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 (δ18ONaHCO3_Pi) and HCl extracted phosphate (δ18OHCl_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 δ18ONaHCO3_Pi in most topsoil samples were within the isotopic equilibrium range (13.63 ‰17.14 ‰). Values of soil δ18OHCl_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 δ18OHCl_Pi in GVP soil. The significant positive correlation of δ18ONaHCO3_Pi values between topsoil and subsoil implied that the labile Pi 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 δ18ONaHCO3_Pi and δ18OHCl_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. •Phosphate oxygen isotope technology (δ18OP) is a powerful tool for tracing P cycling.•The main inorganic P (Pi) pool in GVP soils was the HCl-extracted Pi.•Significant positive correlation of δ18ONaHCO3-Pi was found between topsoil and subsoil.•Higher δ18OP indicated more intensive biological cycling of P in solar greenhouse soil.•Secondary minerals in GVP soil alters the original characteristics of δ18OHCl-Pi.