Introduction

Phosphate-solubilizing bacteria (PSB) function as potential microbial indicator in arid ecosystems; however, the pedo-climatic determinants of their population dynamics within rapidly desiccated lacustrine sediments remain largely unresolved. The recent accelerated drying trajectory of Hamoun Wetland (southeastern Iran)—recognized as a significant source of atmospheric dust—has concurrently intensified multiple edaphic stressors (e.g., elevated osmotic potential, salinity, and sodicity/alkalinity), while selectively fostering the establishment of a suite of five resistant species to these conditions. Vegetation modulates rhizosphere microbial niches via root exudation, microclimatic buffering, and wind-erosion mitigation. However, the direct and indirect effects of soil physicochemical parameters and plant cover on PSB populations have not been quantified in Hamoun Wetland. This study, therefore, aims to (i) quantify PSB abundance in the rhizosphere of the five dominant species versus bare soils, and (ii) more importantly, model the key determinants of PSB density and partition their total effects into direct and indirect pathways.

Materials and Methods

The field investigation was carried out in 2022 within the Hamoun Wetland basin, northern Sistan and Baluchestan Province, Iran—a region with an aridic soil moisture regime and hyperthermic soil temperature regime, where mean annual precipitation is 50 mm and potential evaporation fluctuates between 4000 and 5000 mm year⁻¹. Soil sampling (0-20 cm depth) was carried out across 25 geomorphic landscapes, comprising five dominant plant species (Tamarix stricta, Cyperus rotundus, Aeluropus lagopoides, Halocnemum strobilaceum, and Phragmites communis) alongside adjacent bare soils as controls. A total of 450 soil samples were collected to determine PSB populations and soil physicochemical properties. Soil physicochemical properties such as texture (clay, silt, and sand percentages), bulk density (BD), and saturation percentage (SP), pH, electrical conductivity (EC), sodium adsorption ratio (SAR), organic carbon (OC), calcium carbonate equivalent (CCE), total nitrogen (TN), available potassium (AK), and available phosphorus (AP) were measured by standard methods. Also, PSB populations were enumerated by the plate-count technique on Pikovskaya's medium. Data were analyzed using one-way ANOVA based on a randomized complete block design (RCBD), followed by Fisher's LSD post-hoc test after verifying normality (Shapiro–Wilk) and homogeneity of variance (Levene's test). Stepwise multiple regression and subsequent path analysis were performed to disentangle the direct and indirect effects of edaphic determinants influencing PSB population density.

Results and Discussion

The results of the analysis of variance showed that vegetation cover significantly influenced PSB viable counts (p < 0.001), explaining 27.5% of the total variance. Compared with bare soil (control), all five dominant plant species (T. stricta, H. strobilaceum, P. communis, A. lagopoides, and C. rotundus) significantly increased PSB populations in their rhizosphere. Among them, the highest PSB abundance occurred under T. stricta, which did not differ significantly from H. strobilaceum and P. communis, whereas the lowest was recorded under C. rotundus, showing no significant difference compared to A. lagopoides. The species-specific response is presumed to stem from interspecific differences in rhizodeposition patterns and the chemical profile of root exudates, which collectively govern the selective proliferation of microbial guilds. Pearson correlation coefficients demonstrated that total nitrogen (TN) exhibited the strongest positive correlation with PSB population (r=0.39, p<0.001), whereas available phosphorus (AP) displayed the most pronounced inverse relationship (r=-0.40, p<0.001), alongside a weaker yet significant negative correlation with soil pH (r=-0.17, p<0.05). The positive linkage between TN and PSB abundance is plausible, given that nitrogen constitutes a structural constituent of proteins, nucleic acids, and enzymes, and its scarcity in hyper‑arid calcareous soil often imposes a primary constraint on microbial proliferation. Conversely, the negative correlation between AP and PSB population, while counterintuitive at first glance, may be interpreted as reflecting concentration-driven negative feedback: high soluble phosphorus downregulates phosphatase and organic-acid genes, reducing bacterial solubilizing activity as an energy-saving adaptation. In these predominantly alkaline soils, decreasing pH optimizes conditions for microbial growth, thereby increasing the PSB population. Stepwise multiple regression, performed on twelve edaphic predictors, identified TN (B=11.641, β=0.258, p=0.004) and AP (B=-20.010, β=-0.378, p<0.001) as the only statistically significant predictors among the twelve evaluated soil physicochemical properties. Nevertheless, the adjusted R² (0.224) indicates that approximately 78% of the variability in PSB density remains unexplained by the measured edaphic properties, strongly implicating unaccounted covariates—including root‑exudate fingerprints, microclimates, and inter‑microbial interactions—as predominant modulators, suggesting that TN and AP serve as contributing edaphic associates rather than definitive ecosystem-scale determinants. Path analysis further partitioned the correlation structure, showing that TN yielded a direct path coefficient of 0.277 and an indirect path through AP of 0.113 (total path coefficient = 0.390), whereas AP exhibited a direct negative path coefficient of -0.299 and an indirect path through TN of -0.106 (total path coefficient = -0.405). These observational path coefficients align conceptually with the metabolic-economy hypothesis, although cross-sectional data preclude definitive causal claims regarding in situ biochemical regulation.

Conclusions

This study conclusively establishes that PSB abundance in the Hamoun Wetland is primarily governed by vegetation type, total nitrogen (TN), and available phosphorus (AP). The rhizosphere of all five studied species significantly stimulated culturable PSB populations relative to the bare soil (control); however, T. stricta, H. strobilaceum, and P. communis supported markedly higher bacterial densities than C. rotundus and A. lagopoides, emphasizing species-specific rhizodeposition as a primary determinant of beneficial rhizosphere microbiome assembly. The severe depletion of viable PSB in bare soils reflects a profound loss of essential biological agents driving phosphorus solubilization—a deficit that, alongside the anticipated impairment of other unmeasured mutualists such as arbuscular mycorrhizal fungi (AMF), poses a severe bottleneck for spontaneous plant recolonization. Although these findings specifically delineate culturable bacterial solubilizer dynamics rather than broader meta-community shifts or net phosphorus fluxes, they underscore critical ecological linkages in degraded wetland soils. To break this degradation cycle, we propose a triad of evidence-based interventions for rehabilitating the Hamoun Wetland: prioritizing native, PSB-stimulating plant species (particularly T. stricta, H. strobilaceum, and P. communis) in revegetation programs; implementing nitrogen bioaugmentation using adapted N2‑fixers; and strategically regulating exogenous phosphorus inputs. Collectively, these coordinated strategies present an actionable framework to restore soil microbial functionality, enhance biogeochemical resilience, and facilitate long-term ecological rehabilitation in hyper-arid wetland ecosystems.