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.