Investigation of phosphorus transport dynamics using high-frequency monitoring at a subsurface-drained field in the Western Lake Erie Basin

To minimize the environmental impact of phosphorus (P) loss from subsurface-drained fields to freshwater water bodies like the Great Lakes, a detailed understanding of P transport dynamics is vital. The main objective of this study was to investigate P transport dynamics using high-frequency monitoring. We used the HydroCycle-PO4 instrument to measure total reactive P (TRP) concentration at a high resolution from a subsurface-drained farm with continuous no-till and Blount loam soil. We used a dataset containing hourly TRP concentration and hourly drainage discharge measurements in the analysis. Results showed that there was a good relationship between TRP concentration and drainage discharge (R-squared = 0.60) such that TRP had a transport-limited chemodynamic pattern, that is TRP concentration tended to increase with increase in flow during events. A 1% increase in drainage discharge resulted in a 1.36% increase in TRP load, indicating a significant increase in P concentration during high flows. We found that flow events substantially contributed to P loss (89%) because of capturing the rapid increase in P concentration during high flows. The rate of increase in P concentration during the rising limb ranged from 0.02 to 0.66 mg/L per hour. The highest 7.7% of drainage flow transported 75% of the TRP load during the monitoring period. The hysteresis pattern tended to be positive (clockwise) during the study period, indicating that preferential flow was a pathway for TRP loss. Most flow events (30 out of 36) displayed a flushing effect in which P concentration increased with rise in drainage discharge. In conclusion, high-frequency P sampling showed that management and conservation practices should target flow events to reduce P loss.