Phosphorus saturation of Finnish soils: evaluating an easy oxalate extraction method

Abstract

The aim of this study was to test whether phosphorus saturation of surface sorption sites of (oxyhydr)oxides of aluminium (Al) and iron (Fe) in Finnish soils can be assessed using a single oxalate extraction and, if so, whether the results are closely related to the P forms likely to influence the P concentration in runoff waters. Ten soil samples with varying clay content and P status were studied. Desorption tests were conducted by submitting the soils sequentially to nine anion exchange resin (AER) extractions. Sorption of P was studied by shaking the soils in P standard solutions (0-250 ppm). Soil inorganic P was characterised by sequentially extracting P from the fractions assumed to be connected to Al and Fe compounds and present as the stable apatitic form. The desorption studies and the fractioning of inorganic P suggested that oxalate solution dissolves apatitic P and/or other relatively stable P-bearing compounds, probably referring to the sum of inorganic P fractions rather than labile P. The amount of P desorbed in the nine AER extractions was about 80-280 mg/kg, whereas oxalate extracted about 490-1100 mg P/kg, which approximated the sum of the inorganic P fractions. Therefore, in soils high in apatitic P, oxalate-extractable P does not seem to be a reliable measure of the P saturation of Al and Fe oxide surfaces that regulate the P concentration in soil solution and runoff water.;

Identification of inositol hexakisphosphate binding sites in soils by selective extraction and solution 31P NMR spectroscopy

“…The oxalate solution is used to extract amorphous Al and Fe oxides as well as P bound to those oxides, but does not extract more crystalline metals (García-Rodeja et al, 2007;Gleyzes et al, 2002;Jan et al, 2013;McKeague et al, 1971a;McKeague and Day, 1966). It also extracts amorphous Al and Fe associated with organic matter (McKeague et al, 1971b) and calcium-bound P due to pH dependent dissolution of Ca compounds (Lookman et al, 1997;Uusitalo and Tuhkanen, 2000), although the acidic pH in the soils used in this study suggests that Ca compounds are unimportant as sorption sites for P in these soils. The identification of IP 6 in oxalate extracts of soils by solution 31 P NMR spectroscopy is to our knowledge the first evidence for the chemical nature of P in oxalate extracts of soils and shows that oxalate extractable P can contain considerable concentrations of IP 6 .…”

Laboratory drying of organic-matter rich soils: Phosphorus solubility effects, influence of soil characteristics, and consequences for environmental interpretation

“…TOC and TN were analysed by dry combustion on a LECO CNS-1000 elemental analyser after pre-treatment with sulphurous acid to burn off any carbonates. OM was determined by loss on ignition, after combustion at 550°C for 4 h. Exchangeable bases and Mn were determined by extracting soils with ammonium acetate at pH 7.0 (Allen, 1989), while ammonium oxalate-extractable aluminium (Al ox ), iron (Fe ox ) and phosphorus (P ox ) were quantified following Uusitalo and Tuhkanen (2000). Extraction solutions were filtered through Whatman No.…”

Incremental acidification reveals phosphorus release dynamics in alkaline vertic soils

“…An anion sink can be used to generally mimic the removal of P from solution by plants (Myers et al, 2005). The replenishment of solution-P or labile-P by desorption has been described by maintaining a low solution P concentration with repeated ion sink extractions, using anion exchange membranes (AEM) (Sato and Comerford, 2006;Uusitalo and Tuhkanen, 2000) or iron oxide (Lookman et al, 1995;McLaren et al, 2014a). A combination of repeated ion sink extraction and soil acidification may illustrate the release of reserve-P.…”