Organic amendments affect phosphorus sorption characteristics in a paddy soil

Continuous excessive application of phosphorus (P) fertilizer and/or manure in areas with intensive agriculture can lead to an accumulation of P in soils and progressive saturation of soil sorption capacities, thus resulting in increased P loss from soil to aquatic ecosystem. An 8-year field experiment with four fertilization patterns (chemical fertilizer only, fertilizer plus straw, fertilizer plus 7.5 t ha−1 manure, and fertilizer plus 15 t ha−1 manure) was conducted in the Taihu Lake region of eastern China to investigate the effects of incorporation of straw and manure on P accumulation and the changes of P sorption capacity of a paddy soil. Degrees of P saturation, contents of Olsen-P and total P in manure treated soils were significantly higher than those treated with fertilizers alone or with straw retention (P < 0.05). Phosphorus accumulation in studied soil was less labile in straw treatment than in manure treatments, but it remained extractable by ammonium oxalate. The Langmuir P sorption maximum (Smax) ranged from 24.7 mmol kg−1 to 43.2 mmol kg−1, and was highly correlated with oxalate extractable Al (Alox, r = 0.85) and Fe (Feox, r = 0.79). Moreover, path analysis showed that the direct effect of Alox on Smax was significant (P < 0.05) while that of Feox on Smax was insignificant; and that Alox could also exert influence on Smax via its indirect effect through Feox and organic matter. Thus, Al was the most important soil property associated with Smax in the paddy soil based on path analysis results.