Discussion
Our study confirms that even in harsh dryland soils, microbial
inoculation improves plant growth (Fig 1). The results suggest a
mutualistic relationship between microorganisms and plants, where the
plants benefit from an increase in growth as has been found in mesic
systems (Hoeksema et al., 2010; Nuenkamp et al., 2019; Rubin et al.,
2017; Schutz et al., 2018). The magnitude of the effect seen in drylands
(a 43% increase in plant growth with inoculation) is similar to the
study of Rubin et al. (2017), who found that the effect of microbial
inoculation on plant growth under drought conditions is higher than
under well-watered conditions. Thus, our findings are consistent with
the stress-facilitation hypothesis of community interactions which
suggests that environmental stress increases facilitative interactions
between organisms (Bertness and Callaway 1994; Hammerlund and Harcombe
2019).
We did not find evidence to support the hypothesis that inoculation with
native microbes is more effective than commercially available
inoculants. Our results suggest that any advantage native microbes have
as a result of being adapted to the abiotic conditions of the site might
be counterbalanced by the fact that commercial inoculants have
presumably been developed to contain highly effective strains (Tsoi et
al., 2019). The lack of difference between native and commercial
inoculants (Fig 1) presents an opportunity for more effective dryland
restoration programs. Currently, commercial inoculants are more
expensive than native microorganisms (Kaminski et al., 2019), but have a
much lower barrier to entry compared to developing native inoculants
(Bell et al., 2019). Our results suggest that restoration practitioners
who have low technical knowledge of microbial isolation and culturing
can simply use readily available commercial inoculants.
Our hypothesis that inoculating biodiverse microorganisms will show a
stronger effect on plant growth than inoculating a single microbial
inoculum was not supported (Figure 1C). This observation is contrary to
the idea that increasing biodiversity enhances ecosystem function
through complementary and synergistic associations between individuals
(Wagg et al., 2014). However, there is evidence that microorganisms’
effects on ecosystem functionality are more strongly correlated with
microbial functional diversity than with species richness (Kitz et al.,
2015; Nielsen et al., 2011; Zhou et al., 2020). That is, increasing
species richness without concern for functional diversity might not
provide extra benefits to plant growth (Bender et al., 2016).
Our results show no clear benefit to the added expense and complexity of
including more diverse species in the inoculant (Fig 1). That is,
single-species inoculum provides a cost-effective and simple approach to
improving plant growth in dryland ecosystems. Maybe instead of
developing complex inoculants, we should be focusing our efforts on
identifying groups of inoculants that perform similar functions first.
By understanding individual functional groups of inoculants that produce
similar effects, we can avoid redundant diversity and create more
effective inoculant combinations with known results.
We found no evidence to support our hypothesis that the magnitude of the
effect from fungal inoculation is stronger than that of bacteria
inoculation. This result is contrary to the previous studies of Porter
et al. (2020) and Morris et al. (2007), who found that fungi had a
greater effect on plant growth than bacteria. However, there is evidence
that greenhouse studies maximise fungal effects on plant growth leading
to an overestimation of results when compared to field studies (Hart et
al., 2018; Rubin et al., 2017). This example highlights how caution must
be exercised when interpreting glasshouse studies in the context of
microbial influence on plant growth, especially within environments that
differ greatly from typical greenhouse climates.