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 (FZB42) 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.
Highlights
The world’s total mineral NPK fertilizer consumption is projected to reach approximately225,000,000 metric tons by 2030, representing an average increase of 20% against levels recorded in 2010 [1], with high demands in horticultural production systems
At 49 days after sowing (DAS), both, compost and poultry manure (PM)-compost fertilizers increased plant growth (Figure 1) and shoot biomass production (Figure 2) to a similar extent (+82%) as compared with the unfertilized control (NoFert) variant, but did not reach the biomass of the plants supplied with mineral nitrate +
Inoculants (CFB) significantly increased the shoot biomass production by 123% in the unfertilized variants, and by 42% in the compost amended with poultry manure (p < 0.05)
Summary
The world’s total mineral NPK fertilizer consumption is projected to reach approximately225,000,000 metric tons by 2030, representing an average increase of 20% against levels recorded in 2010 [1], with high demands in horticultural production systems. Approaches to convert waste materials into resources, and using fertilizers based on organic and inorganic waste materials could offer more sustainable and cost-saving perspectives. Apart from potential contaminants (i.e., heavy metals, antibiotics, abundance of antibiotic resistance genes, or pathogenic microorganisms), the low solubility of plant nutrients, and the large proportions of nutrients sequestered in organic binding forms that are not readily available for plant uptake, are major challenges for the use of organic and inorganic waste materials as fertilizers in agricultural and horticultural practice [4,5]. A fertilization management plan that is adapted to the crop demand is even more complicated as compared with mineral fertilizers, and is associated with a high risk of nutrient losses into the environment [4] and low fertilizer use efficiency
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