Phosphorus status and its sorption-associated soil properties in a paddy soil as affected by organic amendments

Yet the transformation and contribution of amorphous aluminum and iron which are usually indicated as oxalate-extractable forms (Al-ox and Fe-ox) to phosphorus (P) sorption in paddy soils, under long-term organic amendments combined with alternating flooding and draining, were not fully understood. The aim of this study was to investigate the effects of organic amendments on P status and P sorption-associated attributes including Al-ox and Fe-ox in a paddy soil. We selected 26 study sites that varied in three fertilization regimes (i.e., MF, straw, and manure) and three management histories (i.e., 2, 8, and 13 years) in the experiment station. Soil samples were assayed for (1) P accumulation status including total P, Olsen-P, oxalate-extractable (P-ox) and degree of soil saturation with P (DPS), and (2) soil properties associated with P sorption including pH, soil organic matter (SOM), Al-ox, and Fe-ox. Manure application presented significantly greater values for DPS (16.3 %, P < 0.01), Olsen-P / TP ratio (4 %, P < 0.01), and Olsen-P / P-ox ratio (6.8 %, P < 0.01). Moreover, it increased the contents of Al-ox and Fe-ox in the paddy soil, especially the content of Al-ox. Phosphorus accumulation in the paddy soil was not significant in straw treatment vs. MF treatment. Both Al-ox (P < 0.01) and Fe-ox (P < 0.05) exhibited significantly linear relationships with SOM, suggesting that the organo-Al(Fe) complexes might widely generate in the paddy soil, which have strong affinity to P. In comparison with Fe-ox, Al-ox showed greater correlation coefficients with soil P forms (varying from 0.56 to 0.81) and higher predictability for DPS of the paddy soil (R (2) = 0.3679, P < 0.01). These results suggest that Al-ox is the primary soil property associated with P sorption in the paddy soil under long-term organic amendments and repeated redox circulations.