Abstract
The molecular speciation of phosphorus (P) in forest soils is of strategic importance for sustainable forest management. However, only limited information exists about soil P speciation in boreal forests. We combined P K-edge XANES spectroscopy, wet chemical P extractions, and X-ray diffraction analysis of soil minerals to investigate the vertical distribution of P species in seven podzolised forest soils differing in soil properties and climatic conditions. The results showed that the total P stock was on average, 4.0 g m−2 in the Oe horizon, 9.5 g m−2 in the A and E horizons, and substantially higher (117.5 g m−2, and 109.3 g m−2) in the B and C horizons down to 80 cm depth, respectively. Although the Oe horizons contain a minor total P stock, 87% of it was stored as organic P. The composition of P species in the P-depleted A/E horizons was highly variable depending on the site. However, of the P stored in B and C horizons down to 80 cm, 58% was adsorbed P, mostly to Al, while apatite accounted for 25% of P, most of which was found in the C horizons. The apatite stocks in the A/E, B, and C horizons (down to 80 cm) accounted for 2.5%, 20%, and 77.2%, respectively, of the total apatite for all the mineral soils studied. These figures can be explained, first, by the dissolution of primary mineral apatite caused mainly by acidification. Second, P uptake by plants and microorganisms, and the associated formation of the Oe horizons, led to the formation of soil organic P. Further, the formation of organo-metal complexes and podzolization led to the translocation of P to the B horizons, where P accumulated mostly as P adsorbed to imogolite-type materials (e.g. allophane) and ferrihydrite, as shown by P K-edge XANES spectroscopy. In conclusion, this study shows that despite the young age of these soils (<15,000 years), most of the primary mineral apatite in the upper 30 cm has been transformed into organic P, and Fe-, Al-bound PO4. Moreover, the subsoil P, mainly consisting of adsorbed P to Al, and apatite, dominates the P inventory and probably serves as a long-term buffer of P.
Highlights
Phosphorus (P) is an essential macronutrient whose availability in soil might constrain forest productivity, succession, and functioning (Elser et al, 2007, Jonard et al, 2015, Lang et al, 2017, Talkner et al, 2015, Vitousek et al, 2010)
Soil phosphorus speciation was investigated in seven forest sites across Sweden that are located along a climate gradient ranging from 56 to 64oN (Fig. 1)
These results suggest a high level of consistency between the X-ray absorption near-edge structure (XANES)-derived P speciation results and the information gained from wet chemical extractions
Summary
Phosphorus (P) is an essential macronutrient whose availability in soil might constrain forest productivity, succession, and functioning (Elser et al, 2007, Jonard et al, 2015, Lang et al, 2017, Talkner et al, 2015, Vitousek et al, 2010). Most Swedish forest soils were developed from glacially deposited, nutrient-poor parent material (Andersson et al, 2014) In these young boreal systems, the release of organic acids from the surface horizon and plant roots triggers silicate weathering, which leads to the formation of an illuvial E horizon (Lundstrom et al 2000; Smits et al 2014). Organic-acid-Al/Fe complexes migrate to the B horizon, where they precipitate mainly as imogolite-type minerals (ITM) (allo phane/imogolite), and as iron(III) (hydr)oxides (ferrihydrite and goethite) (Buurman and van Reeuwijk, 1984; Gustafsson et al, 1995; Gustafsson et al, 1999; Karltun et al, 2000; Lundstrom et al, 2000). With time a large part of the biogeochemically active P may end up as adsorbed P in the B horizon (Adediran et al, 2020, Prietzel et al, 2016, Wood et al, 1984)
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