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.