Phosphorus forms and P-sorption properties in three alkaline soils after long-term mineral and manure applications in north-eastern Italy.

In areas of intensive agriculture continuous applications of mineral and organic fertilizers can lead to an accumulation of phosphorus (P) in the soils and progressive saturation of their sorption capacities, increasing the risk of P losses to aquatic ecosystems. The long-term evolution (44 years) of soil P forms has been studied in a factorial experiment combining three soils (sandy, clay and a peaty) with three types of fertilization (no fertilizer, farmyard manure and mineral fertilizer). The P distribution in the calcium P (PCa), iron P (PFe), iron + aluminium P (PFe+Al) forms, and the effect of treatments on P availability (oxalate extractable-P, Mehlich-3-P, Olsen-P, water extractable-P) were determined. Two sorption P indices and the degree of soil saturation were also calculated. Soil P forms were affected by both quantity and quality of P inputs. Farmyard manure caused a more consistent increase in soil P content, affecting the water soluble and labile P forms (Mehlich-3-P and Olsen-P) in particular. Readily assimilable and potentially mobile P forms were positively correlated with the amount of high molecular weight humic fractions, thus stressing the importance of the type of incorporated materials and their evolution in determining soil P behaviour. Soils treated with farmyard manure, and to a lesser extent with mineral fertilizer, exceeded the threshold values of 20 mg kg−1 for Olsen-P and 150 mg kg−1 for Mehlich-3-P, thus evidencing a potential risk of P loss to water bodies. While in absolute terms the more consistent accumulations were observed in clay and peaty soils, the P content of sandy soils is of particular concern, given their lower P sorption capacity and higher degree of saturation. Among the soil P tests, Mehlich-3-P proved to be the best indicator to assess Italian soil conditions for both agronomic and environmental purposes.