Dynamics of dissolved reactive phosphorus loss from phosphorus source and sink soils in tile-drained systems

Understanding the processes controlling dissolved reactive phosphorus (DRP) loss in tile-drained systems is essential to better define critical source areas and inform nutrient and conservation practice recommendations. Concentration-discharge (C-Q) relationships have been used to infer solute sources, reactivity, proximity, and transport mechanisms governing solute fluxes. In this study, we compare DRP loads and flow-weighted mean DRP (FDRP) concentrations from P source and sink soils at an annual and event scale, and use power law analysis coupled with index-based approaches in order to identify the predominant DRP C-Q behavior in these soils at a daily and event scale. Results indicate that, in general, P source soils consistently lost higher DRP loads and exhibited greater FDRP concentrations when compared to P sink soils when examined at both annual and event scales. At the daily scale, C-Q patterns were linked to the soil P status whereby a chemostatic (b = 1) and dilution (b < 1) behavior was observed for P source and P sink soils, respectively. At the event scale, C-Q patterns were linked to soil P status with a potential contributing influence of flow path connectivity and mixing of event water with matrix and shallow groundwater. Events in P source soils had variable hysteretic behavior with 21%, 7%, 6%, 9%, and 15% exhibiting anticlockwise with dilution, anticlockwise with flushing, clockwise with dilution, clockwise with flushing, and no hysteresis behavior, respectively. These variable C-Q responses suggest that, in addition to discharge and soil P status, rapid exchanges between P pools, the magnitude of discharge events (Q), and the relative number of days to discharge peak (RL) also regulated solute delivery. On the other hand, the predominant nonhysteretic C-Q behavior of events in P sink soils (67%) suggests that DRP loss from these soils can be discounted. These findings highlight the need for nutrient and conservation practices addressing P draw down, P sequestration, and P supply according to crop need, which will likely be required to convert P sources to sinks and to avoid the conversion of P sinks to P sources.