Direct and Residual Effects of Water Deficit Stress, Different Sources of Silicon and Plant-Growth Promoting Bacteria on Silicon Fractions in the Soil

Abstract

Silicon (Si) and plant growth promoting bacteria (PGPB) can effectively reduce the negative effects of water deficit stress and augment plant growth. Few studies have simultaneously investigated the effects of silicon nano-particles (Si-NPs) and plant growth-promoting bacteria (PGPB) on water deficit stress. Hence, the present study aimed to investigate the effects of different levels of water deficit stress (0.8, 0.6, and 0.4 FC), different resources of Si (Si: 100, and 200 mg kg−1 Si-NPs, and 200 mg kg−1 potassium silicate) and native PGPB (control, Pseudomonas19.sp, Bacillus76.sp, and a bacterial consortium of both) on Si fractions in the soil under canola-wheat cultivation. A factorial experiment was carried out in three replications. With increasing water deficit stress from 0.8FC to 0.4FC, the soluble and exchangeable silicon levels decreased significantly from 46.15 and 96.49 to 21.42 and 60.79 mg kg−1, respectively, while amorphous silicon increased from 233.53 to 356.00 mg kg−1. The results indicated that under water deficit stress conditions, both direct and residual effects of PGPB and Si-NPs positively affected the soluble, exchangeable, and amorphous fractions of Si. The highest levels of soluble (98.03 mg kg−1) and exchangeable (108.6 mg kg−1) Si in the soil in which canola was harvested were recorded for the treatment of 0.8 FC + 200 mg kg−1 Si-NPs + Pseudomonas19.sp. These were 78.78% and 57.73%, respectively, higher than those of the control treatment (0.8FC, without bacterial inoculation, and silicon). Moreover, after wheat harvesting, the highest levels of soluble (16.14 mg kg−1) and exchangeable (23.88 mg kg−1) Si in the soil were observed in the treatment of 0.8 FC + 200 mg kg−1 Si-NPs + Pseudomonas19.sp. These were 36.72 and 52.54%, respectively, higher than those of the control. Overall, the application of Si-NPs and PGPB significantly positively affected the absorbable components of Si in the soil, which was in favor of plant growth.