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

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.

Internal nitrogen and phosphorus loading in a seasonally stratified reservoir: Implications for eutrophication management of deep-water ecosystems

Water eutrophication is a serious global issue because of excess external and internal nutrient inputs. Understanding the intensity and contribution of internal nitrogen (N) and phosphorus (P) loading in deep-water ecosystems is of great significance for water body eutrophication management. In this study, we combined intact sediment core incubation, high-resolution peeper (HR-Peeper) sampling, and analysis of N and P forms and other environmental factors in the water column and sediments to evaluate the contributions of internal N and P loading to water eutrophication by N and P fluxes across the sediment-water interface (SWI) of the Panjiakou Reservoir (PJKR), a deep-water ecosystem where eutrophication threatens the security of the local drinking water supply in North China. The results indicated that the PJKR showed obvious thermal and dissolved oxygen (DO) stratification in the warm seasons and full mixing in the cold seasons. The mean DO concentration was 9.9 and 3.55 mg/L in the epilimnion and hypolimnion, respectively, in warm seasons and 10.7 mg/L in cold seasons. The sediment acted as a source of soluble reactive phosphorus (SRP), NH₄⁺-N, and NO₂^--N and a sink of NO₃^--N. The SRP fluxes were 5.28 ± 4.34 and 2.30 ± 1.93 mg m^-2·d^-1 in warm and cold seasons, respectively, and the dissolved inorganic nitrogen (DIN) fluxes were -0.66 ± 47.84 and 44.04 ± 84.05 mg m^-2·d^-1. Seasonal hypoxia accelerated the release of P rather than N from the sediments in warm seasons, which came mainly from Fe-P and Org-P under anoxic conditions. The strong negative NO₃^--N flux (diffusion from the water column to the sediment) implied an intensive denitrification process at the SWI, which can counteract the release flux of NH₄⁺-N and NO₂^--N, resulting in the sediment acting as a weak dissolved inorganic nitrogen (DIN) source for the overlying water. We also found that internal N loading accounted for only ∼9% of the total N loading, while internal P loading accounted for 43% of the total P loading of the reservoir. Our results highlight that efforts to manage the internal loading of deep-water ecosystems should focus on P and seasonal hypoxia.

Long-term cover crops improved soil phosphorus availability in a rain-fed apple orchard

The objective of this present study was to understand the distribution patterns of various forms of soil phosphorus (P) and the biotic and abiotic factors affecting the soil P fractions under long-term cover crops. Here, we investigated the characteristics of soil P forms, community structure of P-solubilizing bacteria (using 16S rRNA) and the related enzyme activity under clean tillage (CT), 14 years of white clover (WC, Trifolium repens L.) and orchard grass (OG, Dactylis glomerata L.) cover crops in a rain-fed apple orchard on the Weibei Loess Plateau, China. Relative to CT treatment, long-term cover crops enhanced the bioavailability of soil P by increasing the contents of total phosphorus (TP), microbial phosphorus (MBP), organic phosphorus (Po) and certain forms of inorganic phosphorus (e.g. Al-P, Ca₂-P, Ca₈-P and Fe-P) in the surface soil, in addition, WC treatment also increase the available P (AP) contents in the topsoil. A redundant analysis (RDA) showed that soil organic matter (SOM), NH₄⁺-N and pH were the key environmental factors affecting the morphological changes of soil P. In addition, the effects of long-term cover crops on soil P forms were mainly concentrated in the topsoil, and the WC treatment had a greater impact on soil P composition than the OG treatment. Interestingly, long-term cover crops effectively increased the abundances of P-solubilizing bacteria, such as Streptomyces, Sphingomonas, Nocardioides and Haliangium, and enhanced the alkaline phosphatase (ALP) activity. Overall, long-term cover crops were an effective strategy to activate soil P as they improve the soil environment.

Investigation of differential levels of phosphorus fixation in dolomite and calcium carbonate amended red soil

BACKGROUND

The pH adjustment of acidic red soils with lime materials is beneficial for the reduction of phosphorus (P) fixation. However, the reasons for varying levels of P activation after adding different lime materials have not been fully investigated. Therefore, this study examined changes in soil labile P and P forms after phosphate application to calcium carbonate (CaCO₃ ) and dolomite amended red soil during a 120-day incubation period. Also change of P sorption properties in the amended soil samples from day 120 were examined through a sorption-desorption experiment.

RESULTS

The increase of soil H₂ O-P and NaHCO₃ -P in the CaCO₃ and dolomite amended soil treatments was mainly ascribed to the decline of the NaOH-P. However, when compared with the control treatment after 120 days, soil Olsen-P significantly increased by 34% and 66% in the CaCO₃ and dolomite treatments. The Hedley P fractionation results demonstrated that the CaCO₃ application caused a notable increase of HCl-P (stable Ca-P), which was 88.4% higher than that in the dolomite treatment. However, the formation of stable P was strongly suppressed in the dolomite treatment due to the presence of magnesium (Mg), which was identified by the negative relationship between M3-Mg and HCl-P. In line with these findings, P sorption-desorption work showed weaker P binding energy in the dolomite treatment relative to the CaCO₃ treatment.

CONCLUSION

In terms of increasing P availability in red soil, this study suggests that dolomite should be used to substitute CaCO₃ in order to reduce the soil P fixation. © 2021 Society of Chemical Industry.

Soil organic matter and phosphorus dynamics after low intensity prescribed burning in forests and shrubland

Various different factors have led to the accumulation of biomass in forest soils in the Mediterranean-climate region in the last few decades, thus exacerbating the effects of wildfires. Although prescribed burning is used to decrease the fuel load and reduce the currency of mega-wildfires, the impacts on soil organic matter (SOM) and nutrient cycling, and therefore on forest ecosystem sustainability, are uncertain. The present study was designed to cover a range of conditions and therefore to assess the variability in the responses in similar geographical areas. Three prescribed burning treatments producing different levels of soil burn severity were conducted in two different types of forests (Pinus nigra and Pinus pinaster) and one (previously treated by prescribed burning) shrubland ecosystem (Cytisus oromediterraneus), all characterized by different fuel loads and depths of soil organic layer, in Central Spain. After the treatments, the SOM content, its thermal properties, and the distribution of Phosphorus (P) forms (³¹P NMR spectroscopy) were measured in the soil organic layer and mineral soils (0-2 cm depth), and the results were related to the temperatures reached. The prescribed burning les to low-moderate perturbations in SOM quality and Carbon (C) and P dynamics. The organic P, which in the unburnt plots represented 70% of the extractable P, was greatly depleted (by 56 and 95% with respect the initials values). This effect was concurrent with decreases in the most thermolabile SOM fractions, suggesting that organic P is readily mineralized, even at relatively low temperatures. Release of large amounts of soluble orthophosphate may occur when the prescribed burning leads to a high level of soil burn severity. The findings show that prescribed burning treatments should be planned carefully in order to prevent long-term perturbation of C and P cycling.

The spatio-temporal change in soil P and P-solubilizing bacteria under clover mulching in apple orchards of Loess Plateau

Cover crop is an effective practice for improving soil quality and increase soil nutrients. However, the spatio-temporal change of soil phosphorus (P) components and P-solubilizing microorganisms in the process of grass succession is not evident. Here, we studied the variation of soil P components and P-solubilizing bacteria at 0-60 cm soil layer under clean tillage (CT) and white clover (WC, Trifolium repens L.) grown for 5, 9, and 14 years in an apple test station on the Loess Plateau, China. This study suggested that clover cover could effectively increase the total P, available P (AP), microbial P, organic P (Po), and inorganic P (Al-P, Ca₂-P, Ca₈-P and Fe-P) in topsoil (0-20 cm) and AP, Po and inorganic P at 20-40 cm soil layer to improve the soil P bioavailability. The effects of WC living mulch on the soil P forms were more significant with the increase in grass growing years, but this effect was difficult to extend to deep soil. In addition, the WC treatments were beneficial to the growth of P-solubilizing microorganisms in surface soil and improved the alkaline phosphatase activity at 0-40 cm soil layer, mainly including Bacillus, Bradyrhizobium, Nocardioides, Sphingomonas and Streptomyces. This study provided a perspective on the dynamic changes of soil P forms and P-solubilizing microorganisms and under long-term cover crop.

Changes of riparian soil-plant system phosphorus responding to hydrological alternations of Three Gorges Reservoir

The anti-seasonal hydrological alternation in the Three Gorges Reservoir (TGR) significantly impacts the release of phosphorus (P) from the riparian soil-plant system, posing a threat to the aquatic environment. To investigate this issue, riparian soils and plants in three tributaries of the central TGR were sampled at three distinct stages: early exposure, final exposure, and soon after inundation receded. Soil properties, P forms, and plant P content were analyzed. A significant decrease in exchangeable P and organic P during exposure, and a decrease in aluminum/iron-bound P during inundation were observed. These changes were linked to the mineralization of organic matter and the reduction of iron oxides. Compared to bioavailable inorganic P, bioavailable organic P contributed more to the total soil P release during the exposure-inundation cycle. Plant P uptake accounted for 76.08% of the bioavailable P released by the soil during exposure. During inundation, plant P release significantly exceeded soil P release. Therefore, the soil-plant system could act as a P "sink" during exposure and a P "source" during inundation. The hydrological alternation of the TGR was the primary driver of this "source-sink" transformation. To mitigate P release in riparian zones, recycling plant materials and establishing monitoring sites are recommended.

Carbon-containing additives changes the phosphorus flow by affecting humification and bacterial community during composting

Phosphorus recycling from organic wastes to prepare a fertilizer by composting is promising. The aim of this study was to compare the effect of diverse carbon-containing additives (T1, glucose; T2, biochar; T3, woody peat) on phosphorus (P) fractions transformations, humus formation and bacterial community succession in chicken manure composting. Results showed that orthophosphate monoester was significantly related to the humification process, and glucose or woody peat addition increased the P in humus. Lentibacillus was a key carbon cycle bacteria related to organics stabilization affected by carbon-containing additives. Redundancy analysis and variation partitioning indicated that phosphatase enzyme activity driven by bacterial community and humic substance had 59.7% contribution to P fractions dynamics. The findings highlight an efficient humus-regulation P stabilization way, notably in composting adding glucose to form humus with a better binding ability to labile P forms and phosphatase.