Pathways of Soil Phosphorus Transformations after 8 Years of Cultivation under Contrasting Cropping Practices

Abstract

The knowledge of P transformations is essential to understand the P behaviour in soils. Organic (Po) and inorganic (Pi) P fractions were obtained from the 0‐ to 30‐ and 30‐ to 90‐cm layers of a Labarre silty clay (fine, mixed, frigid, Humic Cryaquept) by a sequential extraction. Barley (Hordeum vulgare L.) monoculture and 3‐yr barley‐forage rotation as main plots, and two nutrient sources (mineral fertilizer and liquid dairy manure) as subplots were studied. Path analysis was used to describe soil P transformations between the falls of 1989 and 1997. Nutrient source was the main factor affecting P transformations in the 0‐ to 30‐cm soil layer, whereas crop rotation predominated in the subsoil. Pathways differed more for Po than for Pi pools. In the 0‐ to 30‐cm soil layer, 86 to 97% of the variation in resin‐P was the direct effect of added Pi The NaHCO3‐Pi and Po were sensitive to crop rotations and nutrient sources, and were transitory pools of soil P. The NaOH‐Pi was a sink for added Pi and a source of NaHCO3‐Pi Labile Pi tended to store as NaOH‐Po, which was the sink for added Po and immobilized soil Pi in the mineral fertilizer systems. Conversely, the NaOH‐Po was mineralized and contributed to labile Pi in dairy manure systems. The NaOH‐Po acted as store of subsoil Po in the monoculture but was the main source of labile P in the rotation systems. The P transformations in this Cryaquept were strongly influenced by nutrient sources and crop rotations.