Weaker temporal variation of soil moisture can improve crop water use efficiency (WUE), but its physiological mechanism was still unclear. To explore the mechanism, an organized experiment was conducted in Beijing from June to September. From the jointing stage to maturity stage of maize, stable soil moisture (SSM) and fluctuating soil moisture (FSM) were established with Pressure Potential Difference-Crop Initiate Drawing Water (PCI) and manual irrigation (MI), respectively, to explore the physiological mechanism of SSM to improve maize WUE. Among them, PCI treatments were set with 3 pressure differences of -5, -10, and -15 kPa, and MI treatment was watering every 3 days with the irrigation amount of 9.3 mm. The results showed that (1) after water treatment, the average soil water content of PCI-5 kPa, PCI-10 kPa, PCI-15 kPa, and MI treatments were 53% field capacity (FC), 47, 38, and 78% FC, respectively. It was SSM with weak temporal variation under PCI treatments, and FSM with medium temporal variation under MI treatment. (2) PCI treatments reduced the content of proline, malondialdehyde, and abscisic acid in each organ of maize. (3) Compared with FSM 78% FC, the maize root activity at the filling stage of 53% FC SSM and 47% FC SSM increased significantly by 57.1 and 28.6%, respectively, and the carbon isotope discrimination value (Δ13C) in bracts of the two treatments increased by 18.3 and 10.4%, respectively. (4) There was a very significant positive correlation between WUE based on biomass (WUEb) and Δ13C in bracts. In conclusion, a large temporal variation of soil moisture was an important factor that caused water stress in maize. Under SSM treatments, the accumulation of abscisic acid, proline, and malondialdehyde was synergistically reduced. SSM improved the WUE of maize by alleviating short-term soil water stress caused by the fluctuation of soil moisture.

Temporal variation of soil moisture is one of the influencing factors affecting crop water use efficiency (WUE). Compared with fluctuating soil moisture (FSM), stable soil moisture (SSM) with weaker temporal variance has the potential to improve the WUE of crops. However, response of crop rhizosphere microbiome to soil moisture temporal variation remains unclear. In this study, we performed pot experiments on romaine lettuce (Lactuca sativa L. var. longifolia) to compare the effects of different soil moisture temporal variation on plant growth, yield, water use efficiency (WUE), and rhizosphere bacterial and fungal community structures, via manual irrigation and negative pressure irrigation to create FSM and SSM conditions, respectively. The results indicate that SSM improved the growth and WUE of romaine lettuce. Moreover, the rhizosphere microbial community composition of romaine lettuce differed under SSM and FSM conditions. Under SSM, bacterial Bacillus, fungal Aspergillus and Chaetomium were enriched in the romaine lettuce rhizosphere, whereas some taxa such as bacterial Devosia, Lysobacter, Blastococus and Bacillus, fungal Alternaria were reduced; these taxa could therefore be biomarkers in future research. Partial least squares path model (PLS-PM) analysis revealed that rhizosphere microbial communities were indirectly affected by the soil moisture temporal variation, as evidenced by the improvement in plant growth. Our results suggest that the rhizosphere microbial communities of romaine lettuce primarily respond to changes in the soil moisture temporal variation through the plant-microbiome interaction but are not directly affected by soil moisture.