TY  - THES
T1  - Fertilization strategies to improve the plant growth-promoting potential of microbial bio-effectors
A1  - Mpanga Kwadwo,Isaac
Y1  - 2020/07/01
N2  - The use of plant growth-promoting microorganisms (PGPMs) as inoculants to support nutrient acquisition of crops is discussed as a promising strategy for improving fertilizer use efficiency, to enable crop production with less input of fertilizers, and to reduce detrimental environmental side effects related with high inputs of mineral fertilizers. However, the efficiency of PGPM-assisted cropping systems is still biased by the limited reproducibility of the expected effects under real production conditions. This can be attributed to the sensitivity of plant-PGPM interactions to environmental stress factors particularly during the phase of establishment and to limited knowledge on positive or negative interactions with the native soil microbiome and the application conditions required for successful rhizosphere colonization as a pre-requisite for beneficial plant PGPM interactions. 
This study demonstrated that the combination with compatible fertilizers offers an option to promote the establishment of PGPM effects as a potential management option to improve the performance of PGPM-assisted production strategies. In a range of model experiments with maize with a limited inherent potential for root-induced P-solubilization, it was demonstrated that the acquisition of sparingly soluble Ca-phosphates could be synergistically improved by a combination of PGPM inoculants with ammonium fertilizers, stabilized with nitrification inhibitors (Chapter 4). The effect was demonstrated for PGPMs based on 15 different fungal (genus: Trichoderma, Penicillium) and bacterial (genus: Bacillus, Paenibacillus, Pseudomonas, Streptomyces) strains and strain combinations, which were largely ineffective in combination with nitrate fertilization. On average over all experiments, the PGPM-ammonium combinations with sparingly soluble Ca-P supply reached about 84% of the shoot biomass production and 80% of the shoot P accumulation as compared with positive controls fertilized with soluble P. The soil pH-buffering capacity, particularly on neutral to alkaline soils, was identified as a limiting factor, counteracting the plant growth-promoting potential of the selected inoculants with a proven ability for Ca-P solubilization on artificial growth media. Accordingly, plants supplied with nitrate fertilization were severely P deficient and the weak host plants were unable to establish a functional association with the microbial inoculants. By contrast, stabilized ammonium fertilization triggered root extrusion of protons for charge balance of ammonium uptake, associated with rhizosphere acidification, contributing to P solubilization. This increased the P-nutritional status and vitality of the host plants, which enabled the establishment of PGPMs in the rhizosphere. Interestingly in this scenario, the contribution of the PGPM inoculants to plant P acquisition was only marginally expressed but the PGPMs stimulated root development, contributing to an improved nutrient acquisition in general (Chapter 4.1). 
A closer look on the related modes of action (Chapter 4.2) revealed that ammonium fertilization stimulated the production of auxin as a key regulator for root growth, both, by the bacterial inoculants and by the roots of the host plants. While ammonium supply without PGPM inoculants had no effects on total root length, the length of the root hairs and the diameter of rhizosheaths formed by root hair-adhering soil was increased, leading to an extension of the root surface area involved in rhizosphere acidification and spatial acquisition of nutrients. Moreover, root hairs have been reported as preferential infection sites for various inoculants investigated in this study, and accordingly increased root colonization of the fungal inoculant Trichoderma harzianum OMG16 was recorded in combination with ammonium fertilization. By contrast, there was no evidence for increased organic acid production or a contribution of the inoculants to the acquisition of organic P sources by the release of phosphohydrolases in the investigated strains. Increased rhizosphere acidification after PGPM inoculation in combination with ammonium fertilization was observed exceptionally only in one experiment conducted on a moderately acidic sandy soil with a low buffering capacity. However, soil pH was identified as a critical factor determining the expression of the synergistic PGPM-ammonium effects on Ca-P solubilization, which declined with increasing soil pH (Chapter 4.3). Highly-buffered calcareous soils counteracted ammonium-induced rhizosphere acidification and P mobilization as a pre-requisite for PGPM-establishment in the rhizosphere. Under these conditions, successful experiments with applications of granulated fertilizers, based on stabilized di-ammonium phosphate and PGPM inoculants, suggest that placement of starter fertilizers leading to a more concentrated ammonium effect may offer an option to overcome this problem. First field experiments suggested that beneficial effects of ammonium-assisted PGPM inoculation on P acquisition can be expected particularly on soils with low P availability and the approach was patented in 2018.
As a second approach, the combination of PGPMs with fertilizers based on products of organic waste recycling, such as municipal waste compost or composted poultry manure (PM compost), applied with the same P dose, were investigated with tomato as model plant on low P soils with contrasting pH in Ghana (Chapter 5). Interestingly, on both soils, PGPM inoculation increased the P use efficiency and early plant growth only in the combination of compost with PM but not with sole compost application. Additional supplementation with ammonium on the moderately acidic soil increased plant biomass production in PGPM inoculated plants to the same level as soluble superphosphate fertilization. Similar to the ammonium-PGPM combinations, root growth stimulation was a major PGPM effect, which improved nutrient acquisition in general. Large-scale greenhouse and open-field tomato production trials conducted in Romania and Hungary revealed reproducible effects on yield and fruit quality over three years by PGPM combinations with manure-based fertilizers (Chapter 6).
Taken together, the thesis demonstrated that the selection of compatible combinations of fertilizers and PGPM inoculants is an essential factor for the successful establishment of beneficial plant-PGPM interactions in the rhizosphere. Combinations with stabilized ammonium fertilizers or with products based on organic waste recycling, such as composted manures, have been identified as two promising examples with potential for the development of PGPM-assisted production systems.

KW  - Phosphor
CY  - Hohenheim
PB  - Kommunikations-, Informations- und Medienzentrum der Universität Hohenheim
AD  - Garbenstr. 15, 70593 Stuttgart
UR  - http://opus.uni-hohenheim.de/volltexte/2020/1750
ER  -