Soil phosphorus dynamics in a Vertisol as affected by cattle manure and nitrogen fertilization in soybean-wheat system

Repeated application of phosphorus (P) as superphosphate either alone or in conjunction with cattle manure and fertilizer N tray affect the P balance and the forms and distribution of P in soil. During 7 years, we monitored 0.5 M NaHCO3 extractable P (Olsen-P) and determined the changes in soil inorganic P (Pi) and organic P (P) caused by a yearly dose of 52 kg P ha(-1) as superphosphate and different levels of cattle manure and fertilizer N application in a soybean-wheat system on Vertisol. In general, the contents of Olsen-P increased with conjunctive use of cattle manure. However, increasing rate of fertilizer nitrogen (N) reduced the Olsen-P due to larger P exploitation by crops. The average amount of fertilizer P required to increase Olsen-P by 1 mg kg(-1) was 10.5 kg hat without manure and application of 8 t manure reduced it to 8.3 kg hat. Fertilizer P in excess of crop removal accumulated in labile (NaHCO3-P, and P) and moderately labile (NaOH-Pi and P.) fractions linearly and manure application enhanced accumulation of P. The P recovered as sum of different fractions varied from 91.5 to 98.7% of total P (acid digested, P-t). Excess fertilizer P application in presence of manure led to increased levels of Olsen-P in both topsoil and subsoil. In accordance, the recovery of P, from the 0-15 cm layer was slightly less than the theoretical P (P added + change in soil P - P removed by crops) confirming that some of the topsoil P may have migrated to the subsoil. The P fractions were significantly correlated with apparent P balance and acted as sink for fertilizer P.