Phosphorus-related properties in the profiles of three Italian soils after long-term mineral and manure applications

• Form of P fertilizers represents a factor affecting P build-up in the soil.
• Manure increased the P mobile fraction throughout soil profiles.
• In organic soils, P content was associated to SOC and amorphous Fe and Al.

The long-term evolution (44 years) of phosphorus (P) in 1-m profile of three reconstructed soils (0–80 cm = sandy, SDY; clay, CLY; and peaty, PTY; 80–100 cm = native layers) typical of the Po Valley (northern Italy) has been studied in a factorial experiment with farmyard manure and mineral fertilizer. Calcium, magnesium, aluminium, and iron concentrations were determined as total (CaT, MgT) and pseuto-total (AlPT and FePT) amounts and as extractable forms by Mehlich-3 (M3) and ammonium oxalate (Ox) extracting solutions. The P distributions in total P (TP), organic P (Porg), H2SO4 P (PH2SO4PH2SO4), and the more mobile NH4F P (PNH4FPNH4F) and NaOH P (PNaOH) were also measured.As expected soil type always influenced P sorption related soil properties. CaM3/CaT ratios indicated the presence of easily dissolved/released Ca in PTY while Al and Fe dominated in CLY and PTY soils as crystalline and poorly crystalline oxides forms. Treatments affected the pseudo-total Al and Fe with higher contents in manure than mineral, whereas mineral induced higher oxalate/total ratios of both Fe and Al than manure and untreated. Treatment affected the P content with a pattern strongly influenced by soil type and depth. Despite the comparable P balance between mineral and manure, P forms were generally higher in amended plots than mineral fertilized. Manure increased PH2SO4PH2SO4 down to 50 cm in CLY and down to 80 cm in SDY and PTY. High values of PNaOH were also induced by manure throughout the 0-80 cm of SDY.The occurrence of leaching process has been indicated by the general change in some P sorption related properties and P forms found in the native layers. In particular, both stable (i.e. TP) and labile forms (i.e. PNaOH) increased following 44 yr of fertilization. The phenomenon occurred not only in the overfertilized SDY but also in CLY and PTY where the P balance was negative. Long-term factors influencing the mobility of P within the soil profile are due not only to excessive P inputs, but also the forms of P fertilizer applied.