Inorganic and Organic Phosphorus in Sediments in the Walnut Creek Watershed of Central Iowa, USA

The dynamics of phosphorus (P) reactions in stream water are important because of their potential to trigger eutrophication. This study aimed to explore the nature of P in sediments associated with Walnut Creek, Jasper County, Iowa. The Walnut Creek watershed supports row crop production, grazing, and riparian buffer zones. The alluvial cross section is composed of a sequence of sediments that contribute differentially to the amounts and forms of P entering the stream. Twenty-five sediment samples collected near Walnut Creek (classified as bank, in-stream, and floodplain deposits) were sequentially extracted for P. Across all 25 samples, the inorganic P (Pi) fractions followed the order Fe-bound Pi > Ca-bound Pi > reductant-soluble Pi > Al-bound Pi > soluble and loosely bound Pi. For the organic (Po) fractions, the order was nonlabile Po > fulvic acid-bound Po > humic acid-bound Po > labile Po > moderately labile Po. The ranges of total P (TP), Mehlich-3-extractable P (P-M3), and ammonium oxalate-extractable P (Pox) were 386 to 1134, 5 to 85, and 60 to 823 mg kg−1, respectively. Among the sample groups, the highest concentrations of TP, P-M3, and Pox were measured in in-stream deposits. Total P was significantly correlated with Fe oxides, clay, and soil organic matter, especially in the bank and floodplain deposits. Because of the potential release of P from these sediments, we can speculate that changes in land use within the riparian areas may, at least initially, have little direct effect on soluble or particulate P loads in Walnut Creek.