Microbes and Beneficial Microbes

Attempts to use soil microorganisms with potential for nutrient mobilization as inoculants to improve nutrient acquisition of

crops have a long history. However, a major limitation of these approaches is a frequently limited reproducibility of eff ects

under practical conditions. Testing a range of commercial products based on Bacillus, Pseudomonas, Trichoderma and Penicillium

strains, revealed that the expression of their plant growth promoting potential on maize was strongly dependent on the form of

nitrogen supply. Superior performance was recorded in combination with ammonium-based fertilizers, stabilized by nitrifi cation

inhibitors instead of nitrate dominated fertilization. A closer examination of the ammonium eff ects revealed a stronger stimulation

of root growth induced by the inoculants, associated with an ammonium-induced elongation of root hairs. Accordingly, ammoniumfertilized

maize plants showed higher endogenous auxin (IAA) levels and increased auxin-production potential was demonstrated for

Bacillus and Pseudomonas strains in presence of stabilized ammonium, both, on artifi cial growth media and aft er-isolation from the

rhizosphere. Higher root colonization of maize plants supplied with stabilized ammonium was recorded for a Trichoderma harzianum

strain. Moreover, the synergistic eff ect of microbial and ammonium-induced root growth promotion resulted in a larger root surface

area, involved in rhizosphere acidifi cation due to proton-extrusion in response to ammonium uptake, mediating the mobilization of

Ca-phosphates and micronutrients such as Zn and Mn. Finally, ammonium fertilization suppressed root infection by the seed-borne

pathogenic fungus Fusarium proliferatum. Th e fi ndings demonstrated that plant-PGPM interactions are strongly dependent on the

form of N fertilization off ering management options, which have been recently patented.