Relationship between soil test phosphorus and phosphorus release to solution in three soils after long-term mineral and manure application Diego Pizzeghello*, Antonio Berti, Serenella Nardi, Francesco Morari Dipartimento di Agronomia, Animali, Alimenti, Risorse Naturali e Ambiente (DAFNAE), Università di Padova, Viale dell’Università 16, 35020 Legnaro, Padova, Italy A R T I C L E I N F O Article history: Received 9 March 2016 Received in revised form 23 August 2016 Accepted 15 September 2016 Available online xxx Keywords: Fertilization Carbonates Change point Sorption index A B S T R A C T Continued fertilizations have led to soil phosphorus (P) concentrations that exceed those required for optimal plant growth. In this study, P accumulation and leaching were investigated in 1-m profile of three reconstructed soils (sandy, SDY; clay, CLY; peaty, PTY) typical of the Po Valley (northern Italy) following long-term (44 years) farmyard manure and mineral applications and untreated. Soil test P was determined by extracting with ammonium oxalate (P Ox ), Mehlich-3 (P M3 ) and Olsen (P Ols ) solutions, whereas soil P release was estimated by water extraction (P H2O ). Two single-point sorption indices (PSI 19.4 and PSI 50 ) were used to assess the P sorption capacity in these soils. The relationship between extractable P forms and water P was also modelled as a segmented line. Soil test P resulted as being affected by type of soil, treatment and depth. Farmyard manure increased the P forms by 7.7-fold that of untreated down to 50 cm in CLY and PTY, and 28-fold untreated down to 90 cm in SDY, thus indicating both accumulation and leaching of P. The P sorption indices were affected by type of soil, treatment and depth. P accumulation was also confirmed by increments of PSI 50 in the manure-treated deeper layers, likely due to increased organic matter and/or precipitation and co- precipitation of P with carbonates in these soils. Although mineral fertilization also increased extractable P it was with lower magnitude than manure on P leaching and P sorption indices. From the segmented lines between P H2O and extractable P, the presence of change points in CLY and PTY revealed P H2O concentrations above the environmental threshold values. In the light-textured SDY soils, the low P sorption capacity and absence of change points evidenced high P leaching and potential contamination of surface and ground waters, in particular when manure was used. ã 2016 Elsevier B.V. All rights reserved. 1. Introduction Phosphorus (P) loss by leaching and subsurface pathways is recognized as a significant problem in P-saturated soils, poorly drained soils and those high in organic matter (Djodjic et al., 2004; Vanden Nest et al., 2016). However, losses of P in drainage water from agricultural soils in the Mediterranean region are generally considered low because leaching is limited and the subsurface horizons are rich in P sorbents such as clay, iron oxides and carbonates. Recent evidence has shown that continued fertiliza- tions in calcareous soils have led to available P concentrations in the upper layers that exceed those required for optimal plant growth (Vu et al., 2008, 2009; Hao et al., 2008; Pizzeghello et al., 2011), while changes in some P sorption related properties have been found at depth (Olson et al., 2010; Pizzeghello et al., 2014). It must also be considered that the application of manure, as in the Po Valley (Northern Italy), influences the chemistry of P in the soil and favours its leaching and subsurface transport from soil to aquatic ecosystems (von Wandruszka, 2006). In risk assessment systems for P losses, soil P retention capacity is an important factor controlling its release from soil to water. It is generally determined in the laboratory by equilibrating soil with a range of P concentrations for a set period of time. The amount of P sorbed is estimated as the difference between the amount of P added and that remaining in solution at equilibrium (Sharpley, 1995). The data obtained are fitted into adsorption models and various indices of P adsorption capacity are determined. However, this is a time-consuming process that cannot be adapted to routine laboratory analysis. To overcome this problem, the use of a single- point isotherm was suggested (Bache and Williams, 1971; Mozaffari and Sims, 1994) with the amount of P sorbed taken as a reasonably accurate index of the ability of the soil to retain P. A * Corresponding author. E-mail address: diego.pizzeghello@unipd.it (D. Pizzeghello). http://dx.doi.org/10.1016/j.agee.2016.09.015 0167-8809/ã 2016 Elsevier B.V. All rights reserved. Agriculture, Ecosystems and Environment 233 (2016) 214–223 Contents lists available at ScienceDirect Agriculture, Ecosystems and Environment journa l homepage : www.e lsevier.com/loca te/agee