Bacterial Subspecies Variation and Nematode Grazing Change P Dynamics in the Wheat Rhizosphere

Usman Irshad,Etienne Yergeau

doi:10.3389/fmicb.2018.01990

Abstract

Low phosphorus soils are thought to constitute the majority of soils worldwide and cannot support intensive agriculture without high fertilizer inputs. Rhizobacteria are well-known to modify P dynamics and an increased bacterial diversity normally has a positive impact on various process rates. However, it is not known how variation in bacterial diversity at the subspecies level could influence trophic interactions in the rhizosphere and its consequences on plant P nutrition. We therefore hypothesized that the interactions between closely related P solubilizing bacteria and their grazing nematodes could improve plant P dynamics from an unavailable P source. We isolated four Pseudomonas poae strains and extracted nematodes from a Saskatchewan wheat field soil sample. The potential of all bacterial isolates with and without nematodes for increasing P availability in the wheat rhizosphere was tested in controlled microcosms with Ca3(PO4)2 as sole P source. Liberated P, phosphatase activity, plant P and bacterial abundance based on phnX gene copies were determined. Phosphorus solubilization efficiency of isolates varied between isolates whereas phosphatase enzyme activity was only detected under nematodes grazing and during the first 15 days of the experiment. Nematodes grazing upon individual Pseudomonas poae increased phosphatase enzyme activity, bacterial abundance, but decreased plant P concentration compared to non-grazed system. In contrast, the treatment combining all Pseudomonas poae isolates together with nematodes resulted in significant increases in P availability and plant P concentration. Diverse P-solubilizing efficiency and interaction with nematodes within the same bacterial “species” suggest that P dynamics might be linked to micro variation in soil diversity that would not accurately be picked up using common tools such as 16S rRNA gene sequencing.

Highlights

Low phosphorus soils are thought to constitute the majority of soils worldwide
Nematodes and P in Wheat Rhizosphere of cultivable microbes can solubilize inorganic P and release plant utilizable forms directly into solution (Nazir et al, 2017), efforts to improve efficiency of P use are focused on microorganisms
Nematodes increased in numbers during the 2 months of the experiment, indicating that they were feeding on the inoculated bacteria

Summary

Introduction

Low phosphorus soils are thought to constitute the majority of soils worldwide. Soil P consists of inorganic forms, such as rock phosphate and organic forms derived from plants, animals and microbial biomass decay (Behera et al, 2014). Even when the phosphorus inputs are very high, less than 5% of the soil P is bioavailable to plants (Brady and Weil, 2008). Nematodes and P in Wheat Rhizosphere of cultivable microbes can solubilize inorganic P and release plant utilizable forms directly into solution (Nazir et al, 2017), efforts to improve efficiency of P use are focused on microorganisms. Many P-solubilizing bacteria are found in the Pseudomonas and Bacillus genera (Illmer and Schinner, 1992; Irshad et al, 2011), and they generally mineralize/solubilize P either by secreting organic acids or phosphatases (Gugi et al, 1991; Lemanowicz, 2011). The capacity of Pseudomonas to transform P into bioavailable forms through mineralization or solubilization varies depending on the species and substrates. Different Pseudomonas species can solubilize from 52 to 156 mgL−1 Ca3(PO4) (Illmer and Schinner, 1992). It has been previously shown that closely related isolates, often from the exact same bacterial species, differed significantly in their capacity to solubilize P (Zabihi et al, 2011; Singh et al, 2014; Jiang et al, 2017)

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