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Soil Chemistry

Characterization of Soil Algal Bioavailable Phosphorus in the Minnesota River Basin

^a Water Resources Science Graduate Program, 500 Pillsbury Dr. SE, Univ. of Minnesota, Minneapolis, MN 55455
^b Dep. of Civil Engineering, Univ. of Minnesota, Minneapolis, MN 55455
^c Dep. of Soil, Water and Climate, Univ. of Minnesota, St. Paul, MN 55108
^d Water Resources Center, Univ. of Minnesota, St. Paul, MN 55108
^e Current address: Kieser & Associates–Environmental Science and Engineering, 536 E. Michigan Ave., Suite 300, Kalamazoo, MI 49007
^f Current address: Minnehaha Creek Watershed District, Deephaven, MN 55391
^g BES-565, National Science Foundation, Arlington, VA 22230

^* Corresponding author (brezonik{at}umn.edu)

Soil algal bioavailable P (ABP) is the P fraction that contributes most directly to eutrophication in freshwaters affected by agricultural nonpoint-source pollution. There are uncertainties regarding the algal bioavailability of P sorbed to calcareous glacial till soils in the upper Midwest. The ABP of soil samples with a broad range of pH and calcium carbonate content from six sites across the Minnesota River Basin (MRB) was measured by algal bioassay, and relationships with various soil physical and chemical properties were studied. For soils of the MRB, a major agricultural watershed in the upper Midwest, ABP was significantly correlated (p < 0.001) with Bray P, Mehlich-III P, NaOH P, Oxalate P, and Fe-paper P. Among them, Fe-paper P approximated ABP best, particularly for calcareous soils. For acidic soils, amorphous Fe and Al apparently were the primary P retention agents in soil particles. Soil sorption data were well described by the linearized Langmuir sorption model. Although the P sorption maximum () could not be predicted from basic soil physical and chemical properties, the sorption energy constant (b) was highly correlated with soil pH, clay content, and organic matter (OM) content. A P saturation index (PSI_s) that uses sorptivity ( x b) as the measure of sorption capacity gave the best estimate of soil ABP among the predictors used in this study. Phosphorus saturated index itself can be approximated by the widely available Bray-P value for soils in the MRB.

Abbreviations: ABP, algal bioavailable phosphorus • CCE, calcium carbonate equivalent • EPC, equilibrium phosphorus concentration • MPCA, Minnesota Pollution Control Agency • MRB, Minnesota River Basin • OM, organic matter • PSI_m, phosphorus saturation index based on sorption maximum • PSI_s, phosphorus saturation index based on sorptivity • SRP, soluble reactive phosphorus • STP, soil test phosphorus • TP, total phosphorus

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