Phosphorus dynamics in tile-drain flow during storms in the US Midwest

Excess phosphorus (P) in freshwater systems has been associated with eutrophication in agro-ecosystems of the US Midwest and elsewhere. A better understanding of processes regulating both soluble reactive phosphorus (SRP) and total phosphorus (TP) exports to tile-drains is therefore critical to minimize P losses to streams while maintaining crop yield. This paper investigates SRP and TP dynamics at a high temporal resolution during four spring storms in two tile-drains in the US Midwest. Depending on the storm, median concentrations varied between 0.006–0.025 mg/L for SRP and 0.057–0.176 mg/L for TP. For large storms (>6 cm bulk precipitation), for which macropore flow represented between 43 and 50% of total tile-drain flow, SRP transport to tile-drains was primarily regulated by macropore flow. For smaller tile-flow generating events (<3 cm bulk precipitation), for which macropore flow only accounted for 11–17% of total tile-drain flow, SRP transport was primarily regulated by matrix flow. Total P transport to tile-drains was primarily regulated by macropore flow regardless of the storm. Soluble reactive P (0.01–1.83 mg m−2/storm) and TP (0.10–8.64 mg m−2/storm) export rates were extremely variable and positively significantly correlated to both mean discharge and bulk precipitation. Soluble reactive P accounted for 9.9–15.5% of TP fluxes for small tile-flow generating events (<3 cm bulk precipitation) and for 16.2–22.0% of TP fluxes for large precipitation events (>6 cm bulk precipitation). Although significant variations in tile-flow response to precipitation were observed, no significant differences in SRP and TP concentrations were observed between adjacent tile-drains. Results stress the dominance of particulate P and the importance of macropore flow in P transport to tile-drains in the US Midwest. Although only spring storms are investigated, this study brings critical insight into P dynamics in tile-drains at a critical time of the year for water quality management.