Phosphorus Adsorption and Desorption Potential of Stream Sediments and Field Soils.

Agricultural activities have been traditionally considered as the major source of phosphorus in most watershed studies. Phosphorous release from stream sediments, however, has remained largely unattended. Sediments need to be considered also as they exhibit a dual function as sources or sinks of phosphorous. This duality is mainly controlled by their physical and chemical properties. An additional driving force for phosphorus release is imposed by the geomorphology of the stream system. The objective of this study was to quantify the P sorption and desorption for both stream sediments and upland soils at Upper West Emma Creek watershed near McPherson Kansas. We collected sediments from a variety of stream features banks, pools, riffles and bars. Soils were sampled from wheat, row crop, pasture, and manure-amended fields. Water samples were taken under base flow and storm flow conditions. Our analyses of sediments and soils included equilibrium P concentration at zero net P sorption (EPC₀), maximum adsorption capacity (Pmax), anion exchange extractable P (AEP), and degree of P saturation (DPS). Water samples were analyzed for dissolved reactive phosphorous (DRP). EPC₀ was similar between stream features, with an average of about 0.1 mg P L^-1. AEP of bank sediments increased 3-fold respect to other sediments in the stream system. Additionally, Pmax of bed sediments (i.e., pools, riffles and bars) was 19 mg kg^-1; 10-times less than Pmax of bank sediments and field soils. Bed sediments also exhibited low AEP (8.6 mg kg^-1) and high DPS (41%), which indicates that they do not have much more assimilation capacity remaining. Stream DRP during base flow was in the same range as the EPC₀ of sediments and stream DRP during storm flow was in the general range as the EPC₀ of field soils. Finally, P reduction in stream water must address soil losses to be effective in this watershed.