A flow-through reactor to assess potential phosphate release from agricultural soils Emmanuel Frossard a, ⁎, Paolo Demaria a , Sokrat Sinaj a,b , Michael Schärer a,c a Institute of Agricultural Sciences, ETH Zurich, 8315 Lindau, Switzerland b Agroscope research station Changins-Wädenswil ACW, Route de Duillier 50, Case postale 1012, 1260 Nyon, Switzerland c Federal Office for the Environment, Water Division, 3003 Berne, Switzerland abstract article info Article history: Received 24 July 2013 Received in revised form 7 December 2013 Accepted 11 December 2013 Available online xxxx Keywords: Desorption Flow-through reactor Isotopic exchange Kinetics Phosphate Soil Controlling phosphate (P) release from agricultural soils to water while maintaining optimal plant growth con- ditions remain a major challenge for the development of sustainable agricultural systems. To achieve this, it is im- portant to have a proper knowledge of the amount of soil P that can be mobilized by water and of the kinetics of P release. We evaluated the ability of a flow-through reactor in which 33 P labeled soils can be inserted and leached continuously with deionized water, to assess P release. The experiment was conducted on five grassland soils presenting a large range in P availability. The availability of P in these soils was further modified by submitting them to 0 to 3 plant growth cycles with Italian ryegrass (Lolium multiflorum) with three levels of P added (0, 20 and 40 mg P kg soil -1 ). The P input–output balance, water and oxalate extractable P, the degree of P satura- tion of the soil and the amount of isotopically exchangeable P (E value) were assessed in all samples. A subset of these soil samples was labeled with 33 P, introduced in a flow-through reactor and the release of P and 33 P mea- sured over 14 days. The cumulated amount of P released after 14 days was strongly correlated to the amount of oxalate extractable P, isotopically exchangeable P (E value), and water extractable P. The P release kinetics was modeled with a 2 pools model with each pool following first order kinetics. Plants were able to take up P from both pools. Assuming that the leached P had the same isotopic composition as the pool of soil P it came from it became possible to quantify the amount of isotopically exchangeable remaining in the soil which was called the D value. D decreased during the three first days of the flow-through experiment and then increased linearly with time reaching a maximum after 14 days. This maximum remained lower than the oxalate extractable P. Pro- cesses contributing to this increase were isotopic exchange and possibly also some organic P mineralization. The D value was strongly linearly correlated to E values measured after different exchange times, but for a given ex- change time, the D value was lower than the E value, whereas equality could have been expected. This difference was related to the high rate of 33 P export from the soil at the beginning of the flow-through experiment. The D value was also strongly correlated to the oxalate and water extractable P. In conclusion, we suggest that the use of the flow-through reactor yields relevant information on the amount of P that can be leached from a given soil, and that the D value delivers information on the amount of isotopically exchangeable P remaining in the soil and therefore which could still be leached if sufficient time would be given. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Excessive use of phosphate (P) is a problem in many intensively managed agro-ecosystems, especially in those presenting a high live- stock density (Sutton et al., 2013). Inputs of P in amounts that exceed plant needs lead to P losses to water bodies and to their eutrophication. Haygarth et al. (2005) provide a framework to analyze P losses from agro-ecosystems in which they consider the potential amount of soil P which can be mobilized, the mobilization of P by water, and the P trans- fer to water bodies. Some works attempted to decrease the transfer of P to water bodies, e.g. by installing buffer strips between fields and water bodies. However, the efficacy of such measures has been reported to be variable (Noij et al., 2013). The option of decreasing P mobility by increasing soil sorption capacity for P has also been explored, for in- stance by amending the soil with sorbents rich in Fe or Al (Chardon et al., 2012; Groenenberg et al., 2013; McDowell and Nash, 2012). Al- though promising results have been obtained, the efficiency of such amendments and especially of Fe-rich amendments might not hold on the long-term because of the cycles of reduction/oxidation decreasing the efficiency of Fe oxides as sorbent (Schärer et al., 2007, 2009). In any case, the amount of P that can be mobilized has also to be decreased. This can be achieved by decreasing or stopping P inputs and/or by in- creasing P outputs (Dodd et al., 2012; Koopmans et al., 2004a; van der Salm et al., 2009). These approaches are also promising but depending Geoderma 219–220 (2014) 125–135 ⁎ Corresponding author. Tel.: +41 52 354 91 40; fax: +41 52 354 91 19. E-mail address: emmanuel.frossard@usys.ethz.ch (E. Frossard). 0016-7061/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geoderma.2013.12.015 Contents lists available at ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma