Water Regime Modulates Extractable Phosphorus Redistribution Under Liquid and Solid Phosphorus Fertilization in High-P Alkaline Calcareous Soil

In alkaline soils rich in residual phosphorus, phosphorus availability to plants depends not only on the existing reserve, but also on the redistribution of the extractable fraction within the root-explored zone. This study evaluated the effects of water regime, fertilizer treatment, and wheat presence on the distribution of ammonium lactate-extractable phosphorus (P-AL) within the 2–8 cm layer of an alkaline calcareous Chernozem with high initial P availability (P-AL = 178.1 mg kg−1). The pot experiment compared plant-free soil (S0) and wheat-planted soil (SP), four water regimes (H0–H150; 0–150 L m−2), and four fertilization treatments: an unfertilized control (F0), a solid NPK fertilizer (FS), and a liquid NP fertilizer applied at low and high rates (FL1 and FL2). These treatments represented practical fertilization options and were not equivalent in P input, supplying 51.8, 13.9, and 27.9 mg P pot−1 for FS, FL1, and FL2, respectively. Soil and plant samples were collected at BBCH 21–22, 20 days after fertilization. Water regime was a major factor shaping P-AL redistribution, significantly affecting P-AL at all three analyzed soil depths (p < 0.001), with its effect depending on vegetation condition and fertilization treatment. Wheat presence reduced P-AL relative to S0, and apparent P-AL depletion (ΔP-AL = S0 − SP) was greatest under H0, ranging from 83 to 99 mg kg−1. FL2 produced the largest S0–SP contrasts under H0–H100, whereas under H150 the largest difference was associated with FS. Under high water input, the higher-input solid NPK treatment (FS), which supplied the largest P input and was the only treatment supplying K, was associated with the highest shoot biomass (16.7 g), root biomass (6.92 g), and root P accumulation (25.2 mg pot−1). The results indicate that P fertilization in high-P alkaline soils should be adapted to water regime, fertilizer input and application method, without allowing for direct conclusions regarding phosphorus use efficiency, total plant P uptake, or leaching losses.