In the present study, the root AMF colonization was increased with 4 kg Zn ha− 1 in maize while 2 and 4 kg Zn ha− 1 in residual form for wheat in succession along with inoculation of A. brasilense (Fig. 3A and 4A). Plants mainly absorb Zn in divalent form (Zn2+) (Fernandes et al. 2018) which can also be required for several enzymatic activities, among them synthesis of indole acetic acid (IAA) via tryptophan route (Taiz and Zeiger 2013). Martínez-de la Cruz et al. (2015) reported that auxin has positive impact on branching and volume of plant root system. Our results indicated that Zn rates in association with bacteria has ability to produce IAA and enhance plant growth. In addition, this hormone promoted root system which can indirectly stimulate symbiotic relationship between AMF population and their hosts (Liu et al. 2018).
Bidondo et al. (2011) indicated that bacteria can interact with root microbial community by mycorrhizosphere, inside spores or mycelia as observed in-vitro study. It was also reported that diazotrophic bacteria promoted AMF colonization and spores number that lead to higher acquisition of nutrients especially P and Zn with improved rhizospheric environment (Jangra et al. 2019). Several strains of Azospirillum spp. and Pseudomonas spp. had been reported in degradation of biopolymers (Turrini et al. 2018) and along with Zn application increased root development, formation of new points of infection and auxin synthesis that could lead to higher spores production in successive wheat cultivation (Fig. 4D) and signal AMF colonization (Ludwig-Müller and Güther 2007).
The root architecture system of wheat has low root mycorrhization as observed in the present study (Sup. Table 2). The colonization of AM in wheat roots can be change during growth stages which may have influence on nutritional demand (Ma et al. 2019). In addition, an increase in root colonization was observed from seedling to maturity stage with an increase uptake of 20 g kg− 1 of P and 2 mg kg− 1 of Zn in wheat plants. Such increases were not observed in the present study that probably resulting from already exist optimal levels of soil fertility.
The colonization of DSE in root system of wheat or maize and its interaction with other endophytic microorganisms is still need to be addressed. The DSE colonization occur simultaneously with AMF in plant roots (Ranelli et al. 2015) to deal with biotic and abiotic factors in optimum levels of soil fertility or even increasing doses of zinc that can affect colonization of this fungal group (Lugo et al. 2018). A previous study reported that Zn application may alter DSE (Exophiala pisciphila) colonization in maize root system (Li et al. 2011). The low concentration of Zn provided a non-mutualistic relationship between plant and DSE which may result in low biomass production regardless of inoculation. Although, higher doses of Zn fertilizer increased root colonization and biomass production which indicated that alteration in DSE behavior may be due to increasing Zn rates which is verified in the present study (Fig. 3A, B and Fig. 4A, B).
The bacterium A. brasilense has the ability to produce siderophores and other molecules like salicylic acid that may decrease mycelial growth (Kumar et al. 2018). The present study also showed a decrease in DSE colonization in roots of wheat (Fig. 3B and 4B). However, there has not been reported any antagonistic effect between A. brasilense and DSE in root/ soil system (Newsham 2011). Despite this, Santos et al. (2017) reported a mutualistic association between these microorganisms in most situations but can also develop a pathogenic characteristic in others. The mycelia growth can be inhibited by bacteria action, generating colonization inhibitory effects as observed in the present study for wheat. However, the interactions are still unclear and there is need for further studies on the influence of molecules produced by A. brasilense in root DSE colonization.
The CO2-C released in maize root did not show significant effect with inoculation or Zn fertilization (Fig. 4E, F). While, inoculation of A. brasilense under residual Zn rates increased respiratory activity of wheat and thus increasing CO2-C (Fig. 4E, F). The reason might be due to influence of crop and soil management on microorganisms activities. The respiratory activities of soil microorganisms (bacteria, fungi and other) are responsible for the release of CO2-C and being act as sensitive indicators for soil quality (Saurich et al. 2019). In addition, different crop practices or even external inoculation of microorganism can generate changes in soil microbial activities. Bera et al. (2018) observed an increasing trend in microbial respiratory activities in wheat succession to rice under no-tillage. The respiratory activity was higher from the time of sowing to flowering while decreasing in later maturity. The present results reflected similar behavior for wheat succession to maize at 110 days after emergence. In addition, there was no significant differences in CO2-C release at wheat maturity in succession to maize during 24 hours observation (Li et al. 2019). These results corroborate with the low values of CO2-C as verified in present study (Fig. 3F and 4F), which could lead to stabilized environment where a higher carbon as microorganism biomass is incorporated into soil and a low value of CO2-C is released lost to atmosphere.
The grain yield of maize was significantly different with Zn fertilization and inoculation and adjusted to a non-linear function with increasing Zn rates up to 3.8 kg ha− 1. The initial soil Zn content medium (Table 1) which could meet plant needs and also explain the decreasing trend in productivity with further increase in Zn rates (Fig. 3G). In addition, inoculation with A. brasilense showed an increase in grain production (Fig. 3H). It is possible that A. brasilense favored the development of root system with higher absorption of nutrients and water that has a positive influence on nutritional status of plant (Gómez-Godínez et al. 2019; Galindo et al. 2021).
The assimilation of water and nutrients to spike and shoot are directly related to plant nutritional status (Galindo et al. 2019) and therefore leading to higher grain productivity. Our results showed that grain yield of wheat was not statistically influenced by inoculation of A. brasilense (Sup. Table 1; Fig. 4G, H) but still higher (1637 kg ha− 1 in Table 1) than average wheat production (900 kg ha− 1) of State of Mato Grosso do Sul (Conab 2014). Kazi et al. (2016) stated that inoculation of A. brasilense did not significantly influence grain yield of different wheat genotypes. The reinoculation of these microorganisms was reported an essential management to increase their population and colonize in environment and can compete with less efficient native species (Cassán and Diaz-Zorita 2016).