Impact of Elemental Sulfur on the Rhizospheric Bacteria of Durum Wheat Crop Cultivated on a Calcareous Soil

Previous experiments have shown that the application of fertilizer granules containing elemental sulfur (S⁰) as an ingredient (FBS⁰) in durum wheat crops produced a higher yield than that produced by conventional ones (F), provided that the soils of the experimental fields (F vs. FBS⁰) were of comparable quality and with the Olsen P content of the field&#8217;s soil above 8 mg kg^&#8722;1. In this experiment the FBS⁰ treatment took place in soil with Olsen P at 7.8 mg kg^&#8722;1, compared with the F treatment&#8217;s soil with Olsen P of 16.8 mg kg^&#8722;1, aiming at reducing the imbalance in soil quality. To assess and evaluate the effect of FBS⁰ on the dynamics of the rhizospheric bacteria in relation to F, rhizospheric soil at various developmental stages of the crops was collected. The agronomic profile of the rhizospheric cultivable bacteria was characterized and monitored, in connection with the dynamics of phosphorus, iron, organic sulfur, and organic nitrogen, in both the rhizosoil and the aerial part of the plant during development. Both crops were characterized by a comparable dry mass accumulation per plant throughout development, while the yield of the FBS⁰ crop was 3.4% less compared to the F crop&#8217;s one. The FBS⁰ crop&#8217;s aerial part showed a transient higher P and Fe concentration, while its organic N and S concentrations followed the pattern of the F crop. The incorporation of S⁰ into the conventional fertilizer increased the percentage of arylsulfatase (ARS)-producing bacteria in the total bacterial population, suggesting an enhanced release of sulfate from the soil&#8217;s organic S pool, which the plant could readily utilize. The proportion of identified ARS-producing bacteria possessing these traits exhibited a maximum value before and after topdressing. Phylogenetic analysis of the 68 isolated ARS-producing bacterial strains revealed that the majority of the isolates belonged to the <i>Pseudomonas</i> genus. A large fraction also possessed phosphate solubilization, and/or siderophore production, and/or ureolytic traits, thus improving the crop&#8217;s P, Fe, S, and N balance. The aforementioned findings imply that the used FBS⁰ substantially improved the quality of the rhizosoil at the available phosphorus limiting level by modulating the abundance of the bacterial communities in the rhizosphere and effectively enhancing the microbially mediated nutrient mobilization towards improved plant nutritional dynamics.