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 PO 4 . 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.

Keywords

P speciation; P stocks; Apatite weathering; Podzolization; P K-edge XANES spectroscopy

Published in

Geoderma
2022, volume: 406, article number: 115500
Publisher: ELSEVIER

Authors' information