Soil phosphorus dynamics after ten annual applications of mineral fertilizers and liquid dairy manure: Fractionation and path analyses

Knowledge of phosphorus (P) dynamics in soil is essential for predicting its bioavailability and the risk of P transfer from soil to bodies of water. A comparative study was conducted to assess the changes in P fractions and pathways of P transformation in soil measured after 10 annual applications of mineral fertilizers (MIN) and liquid dairy manure (LDM). Fractionation and path analyses were performed to quantify inorganic P (P-i) and organic P (P-o) forms and the relationships between P pools in the 0 to 15-cm layer of a Labarre silty clay (fine, mixed, frigid, Humic Cryaquept) under barley monoculture. The MIN additions resulted in larger increases in Pi fractions and smaller increases in the NaOH-P. compared with the LDM applications. The LDM, however, produced 1.9 times more total soil labile P than the MIN plot. Path analysis indicated that path coefficients between the source and recipient P pools were P-source dependent. The NaOH-Pi was a primary sink of added Pi and a source of NaHCO3-P-i. The roles of P-o pools were more important than the P-i pools for P transformations. The NaHCO3-P-o was sensitive to P sources and likely acted as a transitory pool rather than as a sink or source of soil P. The NaOH-P-o constituted a sink for added P-o and immobilized labile P-i in the MIN plot; it was conversely mineralized and contributed to labile P-i in the LDM plot. The results of this study stress that the pathways of P transformation in this Cryaquept can be revealed by the descriptive path analysis.