Phosphorus sorption - desorption by purple soils of China in relation to their properties

The understanding of phosphorus (P) sorption and desorption by soil is important for better managing soil P source and relieving water eutrophication. In this study, sorption–desorption behaviour of P was investigated in purple soils, collected from 3 kinds of purple parent materials with different kinds of land cover, in the upper reaches of Yangtze River, China, using a batch equilibrium technique. Results showed that most of the farmed purple soils had P sorption capacity (PSC) values ranging from 476 to 685 mg P/kg, while higher PSC values were observed in the soils from forestland and paddy field. A single-point P sorption index (PSI) was found to be significantly correlated with PSC (R2 = 0.94, P < 0.001), suggesting its use in estimating PSC across different types of purple soils. The PSC of purple soils was positively and strongly related to the contents of amorphous Fe and Al oxides (r = 0.73, P < 0.001), clay (r = 0.55, P < 0.01), and organic matter (r = 0.50, P < 0.05). Furthermore, the constant relating to binding strength was positively correlated with the content of amorphous Fe and Al oxides (r = 0.66, P < 0.01), but negatively correlated with labile Ca (r = –0.43, P < 0.05) and soil pH (r = –0.53, P < 0.01). Some acidic purple soils with high binding energy featured a power desorption curve, suggesting that P release risk can be accelerated once the P sorbed exceeds a certain threshold. Other soils with low binding energy demonstrated a linear desorption curve. The P desorption percentage was significantly correlated with soil test P (r = 0.78, P < 0.01) and the degree of P saturation (r = 0.82, P < 0.01), but negatively correlated with PSC (r = –0.66, P < 0.01).