Objectives In response to the frequent alternation of drought and waterlogging in sandy loam soils of the Huaibei Plain, this study investigated the effects of controlled drainage on the dynamic of soil moisture and phosphorus fractions, as well as on phosphorus uptake and utilization in soybean. The aim was to elucidate the mechanisms by which controlled drainage enhances phosphorus use efficiency and to provide a scientific basis for drainage regulation for the efficient use of soil water and nutrients.

Methods The experiment was conducted at the Xinmaqiao Irrigation Experimental Center of the Anhui Provincial Department of Water Resources. A soybean waterlogging experiments were conducted. Three controlled drainage depths were set: 50 cm (WT50), 100 cm (WT100), and a variable depth of 20−30 cm (WTvar) below the surface. A non-waterlogged condition with the groundwater table depth fixed at 50 cm throughout the entire growth period was used as the control (CK). Soil water content, groundwater table depth, soil phosphorus fractions, soybean dry matter accumulation, soybean P content, and total P accumulation in various soybean organs were measured during the waterlogging-drainage stage, intermittent stage, and the whole growth period. Analyze the changes in soil water content and phosphorus content at the beginning and end of each time period to investigate the temporal variation amplitudes of soil water and phosphorus. Correlation analysis, random forest model, and structural equation model were employed to analyze the regulatory pathways of controlled drainage on phosphorus fertilizer use efficiency.

Results Waterlogging significantly reduced leaf P accumulation and content by 20.2%−34.1% and 16.5%−22.3%, respectively, whereas WT50 showed the smallest reductions in P accumulation in the whole plant, aboveground parts, leaves, seeds, and pods. Waterlogging significantly decreased the partial factor productivity of P fertilizer under WTvar and WT100, while WT50 was less affected. During the waterlogging-drainage stage, the increase in water-soluble P in the 0−20 cm soil layer under WT50 was 22.6% lower than that under WTvar, while WT100 showed a larger decrease with significant differences from both. The reduction in total P in the 0−20 cm soil layer under WTvar and WT50 was significantly smaller than that under WT100. During the intermittent stage, the decrease in available P in the 40−60 cm layer under WT50 was significantly larger than that under WTvar and WT100. The decrease in water-soluble P in the 0−20 cm layer under WT50 was 40.5% smaller than that under WTvar, whereas WT100 exhibited a marked increase. In the whole growth period, the reduction in total P in the 20−40 cm layer under WT50 was significantly greater than that under WT100. The increase in soil water content at the 30-40 cm depth under WT50 treatment was significantly higher than that under WT100 treatment. Pearson correlation analysis showed that phosphorus uptake efficiency and partial factor productivity were significantly and positively correlated with dry matter accumulation of the whole plant, aboveground parts, grains, and pods, as well as with phosphorus accumulation in the whole plant, aboveground parts, and grains. In addition, partial factor productivity of P fertilizer was significantly positively correlated with phosphorus accumulation in pods and leaf phosphorus concentration, but significantly negatively correlated with the cumulative value of groundwater table depth exceeding the threshold (SEW₅₀). Random forest and structural equation models further indicated that controlled drainage affected soybean seed and pod dry matter accumulation as well as soil available and total P retention by altering SEW₅₀, thereby indirectly influencing seed P accumulation and regulating P fertilizer use efficiency.

Conclusions A controlled drainage depth of 50 cm in sandy loam soils of the Huaibei Plain significantly improved soil moisture conditions in the plough layer and enhanced soil phosphorus availability, thereby increasing phosphorus fertilizer use efficiency in soybean.