Surface-to-tile drain connectivity and phosphorus transport: Effect of antecedent soil moisture

Macropores connecting surface soils to tile drains can alter water and nutrient transport through the subsurface. In this study, laboratory rainfall simulations with artificial macropores combined with edge-of-field monitoring were used to evaluate surface-to-tile drain connectivity and phosphorus (P) transport as a function of antecedent moisture conditions. Laboratory rainfall simulations using repacked soil boxes with different macropore layouts (i.e., no macropore, surface-connected macropores, and disconnected macropores) were used to examine changes in water sources and flow pathways to tile drains with varying degrees of connectivity and antecedent wetness. Water, tracer, and P fluxes from a tile-drained field were also monitored to quantify linkages among water flow pathways, antecedent wetness, and P delivery to tile drains. Both laboratory and field results showed that surface-to-tile drain connectivity was important for water transport through the subsurface under both dry and wet antecedent conditions. When soil conditions were dry, discharge was minimal and primarily comprised of event water that bypassed the soil matrix. Increasing wetness resulted in similar event water transport, but greater mobilization of stored pre-event water and greater discharge; thus, the dominant source of tile water and the magnitude of tile discharge were substantially altered with changing antecedent moisture. Field data revealed that changes in drainage water source and discharge with increasing wetness impacted dissolved P transport. Dissolved P concentration decreased and loading increased with increasing wetness. Findings indicate that greater mobilization of pre-event water under wet antecedent conditions acted as both a hydrologic and chemical buffer for subsurface dissolved P transport. Comparison of study results to water quality data from a larger edge-of-field network suggest that relationships between antecedent moisture conditions, water flow pathways, and P transport from the current study are broadly applicable across tile-drained fields. Understanding processes controlling P delivery to tile drains has direct applicability for conservation practice implementation and improving process representation in models.