Impact of Land Management Practices on Subsurface Colloidal Phosphorus Transport

Abstract

Wisconsin's abundant supply of dairy manure serves as a crucial phosphorus (P) source for land cultivation, but overapplication can lead to loss through both erosion and leaching. Despite evolving research on modeling these processes, there's a pressing need for leaching breakthrough curve (BTC) studies and varied flow equation-based modeling. Employing CT scans, flow rate analysis, and BTC analysis, we investigated soil structure and P leaching for three land managements: alfalfa, orchard grass, and tilled corn. Alfalfa, corn, and orchard grass exhibited unique macropore characteristics, influencing flow rates and breakthrough behavior via their differing macropore characteristics. Analysis of solute transport behavior using BTCs revealed preferential flow across varied land management and manure scenarios. CXTFIT simulations elucidated parameter impacts on solute dynamics, though there were some discrepancies between observations in breakthrough and fitted parameters. Alfalfa improved in goodness of fit in the mobile-immobile model over convection-dispersion to a greater degree than corn and orchard grass. This improvement is easily explained using macropore characteristics. On the contrary, the macropore structure of Orchard Grass could explain the increased goodness of fit in the convection-dispersion model for one core but did not explain the preferential flow of this land management as indicated in tracer BTCs. There is potential enhancement through additional measurements like pore size distribution and consideration of sorption properties if CT-derived characteristics are used to make land management recommendations. Further insight into P leaching tendencies was gained through nutrient breakthrough and tracer curves, emphasizing soil pore structure's role in the facilitation of leaching versus retention. Tilled corn exhibited the lowest loss of dissolved reactive P (DRP) as a percentage of total P applied, but there was no significant difference for colloidal P loss. Tillage is suggested as the prominent factor impacting macropore structure and solute flow over crop type. Preferential flow was further found to display nuance in its presentation via P and colloid BTC shape. Therefore, to connect flow parameters to general breakthrough patterns to reduce overall leaching, multiple analytical methods should be conducted. Metrics such as total recovery (zero-order moment), mean arrival time (first-order), variance (second), skewness (third), and kurtosis (fourth) should be measured in future work to fully grasp preferential flow and the differences between solutes in colloidal versus dissolved forms. Future work also requires longer-duration rainfall simulations and further CXTFIT investigations to improve model goodness of fit.