Phosphorus retention by soils and suspended sediments in the Minnesota River Basin

Phosphorus (P) retention by soils and river suspended sediments (SS) plays an important role in determining the fate and transport of P, the major pollutant responsible for eutrophication in freshwater bodies. In this research, laboratory experiments were conducted with soil runoff boxes to determine bioavailable P (BAP) content in simulated storm runoff in ten mostly calcareous soils from the Minnesota River Basin (MRB). Bioavailable P levels in the runoff were well predicted by various soil test P methods and an index of P soil sorption saturation (PSI-s) derived from the P retention capacities of the soils. Also, in an across-basin synoptic survey of soil P retention capacity, it was found that for soils in the MRB, soil sorption data were well described by the linearized Langmuir sorption model. The Langmuir sorption energy constant was highly correlated with soil pH, clay content, and organic content ( R2 = 0.75, p < 0.001). PSI-s gave the best estimate of soil BAP among all the predictors and PSI-s itself can be approximated by the widely available Bray-P value for soils in the MRB.; Phosphorus sorption by river suspended sediments was studied under conditions typical of soluble reactive P and SS concentrations in waters of the Minnesota-Mississippi Rivers system. Results showed that the modified Freundlich model provided an excellent approximation to the sorption isotherms (average r 2 = 0.98) while the Langmuir model showed poor fitting, suggesting the presence of more than one group of sorption sites with different P binding energies on the SS particles. It was also found that the SS's acted as P sources to the river water.; Reviewing the two point-nonpoint source pollution trading programs in the MRB indicated that point-nonpoint source pollution trading in the MRB was in general able to achieve cost savings on pollution control. The most cost-effective nonpoint source pollution control measures were those involving structures with long lifetime. Simulations conducted with the agricultural field pollutant transport model, ADAPT, demonstrated that high residue conservation tillage was more cost-effective than sugar beet spring cover cropping, a widely used BMP in one of the trading programs.