Degree Type

Thesis

Date of Award

2010

Degree Name

Master of Science

Department

Agronomy

First Advisor

John L. Kovar

Second Advisor

Michael L. Thompson

Abstract

Livestock grazing in pastures has been identified as a possible factor controlling sediment and phosphorus (P) delivery to Rathbun Lake and associated water bodies in the watershed. The risk of P loss from soils in this watershed was estimated by Mehlich-3 (M3) extractable P, P sorption indices (Smax and k), degree of P saturation (DPS) and P desorption indices (equilibrium phosphorus concentration (EPC) and phosphorus equilibrium buffering capacity (PEBC)) for seven representative soil samples. Most of selected soils had low risk of P loss as indicated by PM3 and DPS. However, EPC values indicated that some soils could potentially behave as sources by releasing P to the water once they become suspended sediments. Selected soil physicochemical properties, including pH, particle size, total C, total N, total P, FeM3, AlM3, CaM3, MgM3, oxalate-Fe and Al (Feox and Alox), dithionite-Fe and Al (Fed and Ald), were correlated with k, EPC, and PEBC to better understand P sorption-desorption of selected soils. We found that k was positively correlated with Fed (r2 = 0.96, p < 0.001). PEBC was not correlated with any selected soil physicochemical properties. EPC was positively correlated to FeM3 (r2 = 0.72, p < 0.05) and Feox (r2 = 0.62, p < 0.05) suggesting that P desorption was controlled by Fe.

Stream sediments often play an important role in regulating phosphorus (P) concentrations in stream water. Equilibrium P concentration (EPC) is the concentration of P in the solution phase (stream water), at which P is neither sorbed nor desorbed by the solid phase (sediments). In this study, EPC was used as a desorption index to determine whether sediments behave as sinks or sources of P to the stream water in the Rathbun Lake watershed. Eight stream sediments (banks and beds) were collected and EPC determined. Sediment physicochemical properties were correlated with EPC to better understand P desorption of sediments. The EPC of the sediments ranged from 0.02 to 0.12 mg L-1. After comparing EPC of sediments with mean dissolved P concentration in stream (0.08 mg L-1), we found that some sediments could behave as a source of P to water once they became suspended. EPC of sediment increased with sediment pH (r = 0.92, p < 0.01) and sand content (r = 0.78, p < 0.05), but decreased with Mehlich-3 extractable Fe (FeM3) (r = -0.93, p < 0.001). Mehlich-3 P and the degree of P saturation (DPS) predicted a low risk of P loss from sediment. For the Rathbun Lake watershed, these results suggest P desorption increases with sediment pH and sand content while P sorption increases with sediment FeM3 content.

Copyright Owner

Najphak Hongthanat

Language

en

Date Available

2012-04-30

File Format

application/pdf

File Size

71 pages

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