Legacy Effects of Long-Term Brackish Groundwater Irrigation on Bacterial Communities in Wheat Rhizosphere and Yield Performance

This study aimed to investigate the legacy effects of prolonged brackish irrigation on rhizobacterial communities and agricultural productivity in wheat. Here, we conducted pot experiments to investigate the mechanisms through which different irrigation regimes (irrigation using brackish groundwater and normal water) regulate wheat production. We applied four irrigation treatments across different stages of wheat growth (early stages, seedling-to-jointing, and late stages, jointing-to-maturity). This included irrigation exclusively using normal water during both stages (RR), using normal water followed by brackish groundwater (RW), exclusively using brackish groundwater (WW), and using brackish groundwater followed by normal water (WR). Under the premise of retaining 10 seedlings per pot, the average number of effective spikes per 10 plants in the RR, RW, and WR treatments was approximately 1.3, 1.1, and 1.1 times that of WW (19 ± 1), respectively. The spike weight per 10 plants in the RR, RW, and WR treatments was approximately 1.8, 1.5, and 1.3 times that of WW (12.75 ± 1.74 g), respectively. Compared with brackish groundwater irrigation, the use of normal water during the early stages significantly reduced the relative abundance of Pseudomonadota and increased that of Chloroflexota (p < 0.05). The number of effective spikes was positively correlated with the relative abundances of Actinobacteriota, Acidobacteriota, Chloroflexota, and Bacteroidota, but negatively correlated with the abundance of Pseudomonadota (p < 0.05). Irrigation regimes altered the rhizobacterial community structure. However, the legacy effect of long-term irrigation using brackish groundwater resulted in the dominance of stochastic processes in bacterial community assembly and stability of the Shannon diversity across all treatments. The complexity of the rhizobacterial co-occurrence network was lower in the RW treatments than that in the WW treatments (p < 0.05). Structural equation modeling revealed that irrigation using normal water during early stages boosted the number of effective spikes in wheat. This enhancement was achieved by increasing rhizobacterial diversity, reducing rhizosphere sodium, and simplifying the microbial network. This study challenges the “legacy effect” of brackish water irrigation by demonstrating that optimal irrigation timing is key to enhancing crop yield.