Changes in characterization of phosphorus sorption in relation to its forms and soil organic carbon

Phosphorus (P) sorption in soil evidently affects phosphate fertilizer use efficiency. Changes in P sorption and its relation to P forms and soil properties under long-term fertilization (1985-1997) were investigated in continuous winter wheat ( Triticum aestivum L. ) systems on loamy soil ( Ustic Isohumisols) of the Loess Plateau. The treatments consisted of unfertilized (CK), nitrogen (N), phosphorus (P), nitrogen and phosphorus (NP), and nitrogen, phosphorus and farmyard manure (NPM) . The application rate of N, P2O5, M was 120, 60 kg/ hm2, and 75 t/ hm2, respectively. Using the soil samples collected in 1985, 1993, and 1997, soil total P, Ca2-P, Ca8-P, Al-P, Fe-P, O-P, and Ca10-P were measured, respectively. Phosphorus sorption isotherms were constructed for the collected soil samples, too. Organic P level, maximum P sorption capacity (Qm) , degree of P saturation ( DPS ) and EPC0(equilibrium P concentration at zero sorption) were estimated. The results showed that; 1) Qm of CK and N treatment in 1997 was increased by 18% and 14% , compared with that in 1985. In 1997, Qm in P, NP and NPM treatments were reduced by 26% , 13% and 26% , respectively, compared with that in 1985. The affinity constant of P in soil (k) was stable in CKand N treatments, increased in P, NP treatments, and decreased in NPM treatment with increasing of time. DPS and EPC0 in CK and N treatment decreased , however, increased in P, NP, and NPM treatments with increasing of time. There was highly significant correlation between Qm and Ca8-P or A1-P( P0.001)and significant correlation between Qm and Ca2-P or Fe-P ( P0.05) . DPS was highly related to Ca2-P and organic P form ( P0.05). There was significant relationship between SOC and Qm, DPS and EPC0 .