Soil phosphorus cycling in greenhouse vegetable production system: New insights from phosphate oxygen isotope

Phosphorus (P) accumulation in soils of the greenhouse vegetable production (GVP) system is common due to intensive fertilization. However, the mechanism of P cycling in soils containing high P concentrations is not clear. In order to clarify the P cycling in GVP, 10 topsoils (030 cm) and 10 subsoils (3060 cm) were sampled under two types of greenhouses (solar greenhouse and plastic greenhouse) in Shouguang, a typical GVP region of China. The pools of soil inorganic P following Hedley sequential extraction, and the oxygen isotopic composition of NaHCO₃ extracted phosphate (δ¹⁸O_{NaHCO3_Pi}) and HCl extracted phosphate (δ¹⁸O_{HCl_Pi}) were measured. Results showed that P in GVP, particularly in the solar greenhouse soils, accumulated significantly both in topsoil and subsoil. The main inorganic P pool in GVP soils was the HCl extracted, accounting for 50.26 %72.76 % in topsoil and 44.42 %57.89 % in subsoil, respectively. Values of δ¹⁸O_{NaHCO3_Pi} in most topsoil samples were within the isotopic equilibrium range (13.63 ‰17.14 ‰). Values of soil δ¹⁸O_{HCl_Pi} in GVP, significantly higher than that in open field (11.41 ‰ in topsoil and 10.27 ‰ in subsoil), indicated more intensive P biological cycling and more secondary minerals formation altered the original characteristics of δ¹⁸O_{HCl_Pi} in GVP soil. The significant positive correlation of δ¹⁸O_{NaHCO3_Pi} values between topsoil and subsoil implied that the labile P_i in subsoil was mainly influenced by its corresponding topsoil. Partially labile P, without biological cycling after fertilization of the topsoil, quickly migrated to the subsoil causing the P accumulation in the subsoil. The higher values of δ¹⁸O_{NaHCO3_Pi} and δ¹⁸O_{HCl_Pi} in solar greenhouse than plastic greenhouse suggested higher degree of P biological cycling, which were determined by the nutrient status. Overall, the phosphate oxygen isotope technology provides a deeper understanding of soil P cycling in GVP.