Densely rooted rhizosphere hotspots induced around subsurface NH4+-fertilizer depots: a home for soil PGPMs?

Background: Populations of plant growth-promoting microorganisms (PGPMs) inoculated in natural soil typically decline over time due unfavourable biotic and/or abiotic factor(s). Improving subsurface root density may enhance PGPM establishment due to high concentrations of organic nutrients released as root exudates. Placing subsurface root-attracting NH4+-fertilizer depots may form such zones of dense localized rooting (rhizosphere hotspots) that can enhance PGPM survival. Nevertheless, required soil conditions that favour formation of rhizosphere hotspots are unknown. This study aimed to investigate: (1) background soil N-min effect on NH4+-depot-zone root growth; (2) PGPM tolerance to high NH4+ concentrations (+/- nitrification inhibitor, DMPP); (3) ability to solubilize sparingly soluble inorganic phosphates; (4) and establishment in a subsurface NH4+-depot. Methods: We conducted a greenhouse rhizobox experiment using spring wheat (Triticum aestivum L.) to investigate the effect of background Nmin (0, 5, 20 and 60 mg N kg(-1)) on root growth around a 1 g NH4+-N depot. The tolerance of selected PGPMs to 0, 2, 10, 50, 250, 1250 mM NH4-N and 0, 0.1, 1 and 3 M NH4-N +/- DMPP was investigated via in vitro culture tests. Promising candidates were further tested for solubilization of tri-calcium phosphate (Ca-P), rock phosphate (RP) or sewage sludge ash (SA). One PGPM was inoculated in a N-15-labelled (NH4)(2)SO4 + DMPP depot in rhizobox-grown maize (Zea mays L.) and root colonization was measured. Results and discussion: N-min 5-20 mg N kg(-1) improved depot-zone root growth, whereas 60 mg N kg(-1) reduced it. Tested PGPMs were tolerant to up to 1250 mM NH4-N. Pseudomonas sp. DSMZ 13134 and B. amyloliquefaciens FZB42 (not Trichoderma harzianum T-22) solubilized Ca-P and RP via acidification, whereas SA was not solubilized despite marked acidification. Placed 15N-labelled (NH4)(2)SO4 + DMPP depot led to increased localized rooting, rhizosphere acidification, shoot N-15 signal, N and P concentrations and contents than homogenously applied Ca(NO3)(2). Inoculation of Pseudomonas sp. DSMZ 13134 tended to increase shoot N and P concentrations, and shoot N content relative to the control. Higher colonization of Pseudomonas sp. DSMZ 13134 was measured in soil around the NH4+-depot than in corresponding soil zone in treatments with NO3-. Conclusion: These results show the first promising effects of combining fertilizer placement and application of P-solubilizing PGPMs on plant growth.