Introduction
Microbial inoculants can allow land managers to increase plant growth
while avoiding the negative impact of fertilizer application
(Soozandehfar et al., 2023). The use of microbial inoculants is now well
established in mesic agricultural settings (Fox, 2015; Timmusk et al.,
2017; Waltz 2017), and there is growing interest in using microbial
inoculation to assist with the restoration of degraded dryland
ecosystems (Kaushal and Wani, 2016; Zhu et al., 2014). However,
systematic studies examining the effect of microbial inoculants on plant
growth at the field scale in drylands are lacking (Hart et al., 2017).
In this study, we conduct a meta-analysis of dryland ecosystems to
quantify the effect of microbial inoculation on plant growth and
increase understanding of which types of inoculants are most effective.
Our first hypothesis is that plants inoculated with microorganisms in
drylands will have higher growth than uninoculated plants.
Microorganisms are thought to facilitate plant growth by increasing
access to water and nutrients (Canbolat et al., 2006; Lai et al., 2008),
and may also produce phytohormones that stimulate plant growth (Jaral et
al., 2012; van der Heijden et al., 2008; Ruzi and Aroca, 2015 van der
Putten et al., 2016). For example, biomass production in tomatoes was
doubled when inoculated with microorganisms (Almaghrabi et al., 2013).
Also, inoculating wheat with microorganisms increased the shoot and root
biomass by increasing phosphorus uptake from the soil (Karimzadeh et
al., 2021). However, the efficacy of microbial inoculation in field
situations varies substantially (Hart et al., 2018; Ryan et al., 2018;
O’Callaghan, 2016), and it is unclear whether microbial inoculation is
likely to be effective in drylands. High temperatures can reduce
microbial activity (Fang, 2022), so benefits may be reduced in dryland
ecosystems with challenging environmental conditions. Conversely, there
is some suggestion that microbial inoculation is more likely to be
successful in drylands due to organisms shifting to facilitative
strategies in high-stress environments (Bertness and Callaway, 1994).
For instance, inoculating with microorganisms increased seedling
emergence (Dadzie et al., 2022) and biomass production (Shirmohammadi et
al., 2020) in drylands suggesting a facilitation effect of microbial
inoculation on plant growth.
Our second hypothesis is that inoculation with native microorganisms
yields greater growth than inoculation with commercial microorganisms.
The dominant theory is that because native microorganisms have
co-evolved with native plants and are adapted to local conditions, they
will result in greater plant growth than commercial inoculants (Dadzie
et al., 2022; Emam, 2016; Koziol et al., 2022; Middleton et al., 2015).
Some studies have supported the hypothesis that native microorganisms
enhance plant growth better than exotic microorganisms (Koziol et al.,
2022; Chaudhary et al., 2020). However, the massive development of
bio-fertilizers in ecological domains (Timmusk et al., 2017; Islam et
al., 2021) compels a systematic evaluation of the magnitude of effect
between native and commercial inoculants under dryland field conditions.
This information will help ecologists and restoration managers to select
appropriate inoculants within their economic means.
Our third hypothesis is that bulk inoculation of different microbial
species will affect plant growth more than inoculation of single
microbial species. One of the most prominent theories in ecology is the
idea that increasing biodiversity leads to increased resource use
efficiency and functional redundancy, whereas the loss of an organism
will cause a reduction in ecosystem function (Bender et al., 2016;
Philippot et al., 2013; Wagg et al., 2014). This idea has readily been
adopted from plant ecology into microbial ecology with little testing,
despite studies showing that microbial activities and trends do not
follow patterns of macroecological theories (Fierer et al., 2011;
Hendershot et al., 2017). Some studies show a greater effect of
biodiverse microbial inoculants than single taxon inoculants (Bradáčová
et al., 2019; Devi et al., 2022; Jain et al., 2015; Lally et al., 2017).
Other studies have shown a similar magnitude of effect from
single-strain microbial inoculation as from multi-species microbial
inoculations (Bhatia et al., 1998; Onwuchekwa et al., 2014;
Shirmohammadi et al., 2020). Here, we provide a systematic evaluation of
the effect of single-strain and multiple-strain microbial effect on
plant growth in drylands.
Our final objective was to test the hypothesis that inoculation with
fungi will yield greater plant growth than inoculation with bacteria in
dryland ecosystems. Bacteria and fungi are the most widely used
microbial organisms for inoculation studies. Both taxa have been found
to enhance plant growth when inoculated with plants (Coleine et al.,
2022; Zhang et al., 2019). Some previous work suggests that inoculation
with fungi will have greater effects on plant growth due to their
ability to ameliorate stress (Porter et al., 2020) and ability to thrive
in soils with a high carbon: nitrogen ratio (Güsewell and Gessner,
2009). On the other hand, the quick turnover of bacteria can cause them
to perform better than fungi (Hasheem et al., 2016). By understanding
the differences in the effect of fungal and bacterial inoculants on
dryland soils, we can inform future dryland restoration projects to
direct their limited resources more effectively.