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

tThe 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 fertil-izer. Calcium, magnesium, aluminium, and iron concentrations were determined as total (CaT, MgT) andpseuto-total (AlPTand 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), H2SO4P (PH2SO4), and themore mobile NH4F P (PNH4F) and NaOH P (PNaOH) were also measured.As expected soil type always influenced P sorption related soil properties. CaM3/CaTratios indicatedthe presence of easily dissolved/released Ca in PTY while Al and Fe dominated in CLY and PTY soilsas crystalline and poorly crystalline oxides forms. Treatments affected the pseudo-total Al and Fe withhigher contents in manure than mineral, whereas mineral induced higher oxalate/total ratios of both Feand Al than manure and untreated. Treatment affected the P content with a pattern strongly influencedby soil type and depth. Despite the comparable P balance between mineral and manure, P forms weregenerally higher in amended plots than mineral fertilized. Manure increased PH2SO4down to 50 cm in CLYand down to 80 cm in SDY and PTY. High values of PNaOHwere also induced by manure throughout the0-80 cm of SDY.The occurrence of leaching process has been indicated by the general change in some P sorption relatedproperties 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 overfertilizedSDY but also in CLY and PTY where the P balance was negative. Long-term factors influencing the mobilityof P within the soil profile are due not only to excessive P inputs, but also the forms of P fertilizer applied