Phosphorus export dynamics from two Swiss grassland catchments

Phosphorus (P) export from agricultural catchments is often event-driven and variable in time. Accordingly, a proper monitoring requires high-temporal resolution sampling during flow events. We report on a comparative study on the P export dynamics from two grassland catchments in Switzerland, where concentrations of dissolved reactive Phosphorus (DRP) have been monitored at a high temporal resolution for periods of 1.2 and 5 y, respectively. The data set is complemented by measurements on total P (TP), total dissolved P (TDP) and particulate P (PP) obtained from routine sampling ongoing since 1984 in both catchments. In one area, the high-temporal resolution monitoring revealed a pattern of the P export dynamic as expected from other studies. During flow events DRP concentrations usually increased significantly exceeding 1000 mg DRP m−3 during peak flow, while the values remained low between 20 and 100 mg DRP m−3 at base flow. This pattern was partially confirmed in the second catchment. The expected pattern was observed in 1998 and 1999 but was partially replaced by a new one for the following years. In the growing season, DRP only weakly increased with flow rate or remained almost constant. During flow events values larger than 300-400 mg DRP m−3 were rare. In contrast to these relatively low peak flow concentrations, high values (200-300 mg m−3) were recorded at base flow during the years 2000-2002. During the same period, DRP concentrations exhibited strong daily oscillations. Independent sampling and analytical control measurements verify the findings. Point sources like leaks from manure tanks could be ruled out based on the ammonia measurements. We suggest that biogeochemical processes within the brook are responsible for this change in the P export dynamics. Despite these changes in the pattern of the P export dynamic and ecological measures implemented on the farms no significant effect on the annual P loads could be observed during the last 10 years.