Soil Type-Dependent Interactions of P-Solubilizing Microorganisms with Organic and Inorganic Fertilizers Mediate Plant Growth Promotion in Tomato

Abstract

The use of plant growth-promoting microorganisms (PGPMs) as bio-effectors (BEs) to improve the nutrient acquisition of crops has a long history. However, limited reproducibility of the expected effects still remains a major challenge for practical applications. Based on the hypothesis that the expression of PGPM effects depends on soil type and the properties of the applied fertilizers, in this study, the performance of selected microbial inoculants was investigated for two contrasting low-fertility soils supplied with different organic and inorganic fertilizers. Greenhouse experiments were conducted with tomato on an alkaline sandy loam of pH 7.8 and an acidic loamy sand of pH 5.6 with limited phosphate (P) availability. Municipal waste compost, with and without poultry manure (PM), rock phosphate (RP), stabilized ammonium, and mineral nitrogen, phosphorus and potassium (NPK) fertilization were tested as fertilizer variants. Selected strains of Bacillus amyloliquefaciens (Priest et al. 1987) Borriss et al. 2011 and Trichoderma harzianum Rifai (OMG16) with proven plant growth-promoting potential were used as inoculants. On both soils, P was identified as a major limiting nutrient. Microbial inoculation selectively increased the P utilization in the PM-compost variants by 116% and 56% on the alkaline and acidic soil, while RP utilization was increased by 24%. This was associated with significantly increased shoot biomass production by 37-42%. Plant growth promotion coincided with a corresponding stimulation of root growth, suggesting improved spatial acquisition of soluble soil P fractions, associated also with improved acquisition of nitrogen (N), potassium (K), magnesium (Mg), and calcium (Ca). There was no indication for mobilization of sparingly soluble Ca phosphates via rhizosphere acidification on the alkaline soil, and only mineral NPK fertilization reached a sufficient P status and maximum biomass production. However, on the moderately acidic soil, FZB42 significantly stimulated plant growth of the variants supplied with Ca-P in the form of RP + stabilized ammonium and PM compost, which was equivalent to NPK fertilization; however, the P nutritional status was sufficient only in the RP and NPK variants. The results suggest that successful application of microbial biofertilizers requires more targeted application strategies, considering the soil properties and compatible fertilizer combinations.

The role of N form supply for PGPM‐host plant interactions in maize

“…, 42.6%; Tab. , 43.9%) was detectable also in the maize plants with ammonium fertilization in comparison with the non‐inoculated controls, as similarly reported in previous studies ( Mpanga et al., , ). By contrast, PGPM (Proradix) inoculation had no effects on root hair development (Fig.…”

“…2) may also increase the root surface area attractive for colonization by the selected inoculants. Accordingly, root colonization by the Trichoderma harzianum strain OMG16 included in the PGPM products ''Combifector A and B'' and successfully tested as maize PGPM inoculant in previous studies (Mpanga et al, 2018(Mpanga et al, , 2019, was significantly increased, by 210%, in maize plants with stabilized ammonium supply as compared with the noninoculated control (Tab. 4).…”

“…Moreover, in agricultural systems, the amount (Dogra and Dudeja, 1993;Nouri et al, 2014;Huang et al, 2017;Schü tz et al, 2017) and the type of the applied fertilizers can also play an important role. In the latter context, the preferential perform-ance of PGPMs in combination with N-rich organic fertilizers has been repeatedly reported (Abbasi et al, 2015;Thonar et al, 2017;Mpanga et al, 2018;Bradá č ová et al, 2019). Recently, Mpanga et al (2019) found an improved plant performance associated with a wide range of fungal and bacterial PGPMs in combination with stabilized ammonium fertilizers as compared with nitrate fertilization.…”

“…Tab. 1 summarizes the effects in maize (Nkebiwe, 2016;Mpanga et al, 2019), wheat (Nkebiwe, 2016), and tomato (Mpanga et al, 2018;Bradá č ová et al, 2019). Accordingly, Nkebiwe et al (2016) reported increased root proliferation and plant growth promotion by PGPM inoculation combined with ammonium fertilization in maize, both, in laboratory and field experiments.…”

“…This will promote spatial nutrient acquisition, nutrient mobilization via root exudates and rhizosphere acidification, as well as root colonization by PGPMs. To address these questions, a set of experiments was conducted with maize under controlled conditions, using a range of bacterial and fungal inoculants with high rhizosphere competence and proven potential for plant growth promotion in maize Thonar et al, 2017;Vinci et al, 2018aVinci et al, , 2018bMpanga et al, 2018Mpanga et al, , 2019 1A). The open parts of the half-pipes were covered with a transparent plexiglass observation window, fixed with adhesive tape (Fig.…”