From sediment to soil: floodplain phosphorus transformations at the Danube River

Substantial transformations of biogeochemical phosphorus (P) fractions can occur within a few hundreds of years under humid and tropical conditions; however, slower changes are expected under dry and temperate climate. The objective of this study was to infer P transformations over time by comparing suspended sediments in the Danube River to floodplain soils developed from such sediments over different time periods in the continental climate of Central Europe. We analyzed suspended sediments from 20 flood events between 1990 and 2006, and floodplain soils from seven sites along a chronosequence covering about 500 yrs. The studied flood sediments had similar characteristics over the 16-yr observation period. Total phosphorus (TP) averaged 732 mg kg(-1), and biogeochemical fractionation yielded important primary mineral contributions (apatite phosphorus, AP, similar to 80% of TP). The TP concentrations of the floodplain soils were in the range of the Danube sediments and showed little variation along the chronosequence. However, the distribution of P among biogeochemical fractions changed considerably in less than 500 yrs of soil development. The youngest soils (< 20 yrs) were dominated by AP, as was observed for the Danube sediments. In less than 250 yrs of pedogenesis, AP markedly decreased and organic phosphorus (OP) increased, and in less than 500 yrs, OP reached AP levels. This shows that while P biogeochemistry in very young floodplain soils is strongly related to the river sediments, significant transformations can occur in less than 250 yrs of soil development in the dry and temperate climate of Central Europe.