Can the isotopic exchange kinetic method be used in soils with a very low water extractable phosphate content and a high sorbing capacity for phosphate ions? Lalajaona Randriamanantsoa a, b, ⁎, Christian Morel c, d , Lilia Rabeharisoa b , Jean-Marie Douzet e , Jan Jansa a, 1 , Emmanuel Frossard a a ETH Zurich, Institute of Agricultural Sciences, Group of Plant Nutrition, 8315 Lindau, Switzerland b Laboratoire des Radio-Isotopes, Route d'Andraisoro, BP 3383,101 Antananarivo, Madagascar c INRA, UMR 1220 TCEM, 71 Av. E Bourleau, BP 81, F-33883 Villenave d'Ornon Cedex, France d Bordeaux Sciences Agro, UMR 1220 TCEM, F-33175 Gradignan, France e CIRAD PERSYST, UR Systèmes de Culture Annuels, Avenue Agropolis, 34398 Montpellier Cedex 5, France abstract article info Article history: Received 20 August 2012 Received in revised form 26 January 2013 Accepted 30 January 2013 Available online xxxx Keywords: Ferralsol Andosol Phosphate ion sorption Low P availability Isotopic technique The isotopic exchange kinetic (IEK) method allows assessing the rate of orthophosphate ions (Pi) exchange between the solid phase and the solution of the soil with time. However, two challenges have to be tackled when using this technique in soils with a very low water extractable Pi concentration (C P ) and with a high sorbing capacity for Pi. The first is that current colorimetric methods do not allow quantifying Pi concentra- tions lower than 10 μgPL -1 . While the second challenge is that a significant fractionation between P iso- topes may occur in the soil solution system in the presence of soils with high Pi sorption capacity. We assessed here: i) whether concentrating the blue phosphomolybdate complex (BPMC) in hexanol prior to its measurement would allow to lower the detection and quantification limits of Pi, ii) whether a significant isotopic fractionation between 32 Pi and 33 Pi could occur during IEK experiments conducted in the presence of high Pi sorbing substrates (e.g., pure goethite or Malagasy soils) and iii) whether the IEK method when used in conjunction with the hexanol concentration of the BPMC to measure C P would detect changes in Pi isotopic exchangeability in a ferralsol cropped with upland rice following the input of manure or water soluble fertil- izer. The detection and quantification limits of the BPMC concentrated by hexanol were 0.3 and 0.8 μgPL -1 , respectively, using a cell of 10 cm length for the colorimetric measurement. The IEK conducted on Pi amended goethite and on Malagasy soils with 32 Pi and 33 Pi did not show any systematic isotopic fractionation between both isotopes, suggesting that in these soils 31 Pi and 32 Pi or 33 Pi have a similar behavior during isotopic exchange. The analysis of the soils sampled in the field experiment showed a significant increase in the amount of Pi isotopically exchangeable after 1 min only after the application of water soluble P. This increase was paralleled by increases in rice yield and P export by grains demonstrating an increased P avail- ability in this treatment. In conclusion, the IEK method can be used in low P and high Pi sorbing soils as the hexanol concentration method allows measuring very low C P and as the different P isotopes have a similar behavior in the soil/solution system. The IEK experiments conducted in the presence of goethite, however, point out to the necessity of taking into account the dispersion of particles for a proper interpretation of the isotopic data. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Phosphorus (P) deficiency is one of the most important factors limiting crop productivity in tropical regions. A large proportion of the soils of these regions have low to very low water extractable ortho- phosphate (Pi) concentrations which is related either to their very low total P content and/or to their high contents in iron and aluminum ox- ides such as goethite and gibbsite which are known to be strong sor- bents for Pi (Hiemstra et al., 2010; Parfitt, 1978). Tracer techniques based on the use of radioactive P isotopes ( 32 P and 33 P) are powerful tools to assess P availability for plants (Frossard et al., 2011). The isotopic exchange kinetic (IEK) is one of these techniques. It allows measuring the exchange rate of Pi between the solution and the solid phase of the soil (Fardeau, 1996). From an experimental point of view, this method consists of adding into a soil/ water system at a steady-state for Pi a known amount of carrier-free radioactively labeled orthophosphate ions and to follow both the isoto- pic dilution of the radioactive P and the Pi concentration in the solution Geoderma 200–201 (2013) 120–129 ⁎ Corresponding author at: ETH Zurich, Institute of Agricultural Sciences, Group of Plant Nutrition, Eschikon 33, 8315 Lindau, Switzerland. Tel.: +41 52 354 91 43; fax: +41 52 354 91 19. E-mail address: rlalajaona@yahoo.fr (L. Randriamanantsoa). 1 Present address: Institute of Microbiology, Academy of Sciences of the Czech Republic, Vídeňská 1083, 14220 Praha 4 – Krč, Czech Republic. 0016-7061/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geoderma.2013.01.019 Contents lists available at SciVerse ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma