Distribution of phosphorus fractionations in calcareous soils of the Loess Plateau

Twelve soil samples from 0～20 cm soil layer were collected from north to south in the Loess Plateau calcareous soil and analyzed for fractionation of organic and inorganic soil phosphorus by Hedley method, amended by Tiessen and Moir. The results show that contents of different P fractionations in soil are in the following order: HCl-P > Residual-P > NaHCO₃-Po > NaHCO₃-Pi > NaOH-Po > NaOH-Pi > H₂O-P. The content of HCl-P and Residual-P are higher and respectively account for 54.00%～88.96% and 0～39.11% of soil total P. The overall distribution of soil P on the Loess Plateau increases from north to south. In all soil types, the average content of NaOH-Po, residual-P and total P is in the following order: Ust Sandic Entisols < Los Orthic Entisols < Hap Ustic Isohumisols < Eum Orthic Anthrosols; generally increasing from north to south. The average content of H₂O-P and HCl-P is in the following order: Hap Ustic Isohumisols < Los Orthic Entisols < Ust Sandic Entisols < Eum Orthic Anthrosols; which initially decreases from north to south and then successively inversely increases. Significant correlations exist between soil total N and P as well as different forms of P; and also highly significant correlations between soil total N and NaOH-Pi, NaOH-Po, HCl-P and total P. C/N ratio, pH and sand content have negative correlations with total P, and different forms of P; pH and NaHCO₃-Po is in significant negative correlation. pH and H₂O-P, NaHCO₃-Pi, NaOH-Pi and HCl-P are in extremely significant negatire correlation. Sand content and NaHCO₃-Pi, Residual-P and total P are significantly negatively correlated. Calcium carbonate is negatively correlated with other forms of P except NaHCO₃-Po, NaOH-Po, Residual-P. Clay content and other forms of P, including total P have a certain degree of negative correlation, except for NaOH-Pi and NaOH-Po. Multiple regression analysis on different forms of P and available P indicates that H₂O-P is the largest contributor of available P among different forms of P, followed by NaOH-Pi and NaHCO₃-Po. Stepwise regression analysis on the different forms of P and available P further shows that H₂O-P has the highest bioavailability.