Variation of the P transformation with alum and dolomite applications in agricultural facility soils

Leached phosphorus (P) losses from protected vegetable field soils (PVFs) due to frequent irrigation and high fertilizers input is a big issue, which may negatively affect the quality of connected surface and subsurface water bodies. The in-situ stabilization of soil P through P sorption materials (PSMs) application in P-enriched soils can reduce the downward movement of P. However, the transformation and mobility of soil P with different PSMs application are still unclear. In the present study, a three-year field experiment was conducted to investigate the effects of applying alum, dolomite, and the mixture of alum and dolomite (MAD, mass ratio = 1:2) on crop growth, soil available P, P fractions, and P transformations in PVFs by using the Hedley P sequential fraction method and the sequential chemical extraction and P K edge X-ray absorption near-edge structure spectroscopy. Results showed that the cumulative application of 8 100 kg/hm² of alum, dolomite, and MAD for three years had no significant effect on vegetable yield. Compared with the control treatment, the soil CaCl₂-P mass fraction in 0～30 cm soil depth in the alum, dolomite and MAD treatments decreased by 24.98%, 8.39% and 11.13%, respectively. On the contrary, there were no significant difference of soil Olsen-P content in the 0～90 cm soil depth among these treatments. After 3 years of application of PSMs, the soil total P contents in the 0～30 cm soil depth in the alum and MAD treatments were significantly increased by 5.62% and 8.82% compared with the control treatment, while the soil total P contents in the 0～30 cm soil depth of application of dolomite decreased by 0.57%. The results of soil P fractions also showed that HCl-P was the highest proportion of soil P fractions, accounting for more than 56% of the soil total P. The content of NaOH-P in the 0～60 cm soil depth after the application of PSMs was significantly higher than that of the control treatment, indicating that the active phosphorus in the soil and the iron and aluminum in the soil were combined. The X-ray absorption near-edge structure spectroscopy further showed that Ca-P was the main component in calcareous soil, accounting for more than 72% of the soil total P. Compared to the control, the treatments of alum, dolomite, and MAD increased the content of iron phosphate dihydrate by 1.55%, 5.68% and 5.88%, respectively. In addition, the treatment of alum and MAD increased the content of hydroxyapatite. The results of soil physicochemical properties showed that the alum treatment significantly increased the content of soil effective iron (M3-Fe), effective aluminium (M3-Al), effective phosphorus (M3-P) and electrical conductivity, and decreased soil pH value. The MAD treatment had no significant change on the physical and chemical properties of the soil. This demonstrated that it was more suitable to apply the MAD in P-enriched soil. This study provides applicable PSMs for reducing leached P losses in PVFs, which is of great fundament to curtail farmland non-point surface pollution.