Roles of Phosphate Solubilizing Microorganisms from Managing Soil Phosphorus Deficiency to Mediating Biogeochemical P Cycle

Abstract

Phosphorus (P) is a vital element in biological molecules, and one of the main limiting elements for biomass production as plant-available P represents only a small fraction of total soil P. Increasing global food demand and modern agricultural consumption of P fertilizers could lead to excessive inputs of inorganic P in intensively managed croplands, consequently rising P losses and ongoing eutrophication of surface waters. Despite phosphate solubilizing microorganisms (PSMs) are widely accepted as eco-friendly P fertilizers for increasing agricultural productivity, a comprehensive and deeper understanding of the role of PSMs in P geochemical processes for managing P deficiency has received inadequate attention. In this review, we summarize the basic P forms and their geochemical and biological cycles in soil systems, how PSMs mediate soil P biogeochemical cycles, and the metabolic and enzymatic mechanisms behind these processes. We also highlight the important roles of PSMs in the biogeochemical P cycle and provide perspectives on several environmental issues to prioritize in future PSM applications.

Keywords: P biogeochemical cycle; P forms; phosphate solubilizing microorganisms; soil P.

Figure 1

A schematic diagram of soil phosphorus (P) biogeochemical cycles. The red arrows indicate geochemical or geophysical P reactions and cycles. The yellow arrows indicate phosphate solubilizing microorganisms (PSM)-induced P reactions and cycles. The white arrows indicate P flows linking plants, streams, and groundwater.

Figure 2

A schematic diagram of possible inorganic P (P_i) solubilization mechanisms in phosphate solubilizing microorganisms (PSM). Arrows of different colors indicate the possible excretion agents by phosphate solubilizing fungi (PSF, purple), phosphate solubilizing bacteria (PSB, pink), phosphate solubilizing actinomycetes (PSA, aqua), and cyanobacteria (cyan), respectively.

Figure 3

Simplified representation of phosphate solubilizing microorganisms (PSM)-induced dissolution to accelerate lead (Pb) precipitation to form secondary inorganic P (P_i) minerals. ① PSM can dissolve Pi minerals to soluble orthophosphate by releasing organic acids. ② Pb ions in the solution are precipitated to Pb–acid complexes by reacting with organic acids that are released by PSM. ③ In the presence of orthophosphate (H₂PO₄^–, HPO₄^2–), Pb ions are precipitated to the relatively stable pyromorphite. ④ Pb ions in the solution are biosorbed to PSM surface due to the negative charges of the surface functional groups.

Figure 4

A schematic diagram of possible phosphorus (P) and sugar transport in roots, arbuscular mycorrhizal (AM), and phosphate solubilizing microorganisms (PSM) [151,152]. Arrows of different color indicate the possible reaction locations of roots (brown), AM (blue), and PSM (purple). Letters of different color indicate the possible sugar reactions (blue), P cycle (red), and microbial metabolism of PSM (purple), respectively. P_i for inorganic P, P_o for organic P, ST for sugar transporter, PSS for protein secretory system, and NSAPs for non-specific acid phosphatases.