Phosphorus in macropore walls of a Danish Glossudalf

Abstract

The distribution of iron, aluminium and phosphorus fractions in a pseudogley soil (Fine‐loamy, mixed, mesic Oxyaquic Glossudalf), including transects across albic macro‐pores (tongues) in the glossic horizon have been investigated by use of selective extraction techniques. In the glossic horizon a distinct redox‐front is observed between albic ped faces adjacent to the macropore lumen and brown ped interiors. Samples, each representing a thickness of 2–3 mm in transects orthogonal to the macropore and the redox‐front, show up to five times more citrate‐bicarbonate‐dithionite extractable Fe and Al (Fe_d, Al_d) in brown ped interiors than in albic ped faces. Surprisingly, the content of total (P_tot) and inorganic phosphorus (P_i) have a nearly uniform distribution across the transects. In the albic ped faces the contents of citrate‐bicarbonate‐dithionite extractable (P_d) and oxalate extractable phosphorus (P_ox) are both low whereas the amounts of P_ior P extractable by 1 M HC1 (P_HC1) are remarkably high.

Calculations demonstrate that the low content of P_d is not due to phosphate saturation. The large P_HC1 — P_d fraction in ped faces is attributed to occluded P, because the P_HC1 — P_d fraction is equivalent to the P_HC1 — P_ox fraction and oxalate is found able to dissolve apatite. The occluded P is not present within Al and Fe oxides, but probably contained in silicates. In the brown ped interiors most phosphate is bound to crystalline Al and Fe oxides.

Key words:

• Glossic horizon
• occluded phosphate
• Podzoluvisol
• pseudogley
• selective extraction

Notes

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