The Form of N Supply Determines Plant Growth Promotion by P-Solubilizing Microorganisms in Maize

Abstract

Phosphate-(P)-solubilizing microorganisms (PSM) are important drivers of P cycling in natural and agro-ecosystems. Their use as plant inoculants to improve P acquisition of crops has been investigated for decades. However, limited reproducibility of the expected effects, particularly under field conditions, remains a major challenge. This study demonstrates that the form of nitrogen fertilization has a significant impact on the performance of various fungal and bacterial PSM inoculants in maize grown on neutral to alkaline soils with limited P availability. Under these conditions, a high soil pH-buffering capacity frequently limits the efficiency of nutrient mobilization, mediated by plant roots and microorganisms via rhizosphere acidification. In a soil pH range between 7.0 and 8.0, nitrate fertilization promoting rhizosphere alkalinisation further aggravates this problem. Accordingly, in greenhouse experiments, six strains of Pseudomonas, Bacillus, Paenibacillus, Streptomyces, and Penicillium with proven P-solubilizing potential, completely failed to promote P acquisition in maize grown on a calcareous Loess sub-soil pH 7.6 with nitrate fertilization and rock phosphate (Rock-P) as a sparingly soluble P source. However, after replacement of nitrate fertilization by ammonium, stabilized with the nitrification inhibitor 3,4-dimethylpyrazole-phosphate (DMPP), five out of seven investigated PSM inoculants (comprising 12 fungal and bacterial PSM strains) exerted beneficial effects on plant growth and reached up to 88% of the shoot biomass production of a control supplied with soluble triple-superphosphate (TSP). Stabilized ammonium combined with PSM-inoculants improved P acquisition (Trichoderma harzianum T22, Pseudomonas sp. DMSZ 13134), while other strains particularly stimulated root growth (T. harzianum OMG16, Bacillus amyloliquefaciens FZB42), which promoted the acquisition also of other mineral nutrients, such as N, K, and Mn. A similar effect was recorded under field conditions on an alkaline clay-loam soil pH 8.6. The combination of stabilized ammonium with a range of consortium products based on T. harzianum OMG16, B. amyloliquefaciens, micronutrients, and humic acids completely compensated the effect of a TSP fertilization on field establishment, nutrient acquisition, and yield formation in maize, while non-stabilized urea-di-ammonium phosphate fertilization was largely ineffective. These findings suggest that the efficiency of PSM-plant interactions can be influenced by the form of N fertilization, offering promising perspectives for synergistic effects with stabilized ammonium fertilizers.

The role of N form supply for PGPM‐host plant interactions in maize

“…Similar results were reported for P acquisition, plant growth promotion and yield formation of tomato in a drip-irrigated tomato production system with ammonium placement and inoculation of microbial consortia in the Negev desert in Israel (Bradá č ová et al, 2019). The results of Mpanga et al (2019) suggested that the beneficial effects of stabilized ammonium fertilization on PGPM per-formance might be at least partially attributed to an improved root-induced mobilization of P and other sparingly soluble nutrients by the well documented rhizosphere acidification effect of ammonium fertilizers (Neumann and Rö mheld, 2002). Particularly on low P soils with neutral to alkaline pH or after application of sparingly soluble Rock-P fertilizers, this resulted in a starter fertilization effect, improving the vitality status of the plants.…”

“…In the latter context, the preferential perform-ance of PGPMs in combination with N-rich organic fertilizers has been repeatedly reported (Abbasi et al, 2015;Thonar et al, 2017;Mpanga et al, 2018;Bradá č ová et al, 2019). Recently, Mpanga et al (2019) found an improved plant performance associated with a wide range of fungal and bacterial PGPMs in combination with stabilized ammonium fertilizers as compared with nitrate fertilization. Tab.…”

“…Recently, Mpanga et al. () found an improved plant performance associated with a wide range of fungal and bacterial PGPMs in combination with stabilized ammonium fertilizers as compared with nitrate fertilization. Tab.…”

“…Therefore, this study aims at a more detailed dissection of the obvious synergistic effects of ammonium fertilization and PGPM inoculation. Based on literature reports ( Nkebiwe et al., ; Mpanga et al., ), we investigated a range of hypotheses on the potential functions of ammonium fertilization in the establishment of plant‐PGPM interactions: (1) ammonium fertilization induces root‐induced rhizosphere acidification, which may be intensified by PGPM inoculation with a beneficial impact on nutrient availability; (2) a lower rhizosphere pH stimulates rhizosphere acid phosphatase activities involved in mobilization of organic P sources; (3) auxin production and thereby the root growth‐promoting potential of PGPMs is stimulated with ammonium as the most abundant N source; (4) ammonium fertilization has direct beneficial effects on root growth in terms of lateral roots and root hair proliferation. This will promote spatial nutrient acquisition, nutrient mobilization via root exudates and rhizosphere acidification, as well as root colonization by PGPMs.…”

“…Tab. 1 summarizes the effects in maize (Nkebiwe, 2016;Mpanga et al, 2019), wheat (Nkebiwe, 2016), and tomato (Mpanga et al, 2018;Bradá č ová et al, 2019). Accordingly, Nkebiwe et al (2016) reported increased root proliferation and plant growth promotion by PGPM inoculation combined with ammonium fertilization in maize, both, in laboratory and field experiments.…”