Phosphorus forms, transformation, sorption, and release in sediments of Walnut Creek watershed, Iowa

Rahutomo, Suroso
Major Professor
Michael L. Thompson
John L. Kovar
Committee Member
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The dynamics of phosphorus (P) reactions in stream water have received much attention due to their potential to trigger eutrophication. This study aimed to explore the dynamics of P in Walnut Creek, Jasper County, Iowa. The Walnut Creek watershed supports a variety of land uses (row crop production, grazing, and riparian buffer zones), and the alluvial cross section is composed of a sequence of sediments that contribute differentially to the amounts and forms of P entering the stream. The experiments were focused on the distribution and transformations of solid-phase P in the sediments, P sink/source status of the sediments under varying physicochemical conditions, potential P release from the sediments to the stream water at varying redox potentials, and a comparison of methods for determining P concentration in diverse matrices.

Twenty-five sediment samples from Walnut Creek watershed (classified into three groups: bank, in-stream, and floodplain deposits) were sequentially extracted for P. The distribution of P among organic and inorganic solid phases varied among the stream sediments. Across all 25 sediments, 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 386-1,134, 5-85, and 60-823 mg kg-1, respectively. Among the groups, the highest concentrations of TP, P-M3, and Pox were in the in-stream deposits. TP was significantly correlated with Fe oxides, clay, and soil organic matter, especially in the bank and floodplain deposits.

The sink-source status for P mobility of the twenty-five sediments in Walnut Creek watershed was predicted for a variety of physicochemical treatments (including variations in solid-to-solution ratio, redox potential, and shaking energy intensity to design treatments of oxic, anoxic, high shaking energy intensity, and low solid-to-solution ratio). The physicochemical treatments resulted in variation in the values of equilibrium phosphorus concentration (EPC) and phosphorus buffering capacity (PBC), and consequently, changed the predictions of the sink-source status of the sediments. Under all physicochemical treatments, it was more likely that the in-stream deposits would act as sources than would the bank sediments or the floodplain soils. It was predicted that most of the twenty-five sediments would act as sources by releasing P to the stream water under an anoxic environment. Total P, Mehlich-3 extractable P, and ammonium oxalate extractable P were the sediment parameters correlated to the EPC values.

Other experiments in this study used four samples from the Walnut Creek bank sediments (the Camp Creek Member, the Roberts Creek Member, the Gunder Member (all three members of the Holocene De Forest Formation), and Pre-Illinoian Till. In the first experiment, the effects of anaerobic incubation and addition of glucose on PBC and EPC of the bank sediments were investigated. The experiment demonstrated that anaerobic incubation decreased Eh and increased pH, and these effects were exaggerated with presence of readily bioavailable carbon. The PBC and EPC varied among the major alluvial units of Walnut Creek bank sediments. In comparison to the treatment without anaerobic incubation, anaerobic incubation significantly increased PBC values of the Camp Creek and Gunder sediments. However, PBC values for the till dropped significantly from 970 L kg-1 (without anaerobic incubation) to 173 L kg-1 (with anaerobic incubation). Only the Gunder sediment showed a significant increase in EPC when samples were equilibrated under anaerobic conditions. In the treatment of anaerobic incubation under abundance readily bioavailable carbon, in all sediments the PBC values were significantly lower and the EPC values were significantly higher than those in the treatment without anaerobic incubation.

The second experiment investigated the effects of oxic and anoxic conditions on the potential release of P from bank sediments. A reactor was specially designed to enclose sediment samples in 3,500 MWCO dialysis tubing, to be inserted into simulated stream water and rotated at a controlled velocity under oxic and anoxic conditions. The experiment demonstrated that variations in oxic and anoxic conditions governed Eh, pH, dissolved organic carbon, and release of P from sediment to the water. The highest dissolved P concentration in the water were exhibited by the Camp Creek and Roberts Creek sediments under anoxic conditions, indicating a higher potential for these sediments to function as an internal P source to an overlying water column under anoxic environments.

The effects of treatments of anaerobic incubation and the addition of readily bioavailable carbon on transformation of P forms in the bank sediments were investigated in the third experiment. The treatments resulted in changes in Eh and pH that were similar to those of the first experiment. The forms of P in four bank sediments of Walnut Creek were redistributed concomitantly with a decrease in Eh and an increase in pH due to anaerobic incubation and the addition of readily bioavailable carbon. Low redox potential increased concentration of labile Pi which coincided with a decrease in slowly cycling P that was associated with Fe. The distribution of slowly cycling P associated with Ca, stable P, and residue P did not significantly change at varying redox potential. Among the four bank sediments of Walnut Creek, the younger sediments with more organic matter, i.e., the Camp Creek and Roberts Creek sediments, had greater labile and slowly cycling P associated with Fe, reflecting a greater potential to contribute to elevated levels of P in the stream water, especially if subjected to low redox potential in the stream.

Lastly, two experiments were conducted to investigate the use of the malachite green (MG) method for determining P concentrations in diverse matrices. The first experiment compared MG to the molybdate blue-ascorbic acid (AA) method to assess the sequential extraction of P during fractionation analysis, while the second compared MG to inductively coupled plasma atomic emission spectrometry (ICP-AES) for determining Pox. The results showed that the MG and AA methods agreed for determining P concentration in the extracts of water, 1 M HCl, and concentrated H2SO4. However, a slight discrepancy between the two methods was found for extracts of 0.5 M NaHCO3 and 0.1 M NaOH, and concentrated HCl. Furthermore, in the determination of Pox, the MG method resulted in significantly higher values than those obtained by ICP analysis.