Total phosphorus concentrations and compaction in riparian areas under different riparian land-uses of Iowa

Reducing non-point source phosphorus (P) pollution is often necessary to improve water quality in agricultural streams. Soil total P (TP) concentrations and compaction are soil characteristics that can influence P losses to streams. The objective of this study was to compare these two soil characteristics among riparian forest buffers, grass filters, pastures with cattle fenced out of the stream, intensive rotational, rotational and continuously grazed pastures and row-cropped fields in three physiographic regions of Iowa. Soil TP and compaction for the seven riparian land-uses were determined in stream bank and surface riparian soils. Total P concentrations in stream bed material along the seven riparian land-uses were also measured. Total P concentrations in riparian stream bank soils among land-uses ranged from 303 to 398 mg kg−1 in the central region, to 432–518 mg kg−1 in the northeast, to 360–555 mg kg−1 in the southeast. In the surface riparian soils TP among land-uses ranged from 434 to 649 mg kg−1 in the central region, to 493–764 mg kg−1 in the northeast region, to 428–716 mg kg−1 in the southeast region. Finally, the TP concentrations in the stream bed sediments among land-uses ranged 194–307 mg kg−1 in the central region, to 169–461 mg kg−1 in the northeast, to 389–964 mg kg−1 in the southeast. Few soil TP significant differences among riparian land-uses within regions were found. Soil compaction under some of the grazing practices was significantly higher than under the conservation practices. Limited differences in soil TP concentrations and compaction were partially due to the young age of the conservation practices and the rotational and intensive rotational pastures. Past land-use on these sites had been either continuous grazing or row-cropping and those uses were still influencing these soil parameters. Within riparian land-uses, surface riparian soils generally had higher TP than the stream bank riparian soils. Stream bed samples had lower TP concentrations than the riparian soils in the northeast and central regions but were higher than the riparian soils in some of the land-uses in the southeast region. Overall TP concentrations in the stream bank, surface riparian soils and stream bed material were high indicating that these areas can potentially be important source areas of P and the focus should be on reducing the transport of P.