Does vivianite control phosphate solubility in anoxic meadow soils? E. Walpersdorf a, ⁎, C. Bender Koch b , L. Heiberg a, c , D.W. O'Connell a, 1 , C. Kjaergaard d , H.C. Bruun Hansen a a Department of Plant and Environmental Sciences b Department of Chemistry, University of Copenhagen, 2100 Copenhagen Ø, Denmark c Institute of Biology, University of Southern Denmark, 5230 Odense M, Denmark d Department of Agroecology, Aarhus University, 8830 Tjele, Denmark abstract article info Article history: Received 4 August 2011 Received in revised form 20 June 2012 Accepted 1 October 2012 Available online 17 November 2012 Keywords: Wetland Gyttja Vivianite Sorption Iron(II) Phosphate Vivianite (Fe 3 (PO 4 ) 2· 8H 2 O) may precipitate in anoxic wetland soils where it may control orthophosphate (P i ) equilibrium solution concentrations at micromolar levels, and thus be of key importance in reducing exces- sive P from agricultural sources and eutrophication. However, vivianite equilibria and kinetics under in situ conditions are not fully understood and the occurrence of vivianite in wetland soils is rarely documented. In the present investigation we have monitored the temporal (November to June) variation in the pore water chemistry of a wet meadow soil (Sapric Medihemist) including a vivianite-containing gyttja layer. Pore water concentrations of Ca, Fe II , HCO 3 , and NH 4 in the gyttja layer were higher than in adjacent horizons. In contrast, dissolved P i concentrations were the lowest observed in the profile and showed only minor fluc- tuations (between 0.1 and 6 μM). Pore water composition in the gyttja layer was close to equilibrium with vivianite (saturation index, SI viv , 2.01 ± 0.53) at constant pH (~ 6.8). Dissolution and precipitation experi- ments in the laboratory with soil suspensions from the gyttja layer demonstrated that vivianite solubility equilibria were only slowly restored. Even after 120 days following perturbation the supersaturation was still high (SI viv ~ 6). It seems that vivianite does contribute to P i immobilization in anoxic soil horizons, but due to slow precipitation kinetics such soils cannot maintain P i concentrations at levels below critical thresh- olds for eutrophication (~ 1 μM), except if pore water geochemistry is kept stable. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The geochemistry of orthophosphate (P i ) in anoxic soils and sedi- ments has been intensively investigated for many years without a clear picture emerging (e.g. Brand-Klibanski et al., 2007; Gächter and Müller, 2003; Hoffmann et al., 2009; Reddy et al., 1999; Roden and Edmonds, 1997; Zhang et al., 2003). It is well documented that anoxic conditions lead to reductive dis- solution of iron(III) oxides, followed by release of sorbed phosphate to solution (House et al., 1998; Kjaergaard et al., 2012; Patrick and Khalid, 1974; Ponnamperuma, 1972; Scalenghe et al., 2002; Szilas et al., 1998). This is an integral part of the internal phosphorous cycling in eutrophic lakes having seasonally anoxic bottom waters (Boström et al., 1988; Roden and Edmonds, 1997) and in wetland soils (de Mello et al., 1998; Heiberg et al., 2010; Zak and Gelbrecht, 2007). Knowledge gaps exist relating to the fate of P i released during reductive dissolution, as in most cases the amount of P i released to solution is much less than expected based on the amount of iron(III) oxide reduced (Jensen et al., 1998; Kirk, 2004; Willett, 1989). Possible explanations for this include resorption to redox-stable sorbents (e.g. aluminum oxides, phyllosilicate clays and calcite), assimila- tion by microorganisms and plants, or precipitation as calcium, magnesium or mixed metal phosphates, or as iron(II) phosphates, e.g. vivianite (Fe 3 (PO 4 ) 2· 8H 2 O), (Kirk et al., 1990). Furthermore, iron(III) oxides with a high P i saturation are less readily reduced than oxides without P (Borch and Fendorf, 2008). As iron(II) concentrations are often rather high in the anoxic soils and sediments, precipitation of iron(II) phosphates is likely to take place. Equilibrium modelling indicates that occurrence of vivianite and vivianite-type compounds in wetland soils, lake and river sedi- ments is not uncommon, but direct evidence is rarely presented (e.g. in Hearn et al., 1983; Manning et al., 1991; Nanzyo et al., 2010; Nriagu and Dell, 1974; Postma, 1981; Taylor and Boult, 2007). The solubility of vivianite is given by: Fe 3 ðPO 4 Þ 2 ·8H 2 O⇋3Fe 2þ þ 2PO 3- 4 þ 8H 2 O and has an equilibrium constant of pK sp =35.767±0.076 at 25 °C (Al-Borno and Tomson, 1994). The pH of anoxic soils and sediments is often relatively high (~ pH 7) due to the acid consuming process of iron(III) oxide reduction. At the concurrent high release of Fe 2+ the concentration of phosphate in equilibrium with vivianite and in Geoderma 193–194 (2013) 189–199 ⁎ Corresponding author. Tel.: +45 21366368. E-mail addresses: evawalpersdorf@gmail.com (E. Walpersdorf), cbk@life.ku.dk (C.B. Koch), Lisa@biology.sdu.dk (L. Heiberg), david.w.oconnell@gmail.com (D.W. O'Connell), C.Kjaergaard@agrsci.dk (C. Kjaergaard), haha@life.ku.dk (H.C.B. Hansen). 1 Present address: Ecohydrology Research Group, University of Waterloo, Ontario, Canada. 0016-7061/$ – see front matter © 2012 Elsevier B.V. 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