Testing mechanisms underlying the Hedley sequential phosphorus extraction of soils

Abstract

The Hedley fractionation has become the preeminent measure for estimating the bioavailability of phosphorus (P) in soils. However, mechanisms underlying P extractability have never been tested. We hypothesize that P sequentially extracted by individual steps can either be referred to a specific mineral source (Hypothesis 1) or to its binding strength to minerals (Hypothesis 2). We prepared mineral‐P associations in the laboratory using various secondary mineral phases and P forms (orthophosphate, phytic acid, ribonucleic acid), which were then subject to the Hedley sequential extraction scheme (anion exchange resin in H CO3 - form, 0.5 M NaHCO3, 0.1 M NaOH, 1 M HCl, and concentrated HCl at 80°C). Extracts were analyzed for P as well as for the main mineral‐borne elements by inductively coupled plasma–optical emission spectroscopy (ICP–OES). In order to test if the observed mineral dissolution patterns match those of natural soils, we applied the Hedley fractionation to forest soils comprising various P stocks and measured in addition to extracted P also iron, aluminum, and calcium by ICP–OES. Phosphorus extractability from mineral‐P associations differed between P forms and mineral phases. Adsorbed P always contributed to several or all extracts, Hypothesis 1 was thus not tenable. Aluminum hydroxide, allophane, ferrihydrite, and goethite completely dissolved during Hedley fractionation from the third extraction step onwards. Successive mineral dissolution also occurred for the soil samples. Thus, extracted P represents partly desorbed P from various soil constituents and partly P co‐released upon dissolution of various minerals. Consequently, also Hypothesis 2 could not be confirmed, i.e., the sequential extraction is not suitable to assess different binding strengths between P forms and minerals. We conclude that the method hardly provides information for studies aiming at the mechanistic understanding of P bioavailability in soil.

Goethite-Bound Phosphorus in an Acidic Subsoil Is Not Available to Beech (Fagus sylvatica L.)

“…In a mesocosm experiment, Andrino et al (2019) showed that arbuscular mycorrhiza in symbiosis with tomato were able to acquire orthophosphate and phytate from associations with goethite. Again, more P was plant-available in the treatment with inorganic P. This finding is consistent with the observation that more goethite-associated phosphate than phytate was extractable by an anion exchange resin in HCO 3 − form, which mimics a plant root taking up P (Klotzbücher et al, 2019). In a hydroponic experiment, palisade grass and ruzigrass took up orthophosphate that was associated with goethite or amorphous Al oxide (Merlin et al, 2016).…”

“…The factors limiting P uptake depicted by Lambers et al (2008) might also explain why our results differ from results of Merlin et al (2016) and Klotzbücher et al (2019). In the study of Klotzbücher et al (2019) it was shown that 43% of goethiteassociated phosphate and 6% of goethite-associated phytate were extracted by an anion exchange resin in HCO 3 − form, which mimics a plant root taking up P. The resin was placed together with P-loaded goethite in ultrapure water. Phosphorus release was driven by the chemical gradient of P between goethite and solution, and an equilibrium state was permanently changed by P removal by the resin.…”

“…There is also experimental evidence that oxide-associated P can be available to plants, whereby availability also depends on the P form (Andrino et al, 2019;Klotzbücher et al, 2019;D'Amico et al, 2020). Soil P includes inorganic and organic P; the presumably most persistent P-monoester is phytate (Darch et al, 2014), which might, therefore, predominate in soil.…”

Contrasting Effects of Long-Term Nitrogen Deposition on Plant Phosphorus in a Northern Boreal Forest

“…The remaining residue fraction extracted mainly recalcitrant P. Additionally, acid-digestible Al and Fe were determined on a subset of the soil using microwave digestion as above. We acknowledge that the Hedley fractionation forms are "operationally defined" and are limited in specifically defining different P forms (see, for instance, Klotzbücher et al, 2019) but still a useful approach in comparative studies like ours to investigate the fate of native P in managed systems as suggested in a review by Negassa and Leinweber (2009).…”

Modeling Soil Responses to Nitrogen and Phosphorus Fertilization Along a Soil Phosphorus Stock Gradient

“…This decrease is concurrent with increasing soil N:P ratios in the topsoil (0-30 cm) along the gradient, and decreasing P concentration in the litterfall, including leaf litter and fine roots in forest floor and mineral soil ( Table 1). Such a decreasing trend along the gradient is less remarkable in soil P content (mg P/kg soil) and available P content [mg P/kg soil, resin P and NaHCO 3 extracted P in Hedley fractionation (Klotzbücher et al, 2019)], due to a noticeably lower stone content at MTF than VES ( Table 1). Other soil properties, including soil organic and microbial C contents as well as soil and microbial C:N:P ratios, do not show clear trends along the total soil P gradient.…”