Abstract
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 HPO4 2−/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. 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). 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. Results show promising growth-effects of Pseudomonas sp. DSMZ 13134 on field-grown maize.
Highlights
Among other responses, plants tend to increase root growth to scavenge nutrients from more soil when soil nutrient concentrations are low
We propose that root colonization by plant growth-promoting microorganisms (PGPMs) can be enhanced if PGPMs are inoculated in rhizosphere “hotspots,” developing around NH4-Depot stabilized concentrated (NH4)+based fertilizer depots, due to NH4+-induced dense root growth and high levels of organic nutrients for microbes released as root exudates [15, 27]
(3) Survival and colonization of inoculated PGPMs is higher in the “rhizosphere hotspot” than in the comparable soil volume with respect to plant position that is supplied homogeneously with NO3− fertilizer
Summary
Plants tend to increase root growth to scavenge nutrients from more soil when soil nutrient concentrations are low. Of increasing importance in sustainable agriculture systems is the effective use of crop bio-stimulants [1, 2] and/ or plant growth-promoting microorganisms (PGPMs). Central to the concept of sustainable agriculture is the reduction of environmental costs associated with farming Among others, it requires responsible use of chemical fertilizers. It requires responsible use of chemical fertilizers This can be achieved through use of suitable fertilizer types and application rates timed to crop demand, seasons, and weather conditions with low risk of fertilizer loss to the environment. Fertilizer placement in soil improves fertilizer acquisition by target crop plants as opposed to weeds [16, 17] and reduces the risk of nutrient loss to the environment. Fertilizer placement can improve nutrient content in crop above-ground biomass as well as crop yield in comparison to conventional fertilizer broadcast (meta-analysis with 40 field studies, Nkebiwe et al 2016, submitted unpublished observation)
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