A B S T R A C T
a b s t r a c t
a r t i c l e i n f o
Article history:
Received 28 February 2018
Received in revised form 30 March 2018
Accepted 30 March 2018
Available online xxxx
Editor: Frederic Coulon
Although patterns of microbial diversity and biomass have been described and reviewed at local and regional
scales, a unifying driver, or set of environmental drivers affecting soil microbial biomass (SMB) pattern at global
level is still missing. Biomass of soil microbial community, known as SMB is considered widely as the index of soil
fertility and ecosystem productivity. The escalating soil stresses due to land degradation and climatic variability
are directly correlated with loss of microbial diversity and abundance or biomass dynamics. Therefore, alleviating
soil stresses on microbial communities with ecological restoration could reduce the unpredictability and turn-
over rates of SMB. Thus, the key ecological factors which stabilize the SMB and minimize its turnover, are sup-
posed to play an important role in the soil nutrient dynamics and productivity of the ecosystems. Because of
the existing public concern about the deleterious impacts of ecosystem degradation, there is an increasing inter-
est in improving the understanding of SMB, and the way, it contributes to restoration and functioning of
ecosystems.
© 2018 Elsevier B.V. All rights reserved.
Keywords:
Land degradation
Environmental drivers
Microbial community
Soil microbial biomass
1. Microbial biomass under different ecological regimes and ecosys-
tem types
Functioning of ecosystems rely on the flux of chemical signals, car-
bon, and nutrients across the trophic levels, mainly mediated by
microbial interactions in the soil-plant-animal food web (Seneviratne,
2015). Soil microbial diversity and abundance/biomass play key roles
in the ecosystem sustainability by maintaining essential functions of
soil health, through carbon and nutrient turnover. Even after distur-
bances, an ecosystem with a higher microbial diversity and biomass
may have a higher capacity to sustain the ecological processes through
microbiological buffering. There is now increasing evidence that above-
ground (plant litter quantity and quality) and belowground (soil
Science of the Total Environment 634 (2018) 497–500
⁎ Corresponding author.
E-mail address: jayshankar_1@yahoo.co.in (J.S. Singh).
https://doi.org/10.1016/j.scitotenv.2018.03.373
0048-9697/© 2018 Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
Science of the Total Environment
journal homepage: www.elsevier.com/locate/scitotenv

2. microbial flora and fauna diversity) are key drivers that play fundamen-
tal roles in controlling ecosystem processes and stability (Wardle et al.,
2004). However, understanding ecological linkages between above-
and belowground abiotic and biotic (including SMB) attributes is still
a key challenge for our knowledge on the stability and functioning of
ecosystem processes. Therefore, given the importance of SMB dynamics
in defining ecosystem properties, the understanding as to how SMB re-
spond to unexpected rise in biotic and abiotic drivers and loss of plant
species diversity seems a research priority that would shed light on
the performance of diverse ecosystems.
Among the various ecological drivers, microbes (N2-fixers, P-
solubilizers, growth hormone producers, etc.) and their SMB pools
may be considered as major ecological impetus in controlling the di-
verse ecosystem functions globally (Table 1). Soil carbon (C) and nitro-
gen (N) together with phosphorus (P) and soil pH are the key drivers
that may limit the functioning of various nutrient deprived ecosystems
(Bru et al., 2011; Carnicer et al., 2015). In strongly soil N deprived eco-
systems such as Arctic and Alpine Tundra and Temperate forests, nutri-
ents release after soil micro-flora (SMB) death has been pointed out as
an important source of plant nutrient N (van der Heijden et al., 2008).
So, it is suggested that uptake of N by soil micro-flora might be an adap-
tation that may have developed for short-term storage of N in several
terrestrial ecosystems, thus reducing the loss of N from the soil of nutri-
ent poor ecosystems. It is also important to mention here that the chem-
ical composition of plant residues (litter and fine roots) from diverse
plant species in an ecosystem could be unpredictable and therefore, dif-
ferent microbial communities might be evolved to decay and decom-
pose the variety of litter types. As many microbial communities have
restricted bio-geographic distributions (N2-fixing rhizobia in tropical
forests and mycorrhizae in boreal temperate forests) it is suggested
that variations in the size of SMB can impact to variability of the func-
tioning of various ecosystem types.
The rise in unpredictable climate shift and anthropogenic perturba-
tions are the critical drivers to regulate the existence and survival of in-
digenous microbial diversity, and consequently, the essential soil
functioning of the ecosystem. The macroclimate, topography and soil
quality (pH, organic C and N, moisture, etc.) are the key drivers in con-
trolling SMB dynamics across different ecosystems and locations
(Wardle, 1992). The temporal dynamics of SMB is likely to be the key
factor in determining the extent of release of immobilized labile nutri-
ents due to microbial cell deaths, and the availability of nutrients re-
leased crucial to plant growth and ecosystem functioning (Wardle,
1998). The factors that provide viability to the soil microbial community
therefore, are assumed to enhance soil nutrient conservation in the
form of higher SMB size owing to the reduced release of immobilized
nutrients. While studying the effect of climatic disturbances on ecosys-
tem types, we may expect SMB to have lesser temporal variability
owing to lesser disturbances.
Along a large geographical area, climate variability, land use types
and the dominant vegetation composition might be the key factors for
SMB variability across different ecosystem types. Differences in the
quantity and quality of substrate (organic C and N) inputs caused by
varying plant residue types (litter and fine roots) and the associated nu-
trient specificity can be crucial drivers to influence the SMB across the
ecosystem types. Thus, the higher SMB in soil of scrub stand than
other ecosystems might be attributed mainly to the greater availability
of organic matter via vegetation cover with giant plant species. How-
ever, in case the temporal data on SMB from different ecosystems across
the globe were quantitatively analyzed, more persistent and less dis-
turbed ecosystems had higher SMB values than the most disturbed
one (Wardle, 1998), though the temporal variability of different ecosys-
tem types was substantially comparable. This indicates that temporal
SMB variability is solely governed by the dominant vegetation and
land use type, but the nutrient turnover along with other edaphic fac-
tors can be more effective than those in regulating the temporal SMB
variability. However, it is suggested that shifts in plant species composi-
tion during land use change, can influence the microbial community
composition dynamics and SMB basically by changing soil organic nutri-
ent status (Zhang et al., 2016). But it is still unclear as to why SMB is not
so responsive to large-scale disturbance regimes, such as invasions of
alien species into new territories, climate change due to atmospheric
CO2 elevation, greater soil nitrogen deposition and land use changes
(Wardle et al., 2004), and therefore, further studies in this regard, will
enhance our understanding about temporal microbial community com-
positions and biomass variability.
Although nutrient status, seasonality, soil factors, temperature, and
other factors, are important drivers to control functioning of dry tropical
forest ecosystem, SMB could be one of the vital factors affecting produc-
tivity in the tropical dry deciduous forests, as observed in the Vindhyan
plateau (Singh et al., 2010). In India, the previous investigations regard-
ing SMB across the tropical dry deciduous forest ecosystems have been
conducted by selecting only one or the other sites on a temporal scale,
and without deciphering differences in SMB status and its role in distri-
bution and variations of the dominant vegetation composition. It may
be hypothesized that alterations in soil properties due to land uses
and anthropogenic activities would correspond to differences in the
SMB size and turnover rates and consequently, to loss of vegetation
cover and ecosystem productivity. However, the experimental evidence
for such arguments is still warranted.
In nutrient poor ecosystems, SMB acts as the major reservoir of avail-
able nutrients, and plays very crucial roles in the survival of plants
(Singh et al., 2016a, 2016b; Vimal et al., 2017). In the dry tropical decid-
uous forest ecosystem, nutrient withdrawal from the senescing leaves
and its immobilization in the SMB has been considered as the nutrient
conserving adaptations in response to nutrient paucity, and thus SMB
Table 1
Microbes as key ecological drivers regulating functioning of diverse ecosystems.
Source: van der Heijden et al. (2008) Ecol. Lett. 11: 296–310.
Ecosystem type Microbes Microbial functions
Tropical savannah,
some grasslands and
tropical forests
Symbiotic, free living and
endophytic N-fixing
bacteria
Support plant productivity
by supplying different
limiting nutrients (N) to
the plants.
Endophytes enhance
growth and competitive
ability of their host plants.
Deserts and boreal
forests
N-fixing cyanobacteria Contribute to the N
economy of ecosystems via
N cycling and accumulation
under extremely limited
N2-fixation situations.
Helps in degraded land
restoration.
Tropical, temperate and
boreal forests,
grassland and
savannah ecosystems
Arbuscular mycorrhizal
(AM), Ectomycorrhizal
(EM) and Ericoid
mycorrhizal (ERM) fungi
Enhanced P uptake of host
plant species when plant
productivity is strongly
limited by P availability.
Provide resistance to plants
against diseases, drought
and other environmental
stresses in nutrient poor
and degraded ecosystems.
Natural ecosystems P-solubilizing bacteria May constitute up to 40% of
the cultivable population of
soil bacteria.
However, significance of
bacteria in natural
ecosystems is still in
enigma and further studied
are still warranted.
Tropical forests Nitrifying and denitrifying
bacteria
Indirectly reduce plant
productivity because about
50% of available soil N is
lost from the ecosystem by
nitrification and
denitrification processes
498 J.S. Singh, V.K. Gupta / Science of the Total Environment 634 (2018) 497–500

3. constitutes the key available nutrient source for the survival of plant
communities (Singh et al., 2010; Singh, 2015a, 2015b). The conversion
of forest into cropland and savanna due to plant harvesting may result
a significant alterations in the size of SMB. Since, plant diversity is a
key driver in controlling SMB (Thakur et al., 2015); any alterations in
the soil properties due to deforestation may have significant impact
on SMB stability and its turnover rates. As plant biodiversity is projected
to decline in response to global environmental change factors, we can
also expect indirect effects of key environmental drivers on SMB status
via alterations in plant community compositions. The increasing de-
mand of agricultural land accelerates deforestation and consequently
disturbances in SMB trends had been noted. Although deforestation is
a common practice for the generation of agricultural managed land-
use systems, afforestation has become a much discussed topic in recent
years (Singh et al., 2010). Therefore, we suggest that long-term investi-
gations should be conducted on the SMB patterns, a sensitive nutrient
pool that may respond to plant diversity loss compared to other envi-
ronmental drivers.
2. Microbial biomass in restoration of degraded land soil
productivity
Restoration ecology, a somewhat new thrust research area, still
needs rigorous explorations to achieve satisfactory results (Calmon
et al., 2011). As the changing global climate is one of the major environ-
mental challenges today, there is an urgent need to restore the huge de-
graded land area to make it cultivable for the overall sustainable agro-
environmental development. Size of SMB can provide one of the most
satisfactory estimates of the productivity of any restored degraded
land (do Couto et al., 2016). A lower ratio of SMB to the total soil organic
nutrients seems to indicate good strength of degraded land rehabilita-
tion. Consequently, the ratio of SMB to the total organic nutrients may
be considered as a reliable parameter for the success of degraded land
rehabilitation. The re-established SMB status and its dynamics under
prevailing environmental drivers may provide valuable information
about the restoration progress and productivity potential of derived
lands for agriculture purposes.
SMB, an important driver of ecosystem functioning, may be consid-
ered as the sensitive key biological indicator of perturbations owing to
soil disturbances (Zornoza et al., 2009). Research practices on degraded
land restoration, have mainly focused on the aboveground features with
lesser emphasis on the belowground soil microbial processes and the
patterns of SMB, which could play a vital role in productivity of the de-
rived ecosystems. The soil microbial properties between degraded and
restored lands have been compared (Bini et al., 2013; Singh, 2016),
but the knowledge about key environmental drivers influencing the
patterns of SMB and microbial community structure during restoration,
is still limited. Therefore, the study of SMB patterns and its relationship
to climate and soil drivers can enhance our understanding in rehabilita-
tion of land uses like vast waste lands (e.g. spoiled coal mine areas). It is
suggested that during restoration programs, the belowground microbial
activities and their biomass development and responses must be con-
sidered in implementation. It is proposed that restoration practices
with mixed plant species having high biomass producing potential,
can contribute to speedy recovery of lost microbial diversity and SMB
turnovers (Singh, 2013; Araujo et al., 2013; Singh, 2014). The fast recov-
ery of microbial communities and SMB size due to mixed plantations
cover (Lange et al., 2015) compared to the monoculture plantations
can help in the returning of the huge bulk of diverse types of plant-
derived resources (leaf litter, fine root biomass, root exudates or
rhizodeposits) and favorable soil conditions. In view of these, it is rec-
ommended that plant species having high biomass production effi-
ciency, and favoring the establishment of beneficial microbial
(mycorrhizae and nitrogen-fixers) associations, should preferably be
selected during restoration programs.
Speedy recovery of soil productivity on the deforested area can be
achieved by afforestation of mixed plantations in association with suit-
able bio-inoculants, which can be a good microbial “booster” to enhance
the SMB and ecosystem productivity (Fig. 1). The inoculation of suitable
bio-fertilizers along with mixed plantations, not only increases the pop-
ulation of the microbial strains inoculated, but also enhances diversity
and abundance of microbes native to the soil, through dormancy break-
ing of their quiescent viable spores developed in response to environ-
mental stresses (Seneviratne and Kulasooriya, 2013; Singh and Strong,
2016). Thus, a combination of mixed plantation, stress-tolerant bio-
Degraded land
Microbial flora
and fauna
Plant litters
Root exudates
Crop land Forest ecosystem
Soil microbial biomass
Dissolved organic matter
Re-vegetation of degraded land
Nutrient immobilization
Available nutrients
Nutrient
uptake by
plants and
soil
micro-flora
Solubilization
Decomposition
Mineralization
Fig. 1. Schematic representation showing contribution of soil microbial biomass as source of plant available nutrients to agro- and forest ecosystems.
499
J.S. Singh, V.K. Gupta / Science of the Total Environment 634 (2018) 497–500

4. inoculants, and organic amendments in restoration of the degraded
lands may offer a tripartite association to augment soil nutrient turn-
overs and consequently to reinstate productivity of stressed ecosys-
tems. However, it should be kept in mind that addition of microbial
inoculants into new territories may greatly affect soil plant-microbe in-
teractions, especially when the new microbial species has vastly differ-
ent physiological behaviours from the indigenous micro- flora. No
information is available on how addition of exotic soil microbial inocu-
lants in association with re-vegetation of degraded lands can influences
the exiting native soil micro-biota, although these effects should be
strongest when the alien microbial inoculants has functional attributes
that are not compatibles to the indigenous microbial species. In our
opinion, the future research on SMB turnovers as affected by the key
ecological drivers and its correlation with the ecosystem functioning
holds potential as the complementary criteria for evaluating the rehabil-
itation progress in restoration ecology.
3. Conclusions
Besides the importance of physico-chemical and biological processes
to maintain soil health, SMB may be the ‘keystone’ biological driver of
ecosystem functioning. Loss of beneficial microbial diversity and SMB
associated to land use changes and erratic climate shifts, are the major
reasons for deterioration of soil fertility and ecosystem productivity.
Therefore, researches related to plant-microbe-soil associations to com-
bat problems of soil degradation need to be performed in greater depth.
Today, it is the utmost need to maintain soil health and productivity
with emphasis on restoration of vast area of the degraded lands using
efficient biomass producing novel plant species and microbial inocu-
lants. Finally, SMB is concluded to be an important index for the soil
health and environmental sustainability.
Acknowledgement
JSS is thankful to Head, Department of Environmental Microbiology,
BBA University, Lucknow, India for providing all kind of supports and
the infrastructure facilities.
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