Objective In water-deficit area of northern China, soil water content is a crucial factor affecting plant growth. Studying the soil water status of Populus tomentosa stands under different planting densities and water treatments can provide a reference basis for soil water maintenance of plantations in the North China Plain.

Method Populus tomentosa plantations under five different planting densities (Ⅲ 3 m × 3 m, Ⅱ 3 m × 6 m and Ⅰ 6 m × 6 m) and water (drip irrigation, FI and rainfed, NI) treatments (FI_Ⅲ, FI_Ⅰ, NI_Ⅲ, NI_Ⅱ and NI_Ⅰ) were selected in this study. During the growing season (May, June, August and October) in 2021, soil water content (SWC) within the 6 m-depth soil profile was measured using the drying-weighing method, soil water content conditions and the occurrence of dry soil layer (DSL) were investigated and compared among different treatments.

Result (1) Shallow soil layers (0−30 cm and 30−100 cm, ranging from 5.62% to 15.53%) showed significantly lower SWC than the deep soil layers (100−200 cm, 200−400 cm, and 400−600 cm, ranging from 16.50% to 27.00%) in each treatment. The SWC in all stands increased with depth within the vertical profile of 0−240 cm, showing two peaks at 240−260 cm (26.37%−30.56%) and 360−400 cm (22.79%−33.00%), while the change of SWC at 400−600 cm was relatively gradual. (2) All five stands exhibited the highest soil water availability in October, with an average SWC from 20.16% to 23.16%. In rainfed treatment, soil was driest in June independent of planting density, the average SWC ranged from 13.11% to 14.96%. Drip irrigation treatment reduced the seasonal variation in SWC in the soil layer below 30 cm. (3) Under different water treatments, high density stands exhibited the highest soil water availabilities in the deep soil layers (average SWC of 23.18% for FI_Ⅲ and 21.13% for NI_Ⅲ). However, NI_Ⅱ exhibited the highest soil water compensation at the end of the rainy season (October) of 403.12 mm. In both high and low density stands, SWC in the 0−30 cm soil layer was significantly increased by drip irrigation treatment, the compensation of soil water in the deep layers was also enhanced (the change in water storage was 84.40% in FI_Ⅲ than in NI_Ⅲ, and 173.99% higher in FI_Ⅰ than in NI_Ⅰ). Drip irrigation treatment only significantly improved soil water storage in high density stands (P < 0.05). (4) Both drip irrigation and precipitation effectively alleviated or eliminated the occurrence of recoverable DSL within 2 m-depth under different planting densities.

Conclusion According to the results of this study, a 3 m × 3 m planting density with frequent full irrigation treatment during dry season (April to June) is recommended for the cultivation of large-diameter poplar plantation in the North China Yellow River Plain in order to achieve fast tree growth in the early growing stage (2−4 years) and improve water condition of the deep soil layers. After the occurrence of evident density effect and deep soil water content depletion, management practices like thinning can be implemented to maintain soil water production and enhance the productivity of poplar plantations.