Plant survival depends on the ability of roots to sense and acquire nutrients in soils, which harbor a rich diversity of microbes. A subset of this microcosm interacts with plant roots and collectively forms root-associated microbial communities, termed the root microbiota. Under phosphorus-limiting conditions, some plants can engage in mutualistic interactions, for example with arbuscular mycorrhizal fungi. Here, we describe how Arabidopsis thaliana, which lacks the genetic capacity for establishing the aforementioned symbiosis, interacts with soil-resident bacteria and fungi in soil from a long-term phosphorus fertilization trial. Long-term, contrasting fertilization regimes resulted in an ∼6-fold and ∼2.4-fold disparity in bioavailable and total phosphorous, respectively, which may explain differences in biomass of A. thaliana plants. Sequencing of marker genes enabled us to characterize bacterial and fungal communities present in the bulk soil, rhizosphere, and root compartments. Phosphorus had little effect on alpha- or beta-diversity indices, but more strongly influences bacterial and fungal community shifts in plant-associated compartments compared with bulk soil. The significant impact of soil P abundance could only be resolved at operational taxonomic unit level, and these subtle differences are more pronounced in the root compartment. We conclude that despite decades of different fertilization, both bacterial and fungal soil communities remained unexpectedly stable in soils tested, suggesting that the soil biota is resilient over time to nutrient supplementation. Conversely, low-abundance, root-associated microbes, which collectively represent 2 to 3% of the relative abundance of bacteria and fungi in the roots, exhibited a subtle, yet significant shift between the two soils. [Formula: see text] Copyright © 2018 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Phosphorus (P) fertilizer recommendations in most European countries are based on plant-available soil P contents and long-term field experiments. Site-specific conditions are often neglected, resulting in excessive P fertilizer applications. P fertilization experiments including relevant site and soil parameters were evaluated in order to analyze the yield response. The database comprises about 2000 datasets from 30 field experiments from Germany and Austria. Statistical evaluations using a classification and regression tree approach, and multiple linear regression analysis indicate that besides plant-available soil P content, soil texture and soil organic matter content have a large influence on the effectiveness of P fertilization. This study methodology can be a basis for modification and specification of existing P fertilization recommendations and thus contribute to mitigate environmental impacts of P fertilization.Electronic supplementary materialThe online version of this article (10.1007/s13280-017-0971-1) contains supplementary material, which is available to authorized users.

Soil bacteria belonging to the genus Azotobacter, Pantoea and some unidentified soil isolates were tested in vitro for phytohormone production under laboratory and soil conditions. The German wheat variety Munk was inoculated by several soil bacteria with exogenously applied hormones (IAA, 2,4-D) and a flavonoid (naringenin) with a half of the amount of recommended doses of fertilizers under greenhouse conditions. Most of the soil bacteria tested were able to produce indole acetic acid (IAA), and stimulated a lateral root development and colonization by the addition of 2,4-D and IAA. A formation of paranodules on roots as a result of crack entry invasion was observed with 2,4-D as well as with IAA. We were able to reisolate the organism from the paranodules and could establish the same results. Analyses for root exudates and in vitro phytohormone production by various bacterial isolates were also carried out, revealing that 2,4-D can be replaced either by high IAA producing bacteria or by exogenous application of IAA. Bacterial survival in the rhizosphere as well as the root and shoot weight of wheat plants were positively affected also by the addition of IAA, 2,4-D and naringenin. A. brasilense (Sp7) partly due to the development of a protected niche. These paranodule structures derived from the induction of the initials of the lateral roots are quite dissimilar to root nodules particularly when colonized by non-symbiotic bacteria. Protoplasts of the bacteria (L-forms of Azotobacter, Pseudomonas syringae and Bacillus polymyxa) were suggested to have the ability to penetrate the cell wall and membrane structures of living plant cells and to colonize plant tissues (Cocking et al. 1990). Also some microorganisms colonize in the intercellular spaces as seen in apoplast of stem of sugarcane (Cocking 2003). Despite encouraging results with Azospirillum, Azotobacter etc., some controversy still exists about the mechanism of bacterial root interactions. Reports suggest that the colonization of these bacteria is caused by factors like N 2 fixation, siderophores, ammonia excretion, phytohormones (Lakshminarayana 1993) and antifungal properties etc. (Verma et al. 2001), collectively enhancing the root proliferation, increase in the lateral roots and root hair formation. Dobbelaere et al. (1999) suggested that plant growth substances are one of the key factors observed in plant growth promotion. Tien et al. (1979) reported that the morphological changes due to bacterial inoculations can be mimicked by applying a combination of plant growth substances.Keeping all these factors in mind, we made an attempt to screen different soil bacteria for the production of various phytohormones, their colonization on German winter wheat (var. Munk) with or without exogenously applied natural and synthetic plant hormones and the concomitant changes in root morphology or formation of paranodules under controlled greenhouse conditions in addition to plant growth parameters. Natural hormone like IAA, synthetic-2,4-D (2,4-dichlorophen...

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