Improving fertilizer-depot exploitation and maize growth by inoculation with plant growth-promoting bacteria: from lab to field

BACKGROUND: Among other responses, plants tend to increase root growth to scavenge nutrients from more soil when soil nutrient concentrations are low. Placement of fertilizers near seeds or roots facilitates nutrient acquisition by target crop plants. Nevertheless, nutrient uptake from soil-placed fertilizer-depots depends on increased uptake rates and efficient spatial exploitation of the depot by roots. The aim of our study was to optimize exploitation of subsurface fertilizer-depots by inoculating the depot zone with promising plant growth-promoting microorganisms (PGPMs) as bio-effectors. If included in depots, root-attracting NH₄⁺ or HPO₄²⁻/H₂PO₄⁻ ions may also enhance rooting within the depot, which in turn improves survival and root colonization by inoculated PGPMs; a consequence of high levels of microbial nutrients exuded in densely rooted soil. METHODS: We tested maize (Zea mays L.) in two greenhouse (pot and rhizobox) and two field experiments (2014 and 2015). A core treatment was NH₄⁺-fertilizer placed as a subsurface depot (Depot). In the field, there was also NH₄⁺-fertilizer broadcasted and incorporated in soil (Broad). Depot and Broad were each with PGPM as bio-effector (BE) or without (NoBE). Bio-effectors included: Pseudomonas sp. DSMZ 13134 (BE1) and Bacillus amyloliquefaciens FZB42 (BE2, only in field trials). RESULTS AND DISCUSSION: In pots, Depot with BE1 led to 59 % higher shoot dry matter, 50 % higher shoot N content, and 64 % higher shoot P content than without PGPM. In rhizoboxes, higher root length density (RLD), lower rhizosphere pH, and higher BE1-colonization rate were measured in the fertilizer depot compared to the corresponding zone for controls with homogenous NO₃⁻ supply. Depot led to higher shoot N and P concentrations (+26.6 % N; +20.6 % P) and contents (+11.1 % N; +17.6 % P) than control. BE1 led to higher shoot N concentration (+13.5 %) than NoBE. In the field, fertilizer-depot soil had higher RLD than corresponding non-depot soil. BE1 led to doubled fertilizer-depot RLD in comparison to without (2014). In 2014, Depot led to 7.4 % higher grain yield than Broad (not statistically significant), whereas BE broadcast had no effect. In 2015, Depot led to 5.8 % higher fresh shoot biomass than Broad, below-seed placement of BE1 led to higher fresh (+7.1 %) and dry (+8.0 %) shoot biomass than NoBE. CONCLUSION: Results show promising growth-effects of Pseudomonas sp. DSMZ 13134 on field-grown maize. Graphical abstract Rhizosphere hotspots around placed fertilizer-depots promote PGPR establishement, nutrient aquisition and plant growth