Poor-Soil Rhizosphere Enriched with Different Microbial Activities Influence the Availability of Base Elements

Understanding the ecology of soil rhizosphere is essential to enhancing soil ecosystem and plants productivity. Poor-soil properties can limit rhizosphere microbial composition, interactions and plants productivity. Furthermore, the presence of plant exudates and microbial interation can change the rhizosphere dynamic. In the current study, we used two types of soils, rich nutrient soil represented by potting-soil and poor-soil represented with nutrientdeprived poor-soil. The two types of soil were inoculated with five microbial combinations using plant growth promoting bacteria (PGPB, Pseudomonas fluorescens) and mycorrhiza, and planted with two-day-old sorghum bicolor and Setaria italica (Foxtail millet) seedlings. Soil elements analyses were conducted 60 days after planting. The availability of exchangeable base alkali elements (Ca, Mg, Na) were measured and compared. Na was exclusively affected by PGPB inoculation in both soil types. Ca and Mg increased highest when associated with arbuscular inoculation, exclusively, in comparison to association with PGPB or Ectomycorrhiza in both soil types when planted with sorghum. Whereas, the same elements Ca and Mg, increased highest when associated with arbuscular mycorrhiza inoculation whether combined with PGPB or ectomycorrhiza when in soil planted with Setaria italica. The mycorrhiza increased Ca and Mg availability when combined with PGPB. Naincrease was associated only with PGPB inoculation solely in both soil types. The use of arbuscular mycorrhiza inoculation combined with PGPB is ecofriendly method to enrich rhizosphere in poor-soil and eliminate the need to use any chemical fertilizers.


Introduction
The rhizosphere is the borderline between plant and microorganisms, within biological systems and soil.The rhizosphere is the borderline where interactions affect ecosystem and plant growth and productivity.Soil rhizosphere interactions involve soil microorganism and plant roots exudates.It has been reported that poor-soil nutrient content influences the microbial diversity [1] and plant biomass [2].However, inoculating poor-soil with microbes such as Plant Growth Promoting Bacteria (PGPB) and mycorrhiza have been found to increase microbial activities and soil available elements [2].These plant growth-promoting bacteria, such as some strains of Pseudomonas, can induce plant growth by changing the rhizosphere via volatile organic compounds [3].Mycorrhiza was intensively studied and found to benefit soil and plant productivity and health [2] [4] [5].Poor-soil enrichment with microorganisms increased soil organic matter and productivity of plants [6] [7].In situations of abiotic stress, soil inoculation with arbuscular mycorrhiza alone have improved the ability of the soil to increase water use efficiency for Allium sativum L. [8].Moreover, ectomycorrhiza have great tolerance for heavy metals in contaminated soils [9].Inoculating soil with PGPB such as Pseudomonas fluorescens, reduced soil toxicity and increased sorghum bicolor productivity and mycorrhiza colonization growth [6].Overall, the use of PGPB alone or combined with mycorrhiza increases soil microbial diversity and plant productivity [10].The objective of the current study was to evaluate microbial inoculation ability to increase alkali elements availability in poor soil.

Material and Methods
In the current study, two types of soils were studied under two levels of biological root systems with six microbial combinations.At the first level, we applied six microbial combinations.The first type of soil was Potting-soil (PS) (Miracle Gro Premium Potting Mix by Scotts) distributed in pots 6 kg for each.The second type of soil was Poor-soil (TL), composed of soil poor in lime (CaO), with low holding capacity, and low nutrient such as magnesium (Mg), nitrogen (Na), phosphate ( ) PO − , and potassium (K).The biological systems included two types of plants roots: sorghum bicolor and Setaria italica.
The soil was distributed in (6 × 7 inch) pots, 6 kg for each with four replicates for each group.The inoculum was dissolved with water (9 g/100 ml) for placement in each pot.We used plant growth promoting bacteria (PGPB) Pseudomonas fluorescens obtained from microbial laboratories.The mycorrhizal mix source was MycoApply® Endo (Valentine Country Inc.).

The Experimental Design
The Analysis of correlation was conducted using Excel 2017.We also used the t-independent test to check if there was a significant difference for availability of each element (Ca, Mg and Na) under different microbial treatments within the same soil, in comparison to positive control group in all soil types, whether planted with sorghum bicolor or Setaria italica (α = 0.05).Analysis of t-independent test was conducted using SPSS 16.

Results
The ANOVA results indicated that elements availability was significantly af- potting soil.The t-independent test showed similar results with highly significance influence via PGPB and AM + PGPB groups on Na availability in all soil types.In comparison, the Ca availability was only significantly influenced in poor soil under the same groups (PGPB and AM + PGPB) planted with sorghum bicolor only.Lastly, Ecto + PGPB influenced Mg availability significantly in all soil types (Table 1).
Elements were compared to positive control (C+) to identify the percentage of change in Poor-soil (TL) or Potting-soil (PS) for the two types sorghum and Setaria italica.
In poor-soil planted with sorghum, Ca increased 96% in the two mycorrhiza groups.AM and Ecto + AM then followed with increase reached 85% in groups that were inoculated with PGPB solely or combined mycorrhiza two types.In when planted with sorghum, and increasing over 400% when planted with Setaria italica.In general Ca/ Mg ratio in all groups was high (Figure 1

Discussion
The microbial inoculations enriched the soil rhizosphere and made significant contributions to plants productivity and nutrient dynamic.Rhizosphere microbial activities induce the oxidation-reduction potential, influence moisture and nutrients abundance [12].The activity of microbes in soil increase elements availability via biomineralization [13].The use of PGPB has been reported to increase soil fertility by biomineralization organic matters in soil [14].The biomineralization resulted from microorganism interaction with plants secrete Indol Acetic Acid, 1-Aminocyclopropane-1-Carboxylate deaminase and cytokinin in response to plants root phytohormones [14].In the current study, we noticed high Ca and Na in soil inoculated with dead inoculum (negative control group, C−).The C− poor-soil with no microbial or plant root enrichment remained stable for two months with high Mg and Na and high Ca compared to all the groups.Potting-soil was high in Mg and Na in comparison to all groups.In comparison, the positive control C+ soil, inoculated with living microbial combination resulted in high Ca in poor-soil only and lower in Mg, Na.The soil CaMg ratio was higher than in all the groups indicating a healthy soil composition.
It seems that Na was the only element that increased in PGPB group, while another elements Ca and Mg increased associated with arbuscular mycorrhiza group alone or when combined with ectomycorrhiza or PGPB.The high soil Ca to Mg ratio increase mycorrhiza colonization [15] and plant productivity [16].It has been found that low soil Ca to Mg ratio cause plant premature root senescence and affect mycorrhiza colonization adversely [15].The increase in soil Ca and Mg with stable high ratio for Ca associated with AM group alone or when combined with PGPB.Ca is a signaling element bound to specific receptors between plant root cells and microorganisms [17].The increase of Ca in all the AM inoculated soil groups indicated its importance in plant root and arbuscular mycorrhiza communication to facilitate colonization [17].
The arbuscular mycorrhiza combination with PGPB increased Ca and Mg in presence of plants roots in comparison with positive control.The positive control group, which contained active microbes, was lower in these two elements (Ca and Mg) since this group lack plant root system.The need of plant and soil microbes' interaction is essential to rhizosphere health and ecosystem stability.

Table 1 .
T independent test analysis interaction results for using different microbial combinations in comparison with positive control (C) in two types of soil (poor TL and potting soil PS), using two plant types (sorghum bicolor and Setaria italica), on availability of three elements Ca, Mg and Na with significant association (p < 0.05).
comparison, poor-soil planted with Setaria italica, Ca reached over 100% in all