Effect of long‐term fertilization on the transformations of water‐extractable phosphorus in a fluvo‐aquic soil

Improved information on water‐extractable soil P (P w ) and its distribution in various forms is needed to assess its bioavailability and environmental impact. This study investigated P w in a fluvo‐aquic soil solution in relation to the continuous application of inorganic fertilizer (NPK) and wheat straw–soybean‐based compost for 15 y. Phosphatase‐hydrolysis techniques were used to fractionate organic P (P o ) in water extracts of soil into phosphomonoester (P om ) and phosphodiester (P od ). In comparison with the noncomposted treatments, compost application significantly increased the levels of inorganic P (P i ) and P o . P om was the main form in water‐extractable soil P o (71%–88%), in which sugar phosphate (P os ) occupied 48%–75%, inositol hexakisphosphate (P op ) comprised 13%–23%, and P od only accounted for a small percentage (11%–26%). Long‐term compost application significantly increased the content of P om , P os , and P od , but decreased the P op content; the ratio of P om to P o increased significantly in compost‐treated soil, but the ratio of P op to P o and P od to P o significantly decreased. Thus, the equilibrium of phosphatase involved P transformations shifted to P i in compost‐treated soil. The phosphomonoesterase and phosphodiesterase activities were significantly higher in compost‐treated soil, which favored the transformations of P od into P om and P om into P i . The ratio of P o to P w in water extracts of compost‐treated soil was similar to that of control soils with no fertilizer input (CK), but was significantly lower than in NPK treatment, which demonstrated that a larger increase occurred for soil P i in water extracts of compost‐treated soil. Long‐term compost application in the fluvo‐aquic soil changed the composition of P w , promoted the rate of P transformations in soil solution, and significantly increased soil P bioavailability.