Unravelling Spinach Growth: When Microbial Inoculants Fall Short - Soil Quality And Microbial Communities Across Land Uses

This study investigates microbial inoculants as alternatives to synthetic fertilizers for spinach cultivation. Five bacterial strains (MP1-MP5) were isolated from soil and mature spinach plants, based on nitrogen-free growth and phosphate solubilization abilities. These were applied individually or in combination (1g/kg of soil per week) to containerized topsoil collected in triplicate from agricultural (AG) and campus (CAM) sites. Biotic interactions were studied for unified application. Spinach growth, measured by leaf area 50 days after germination, showed minimal improvement except when three endophytes were combined, suggesting soil limitations. Soil physicochemical and biochemical analyses, along with principal component analysis (PCA), revealed key parameters for CAM soil, including bulk density, electrical conductivity, clay content, total organic carbon, nitrogen, and pH. For AG soil, total organic carbon, pH, sand content, urease, and β-glucosidase were significant. The soil quality index (SQI) was 0.59 for CAM and 0.55 for AG, indicating both as fair soils needing improvement for optimal spinach growth. Effective microbial inoculation requires establishing niches in both soil types. Illumina sequencing of 16S rRNA (V3-V4) showed higher species richness (AG: 1,413 vs. CAM: 427), greater Shannon diversity (AG: 8.292 vs. CAM: 4.068), and Simpson's diversity (AG: 0.991 vs. CAM: 0.800) in AG soil. Bacillus mannanilyticus and Bradyrhizobium elkanii were dominant in CAM and AG soils, respectively, with Streptomyces puniciscadipi also prevalent in AG. Both the soil sample does not have the presence of applied inoculats observed through 16srRNA sequencing. Combining endophytic bacteria (MP-1 Pseudomonas sp., MP-3 Bacillus sp., MP-5 Flavobacterium sp.) improved spinach growth more than other combinations, while soil-isolated bacteria (MP-2 Azospirillum, MP-4 Beijerinckia) struggled in CAM soil and faced competition in AG soil. This study highlights the need for inoculants to successfully establish in soil for effective trait-based applications.