Monday, November 5, 2007 - 9:45 AM
104-3

Biogeochemistry of Phosphorus in Danube Sediments and Floodplain Soils of Progressing Development.

Franz Zehetner, G.J. Lair, F.-J. Maringer, and M.H. Gerzabek. Institute of Soil Research, University of Natural Resources and Applied Life Sciences (BOKU), Peter-Jordan-Str. 82, Vienna, A-1190, Austria

The biogeochemistry of phosphorus in floodplain soils is influenced by the nature of deposited sediments and by their alterations during soil development. The objective of this study was to follow the transformations of biogeochemical phosphorus fractions from suspended sediments in the Danube channel to floodplain soils of progressing pedogenic development. Suspended sediment was collected during 20 flood events between 1990 and 2006, and floodplain soils were sampled in a chronosequence scheme. The studied flood sediments had similar characteristics over the 16-yr observation period. Total phosphorus (TP) contents averaged 730 mg/kg, and the biogeochemical fractionation yielded important primary mineral contributions (apatite phosphorus, AP, about 80% of TP) and minor portions of P associated with hydrous oxides and organic matter (non-apatite inorganic phosphorus, NAIP, and organic phosphorus, OP, respectively). On the floodplain, most soils classify as Entisols, but show progressing development with age towards the Mollisols order. The youngest soils are characterized by low organic matter contents and mostly amorphous hydrous Fe oxides. With progressing soil age and development, organic matter has accumulated and hydrous Fe oxides have become increasingly crystalline. The TP contents of the floodplain soils are in the range of the Danube sediments and show little variation along the chronosequence. However, the distribution of P among biogeochemical fractions has changed dramatically during <500 yrs of pedogenesis. The youngest soils (<20 yrs) correspond with the Danube sediments and are dominated by AP with minor portions of NAIP and OP. After <250 yrs of pedogenesis, AP has markedly decreased and OP increased, and after <500 yrs, OP has reached AP levels and become an equally important P fraction in the topsoil. This shows that while phosphorus biogeochemistry in very young floodplain soils is strongly related to the river sediments, significant transformations can occur within <250 yrs of pedogenesis.

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